Flathead Engine Rebuild

You can enlarge any image by clicking on it.
Ford Flathead Isometric View
This web page is about restoring/resto-rodding a 1950 Ford F-1 pickup, and is one of a series of articles documenting the project. I am providing this in the hope that it is helpful to fellow Ford truck owners, but beware that there are many ways to accomplish the same goals... I have only shown one way. Also, keep in mind, that my comments are specific to this year, make and model, and should not be generalized. Feel free to drop me a line if you see mistakes or need more information, but please understand if it takes me a while to respond. To go back to the main project page click here.

Table of Contents

Use these links to Jump to a section within this page:

Current Status

The flathead rebuild is complete, but minor tuning continues (go to the Flathead Tuning & Maintenance page). The engine was run many hours on a test stand; the first test drive was April 2010.

Our Plan

As noted on the main page for the Resto-Rod, while the truck remains more-or-less original, the engine is getting significant upgrades.  My Dad and I had been researching the art of flathead rebuilding for several years, and had a pretty good idea what we wanted to do long before we actually pulled the engine and started the tear down.

Our goals were...
To accomplish our goals, we planned the following engine upgrades & improvements:

Goal
Upgrades
Better breathing
  1. Relieve engine block ("MCF")
  2. Port intake and exhaust
  3. Offenhauser aluminum intake
  4. Four barrel carburetor (vacuum secondaries)
More compression
  1. Aluminum heads
  2. Dome pistons
Better ignition
  1. Converted to 12V negative ground
  2. Mallory dual-point distributor
  3. Performance wires
Improved oil filtration
  1. "McNicholl's" block mod
  2. Modern spin-on remote oil filter
Modern Crankcase Ventilation
  1. Use PCV system
  2. Eliminate downdraft tube
  3. Relocate oil fill to fuel pump location (use electric fuel pump)
 

What is MCF?

I've mentioned "MCF" a couple of times already, so what is it?  MCF is short for Motor City Flathead.  I'd strongly recommend buying the books identified in the Reference section, where the most of the MCF tricks are revealed.  Oddo and McNicholl's both go into MCF modifications.

What We Started With

The truck had a period-correct Flathead V8, but it's not the original engine.  We know it was originally equipped with a V8 by the VIN number (starts with 98RC), but it has a Mercury V8 (8CM) now, so it's not the original.  According to Aunt Doris (daughter of my great-grandfather), the neighbor across the street wrecked her new '51 Mercury, and that's how the truck ended up with a Merc V8.  Whatever the case, its good news, because the Mercury flatheads have a 4" stroke, instead of the 3¾" stoke that the Ford's had.

   Engine   Engine

Tear down

The engine tear down started during the summer of 2005, and there were no unpleasant surprises.  The engine block turned out to be a 1BA casting, which makes sense because according to Aunt Doris, the engine came from a '51 Mercury.  I was expecting a severely sludged engine, but it wasn't all that bad considering the passive oil filtration system all Ford flatheads had.

On engine stand   Bottom end
no crankshaft   heads pulled

Engine Cleaning, Inspection & Machining

Before running off and spending money on parts, you need to thoroughly clean the engine.  We hot-tanked the engine a couple of times, pressure rinsed it, and even did some mild bead blasting of the cooling passages.  Then have the engine inspected,  i.e. checked for cracks (magnafluxed, FPI, whatever).  Once we knew we had a good block, we had all the usual machining done. 

We were fortunate to have a local machine shop with flathead experience (Car Part’s Machine and Auto Supply, in El Paso, TX).  The owner/operator of the establishment (Joe Hicks) gave us a lot of technical guidance which really helped us (even though he hadn’t done a flathead in quite a while).  Joe has subsequently semi-retired and sold his business to Stafford Performance, but he's still active in it.

The engine was bored to 3.248", alignment checked (no machining required), decked, rods resized, and crank journals ground .010, and balanced.

Parts List

Here's our parts list:

Description
Mfr / Spec. / Part No.
Cost*
Supplier
MACHINING
BLOCK
BORE           3.248"
MAIN JOURNALS  0.010"
ROD JOURNALS   0.010"
DECKED + RESIZED + BALANCED
1100.0
STAFFORD
RECIPROCATING ASSY
PISTONS, 3.24"x4.00", HYPERMAX - HYPEREUTECTIC ALU
910-15525
274.95
SPEEDWAY
RINGS, 3.24" STYLE F
910-15572
119.95
SPEEDWAY
SUPPORT, MAIN CAP - FORD FLATHEAD 1949-53
560-3408
39.95
SPEEDWAY
VALVETRAIN
HEADS, .425"
OFFENHAUSER 1069
465.00
PATRICKS
CAMSHAFT, 3/4 RACE
ISKENDERAN MAX 1
279.95
PATRICKS
LIFTERS, ADJUSTABLE
AT740
159.95
PATRICKS
GUIDES, VALVE - SET OF 16
VC428
62.80
PATRICKS
VALVES - SET OF 16
V967
127.80
PATRICKS
RETAINERS, VALVE SPRING - SET OF 16
6514
20.00
PATRICKS
CLIPS, GUIDE RETAINERS
VK88
16.00
PATRICKS
LOCKS, VALVE - SPLIT LOCKS
VK1152
N/C
PATRICKS
GEAR, CAMSHAFT - ALUMINUM
?
42.95
PATRICKS
INDUCTION
ADAPTER, CARBURETOR - FORD FLATHEAD
135-1935B
19.95
SPEEDWAY
CARBURETOR, HOLLEY 390 CFM 4BBL, VACUUM SCNDRY, ELEC CHOKE
425-8007
369.95
SPEEDWAY
MANIFOLD, INTAKE - SINGLE/QUAD OFFENHAUSER
560-1078
199.95
SPEEDWAY
EXHAUST
BAFFLE, EXHAUST - FORD FLATHEAD
916-15201
34.95
SPEEDWAY
HEADERS, EXHAUST - SHORT TUBE
RED'S HEADERS - 1950 F-1
185.00
PATRICKS
GASKETS & SEALS
GASKET, HEAD - GRAPHITE
910-15848
59.95
SPEEDWAY
KIT, GASKET - FLATHEAD BIG BORE
910-15880
104.95
SPEEDWAY
  * GASKET SET- SHORT BLOCK 910-15881 (INCL IN 910-15880)
-
-
  * GASKET, HEAD - COPPER
910-15895 (INCL IN 910-15880) -
-
  * SEAL, CRANKSHAFT, 1 PC - FRONT
912-S12853 (INCL IN 910-15880) -
-
  * GASKET, EXHAUST - COPPER
910-13599 (INCL IN 910-15880) -
-
  * GASKET, INTAKE
910-21005 (INCL IN 910-15880) -
-
FASTENERS
BOLT KIT, FUEL PUMP STAND (S.S.)
913-03920 3.99
SPEEDWAY
BOLT KIT, EXHAUST HEADERS (S.S.)
913-03955 18.99
SPEEDWAY
BOLT KIT, WATER NECKS (S.S.)
913-03935 6.99
SPEEDWAY
BOLT KIT, OIL PAN (S.S.)
913-03950 19.99
SPEEDWAY
BOLT KIT, TIMING COVER (S.S.)
913-03915 9.95
SPEEDWAY
BOLT KIT, INTAKE MANIFOLD (S.S.) - OFFY
913-03900
24.95
SPEEDWAY
BOLT KIT, WATER PUMP (S.S.)
913-03940 18.99
SPEEDWAY
IGNITION
DISTRIBUTOR, DUAL POINT
MALLORY 2527501
245.00
PATRICKS
COIL, IGNITION
ICR64
49.95
PATRICKS
SPARK PLUG WIRES, SET OF 8 - ACCEL SPIRAL SUPER STOCK
3010
24.95
PATRICKS
COOLING SYSTEM
FAN AND SHROUD ASSY, 2700 CFM ELECTRIC FAN
910-15640
199.99
SPEEDWAY
PUMP, WATER PUMP - DRIVER SIDE
910-15592-DRV
89.99
SPEEDWAY
PUMP, WATER PUMP - PASSENGER SIDE
910-15592-PASS
89.99
SPEEDWAY
GASKET, WATER PUMP - PAIR
910-15537
4.99
SPEEDWAY
MISCELLANEOUS
ALTERNATOR, POWERMASTER 82021
910-67165-PLN
370
SPEEDWAY
GROMMET, PCV
HELP! #42054
4
CHECKER
VALVE, PCV
Fram FV306
5
CHECKER
WATER WETTER
910-15709
7.95
SPEEDWAY
DIP STICK, CHROME
8BA-6750-CH
24.95
C&G
* Approximate

Suppliers

PATRICKS - Patricks Antique Cars & Trucks, Casa Grande, AZ
SPEEDWAY - Speedway Motors
REDS HEADERS - Red's Headers, CA

The Rebuild

We started putting the engine back together in May 2006.  Here's a pictorial diary of the rebuild, with comments where appropriate.

Modify the block for the improved oil filtration system

There are several options for improved oil filtration... we used the one in McNicholl's book (its also in Ron Ceridono's book, but the discussion is more complete in McNicholl's). For this, you need to enlarge the existing hole ("A" in the picture) so that you can slightly enlarge and tap the internal passage in the block (between "B" and "C"), and screw in a plug.  Then you need to drill another hole on the top of the block downstream of the plug ("C" in the picture, see the book for details).  (You should actually do this step before sending it to the machine shop.  In any case, do it before assembling the bottom end because you need to make sure you get all the metal shavings out of the block.)  Note that this does not provide true full-flow oil filtration because the main bearing oil galley is between the oil pump and the hole marked "A". Some people claim this provides about 95% oil filtration, but whatever it is, its a big improvement. (For an even better explanation of this, refer to flathead.org.) If you want true full-flow oil filtration, both MCF and Red's Headers make a kit.  The MCF kit is shown in Oddo's book.

This picture (taken near the end of the rebuild) shows the McNicholl's modification.  Hole "A" was enlarged so that we could drill and tap the internal passage, and a plug (not shown) has been screwed into the passage between "B" and "C", and hole "C" was added ("B" is the original oil pressure tap).  The way the oiling system works now is that the oil pump pumps oil out of hole "A" (remember there is a plug in the passage between B and C now), through the oil filter, and re-enters the engine at hole "C".  Hole "B" remains the oil pressure tap.

full-flow oiling block mod

Fuel pump pushrod bushing

Added May 2011 after a few inquiries.

If you are going to use the mechanical fuel pump, replace the fuel pump pushrod bushing (align the oil hole with the oil passage), and use a new pushrod if its worn.

If you are not going to use the mechanical fuel pump, you must block the fuel pump pushrod hole, otherwise you'll loose oil pressure! Two oil passages meet at the fuel pump pushrod hole. In factory configuration, a bushing is pressed in which has a small oil hole to lubricate the fuel pump pushrod... if there's no pushrod, then you loose oil pressure. This applies to 8BA blocks, and I think it applies to the earlier blocks as well. There are several ways to do this, but in general tapping the hole and screwing in a plug is not recommended - the block is hard to tap and the casting is thin. You can: use the 6025 bushing and braze the hole closed, have a 0.380" diameter aluminum plug made, use a Welch plug, etc., see this link. Sorry, I didn't take a picture, but this is the hole at the back of the block, in the lifter valley.

Paint the block

Paint the block after getting it back from the machine shop to prevent it from rusting.  In the books, you'll see a lot of references to painting the lifter valley with Glyptal.  Glyptal is hard to find these days.  We used high-temperature ceramic engine enamel, Ford Blue (Duplicolor DE1601), for all the internal painted surfaces, and Ford Red (Duplicolor DE1605) for the exterior.  Be sure to do a good job of masking the deck.

painting the block   painting the engine

Drill holes for adjustable lifters

If you're using adjustable lifters, like we did, you'll need to drill a 3/16-5/32" hole in each lifter bore, as low as practical, but above the fillet runout.  This is so that you can insert a punch into the lifter bore, catching one of the lightening pockets on the lifter, thus anti-rotating it so you can turn the adjustment nut.  Thoroughly debur the lifter bore so the lifters don't hang up (remember the lifter needs to rotate about an 1/8 of a turn per cycle to prevent wear).  This picture was taken after we painted the block; just be sure to drill your holes before assembling the bottom end so you can debur and clean out the metal shavings.

drill holes in lifer bore

Relieve the block

Next, we relieved the block.  This a partial MCF relief (the MCF concept but less than 3/16" deep), which reduces compression by about 0.2 points, but the breathing improvement is more than worth it.  Notice that the relief is not taken down to the bore, which helps retain bore rigidity.  This was done by hand with a die grinder.  I use the term "we" loosely here... my Dad did all the work.  Thanks Dad!

The MCF relief   MCF relief

Port the intake and exhaust

More time with the die grinder.  We opened up the intake runners (in the block, not the intake) a little, and port matched it to the Offy intake, which required us to enlarge the openings quite a bit. Beyond that, we smoothed out the intake runner and removed the high edge where the valve guide sticks up into the valve port.  Be careful and don't remove too much material inside the intake runner... there are cooling passages running the full length of the block on both sides of the runner (see Fig. 3 on pg. 74 in Oddo's book)!  If you're installing new valve seats (that is, if your block has valve seat inserts... some don't), do the porting beforehand so you don't nick the seats.  Otherwise, just be careful.

intake porting   relief around valve guide boss

Install exhaust baffle

You can buy a bronze exhaust baffle for the middle exhaust ports. This helps reduce cross-talk between the middle cylinders - they share a common exhaust port, and the exhaust valves have some overlap relative to each other.  Notice the stud which holds the top of the baffle, and the plug which keeps it in place.  One note: the baffle had very large raised lettering that would have interfered with exhaust flow... we ground the lettering off to give the exhaust a nice flow path.

baffle   exhaust baffle in middle port

Reciprocating Assembly & Camshaft

For some reason, I didn't take but one picture of the bottom end!  At least is shows the main bearing cap support that we installed.  We Plasti-Gaged all the bearings and everything was well within spec (if anything a little on the tight end).  The rear seal is a common problem, so we took our time, soaked the rope seal in motor oil for several hours, trimmed it using the template provided, and put a dab of RTV on the cut ends.  We used a new-style one-piece front seal.  After lubing the camshaft (see Hits & Misses), we installed it and a new aluminum timing gear.  The Max 1 camshaft is so named [apparently] because the cam lobes are nearly as big as the cam journals... hence its the max lift possible.  The Isky documentation also refers to this cam as the Max £1.  If you use an aluminum timing gear like we did, make sure you use the correct distributor drive gear.

recip assy recip assy

Assemble the valvetrain

We used Chevy small block valves (or said more correctly, Chevy small blocks use Ford flathead valves [smile]), which required a shim (washer) to get the proper spring height.  When installing the washer, put it on the valve end, not the stem end.  This reduces reciprocating mass.  Unfortunately, I didn't take any pictures while building up the valve/spring/guide assembly, but its pretty straight forward (see exploded view).  You'll need a valve installation tool to install the valve assembly in the block (see pg. 74 in Oddo's book for an illustration of using the tool, pulling the valve guide down so you can insert the retainer).

The Isky Max-1 cam (P/N 811100, see the Isky Flathead Specification sheet 1 and sheet 2) recommended .014" valve lash for both intake and exhaust, which we set.  Adjusting the valves takes a lot of time, and I messed up one of the adjusting nuts in the process, and had to wait for a replacement (more on this below).  Take our advice (it was hard learned):

Heads

We opted for Offy "425" aluminum heads, which yield a nominal 8.25:1 compression ratio for an otherwise stock engine with a 3¼×4" bore and stroke, according to Offenhauser (interpolate in the cart below and you get 8.25).  Between the pistons, gaskets, and block relief, we ended up with a compression ratio right between 8.0 and 8.1:1.  You might ask why we didn't opt for more compression.  The reason is simple:  in a flathead, more compression means less air flow, and the flathead's chief deficiency is poor breathing.  Plus, we were aiming for a mild-mannered street engine which runs on 87 octane, not a dragster.

Offy compression chart

We liked the looks of the engine on the front page of McNicholl's book, so we painted the heads with a gloss black engine enamel, and then hit the top of the fins with a fine grinding wheel to give the fin tips a polished aluminum look. 

We initially used copper head gaskets.  These are nice gaskets, and they are re-usable (yes, they really are!), but they're messy because you have to use a copper spray sealant.  We had the heads off twice and could have re-used them again, but ended up using a graphite gasket the final time because we were tired of the mess.  (See the Hits & Misses section to find out why the heads were off twice!).

The head torque sequence is on pg. 105 of Oddo's book.  We torqued the heads to 65 ft-lb (right in the middle of the Offy recommendation for head bolts (if you use studs, its 50 ft-lb).  You'll need to re-torque the heads (and intake) after the first run-in, and again after a few more cycles between room temp and full operating temperature.
Torque Sequence

Ignition

With regard to the distributor, the factory unit can be rebuilt and/or converted to breakerless ignition, but you should be aware that the factory distributor will require the use of a factory-style Ford/Holley 94 carb. This is because the factory distributor has a vacuum brake, not a vacuum advance, and it must be connected to a venturi vacuum source. Newer carbs do not have venturi vacuum ports. Therefore, if you are using something other than the factory carb, you'll have to replace the distributor, which is probably a good idea in any case. MSD and Mallory both make electronic and breaker point distributors.

We opted for the Mallory dual-point distributor #2527501, which has mechanical advance only. (Refer to the Flathead Tuning page for updates, as I converted this distributor to Pertronix ignition later.) We stabbed the Mallory dual-point distributor and terminated the spark plug ends.  Note that the firing order is slightly different than that of later model Ford V8's... cylinders #2 and #8 are swapped in the firing order.  (The graphic below is from Van Pelt's Flathead website.)

Top End

Both the Offy intake and the Holley carb have provisions for a PCV valve.  We eliminated the downdraft tube, put the PCV valve in its place, and plumbed it to the carb's PCV tap (see Hits & Misses for more info).  See pg. 115 in Oddo's book for the intake torque sequence.

Using an electric fuel pump allowed us to relocate the breather/fill tube to the back of the engine (see photo).  This was just a preference, we could have left it in the stock location, but it does clean up front of the engine, which is busy enough as it is.

Another aesthetic dress up was the use of the spark plug wire looms from Speedway.  These looms have a stainless steel tube for each spark plug wire which terminates at the proper cylinder location.  We mounted the looms on the top of the engine using the intake bolts... you can also put them on the bottom and use the exhaust bolts.  These worked out really nice, but you have to buy a wire set without the distributor ends pre-installed.  We opted to use an electric fan, which allowed us to route the wires to the bank opposite the distributor in front of the generator.  It provides a slightly nostalgiac look (like the old crab-style distributor placement), but if you're keeping the stock fan, you'll need to route the wires on top of the engine. One word of caution, according to Mike Davidson (see References), these can cause interference if using electronic ignition.

Initially, we rebuilt the generator and converted it to 12V to keep the original looks. However, even with a new 12V regulator, we could not get enough out of the generator to run an electric cooling fan at idle, so we decided to "punt" on the generator and instead bought the Powermaster "alternator-that-looks-like-a-generator" from Speedway Motors (see link.) This worked out quite well, the generator is a simple 1-wire hook up, eliminates the voltage regulator, and gives 59 amps at idle and 81 at cruise speed (at least according to the bench test tag). It comes with a new bracket which I had to do a little grinding on, and its a 1/2" bigger in diameter, but we had no clearance issues. (In the pictures, a red strap is a picture of the generator, and the black strap is the Speedway alternator.)

The engine was originally designed with an intake heater (the block has 2 small exhaust passages leading to the intake), and the Offy intake has provisions for this.  An old hot rodder's trick is to block off the exhaust passages to keep the intake cool.  It just so happens that the hole in the block is slightly smaller than the diameter of a penny, and the intake gasket hole is a perfect fit for a penny.  We tried the engine both ways... with and without pennies blocking the exhaust port.  Blocking the intake heater passages gives you really crisp throttle response, but takes a long time to warm up on a cold day.  We tested it during the summer (>90°F) and winter (~25°F), and decided to leave the pennies in, mainly because on a hot day with a hot engine, the engine would stumble on hard acceleration without pennies.  Legend has it that putting the pennies heads up is good luck.

PCV in downdraft tube location   breather    pennies block exhaust heater

Cooling

Speedway sells an electric fan & shroud assembly that fits the F-1 pretty well. The width is perfect, and the height leaves about 3" of the radiator uncovered. This is less than ideal, but a lot better than the unshrouded 16" fan we were using. Its made by Cooling Components, Inc., P/N CCI-40 (see Parts List for the Speedway P/N). The fan pulls 2700 CFM, which should be adaquate. With the fan & shroud installed, there is still about 3" clearance to the engine. It is recommended that a 30A relay be used to run the fan, we ended up using used an adjustable Hayden fan controller which includes A/C control (#HDA-3647 from Summit).

Speedway fan & shroud

Fire In The Hole!

Alas, we mounted the engine on a test stand (another thanks to Dad here, who made it) wired it up, and filled the engine with oil and coolant (read the information below about Motor Oils below for some important information).  With the coil disconnected, we cranked it until we got oil pressure, which took about a minute's worth of total cranking.  We then loosened the screws on the float bowls and ran the electric fuel pump until the bowls were full.  After reconnecting the coil, viola, the engine fired right off!  No backfires, no leaks.  We have 40 psi oil pressure at hot idle (650 RPM), and 16" of steady manifold vacuum.

Click here for a QuickTime movie of the first engine run (without mufflers, LOUD!) [8 MB].
It will take about 5 minutes to download this move using Cable/DSL, and a QuickTime compatible player.
(Get the player from Apple or RealPlayer).


engine on test stand - ready   engine on test stand (right) 

Engine Tuning

Go to the Flathead Tuning page.

Click here for a movie of the engine running on the test stand, tuned somewhat better, with mufflers, at ~1500 RPM [5 MB].

Compression Test

We checked the compression after running the engine for about 20 hours, and are very pleased with the results.  You can see the original worksheet here (also the compression test from May 2011 after 3000 miles).


CYL #
COLD PSI
HOT PSI
1
136
141
2
136
140
3
137
140
4
133
142
5
136
143
6
134
139
7
132
140
8
131
138
AVG
134.4
140.4
HI-LO
6
5
SIGMA
2.199
1.597
 

Hits & Misses

The flathead rebuild really went better than expected.  We had no issues with part fits, and everything works.  But we did learn a few things along the way...

Things that worked great

Important Information about Motor Oils

It is extremely important that you read the Motor Oil Warning sheet that comes with the Isky camshaft.  The warning is NOT specific to Isky cams, but is generally true for all flat tappet cams, even haydraulic ones.  The bottom line is this:  the Federal Government has banned the anti-wear additives that used to be in motor oils, especially ZDDP (which is bad for catalytic convertors). The ban was phased in starting in the 1980's and as of 2004 all Zinc and Phosphates (i.e. ZDDP) were removed from "street legal" motor oils - that is any oil with "API Service Rating SM" or newer.  The OEM manufacturers don't care because all new production vehicles use either roller tappets or are overhead cam followers, which don't need ZDDP. But this is bad news for all us hot rodders who are using flat tappet engines, especially with solid lifters! ZDDP is need for both the break-in and for the long term (Zinc in particular gets in the mircoscopic pours of the cam lobes and reduces wear on contact surfaces). The work around for this problem boils down to a few options:

Currently, I am using the Joe Gibbs Conventional Hot Rod 15W-50 motor oil, althought I may switch to the Valvoline "Not Street Legal" motor oil since NAPA carries it now. Both have high Zinc levels. Initially, I used a conventional SM-rated oil with the Lucas additive, and while I believe this is a good solution, I switched to the Joe Gibbs oil to avoid any possibility of "additive clash". You can order Joe Gibbs motor oils directly, or from select dealers.

Things that didn't work so great

CYL #

INTAKE

(.352 REF)

EXHAUST

(.350 REF)

1
.346
.349
2
.345
.345
3
.345
.345
4
.345
.345
5
.346
.346
6
.346
.340
7
.344
.345
8
.346
.344
HI-LO
.002
.009

   Lifter Damage  

Unexpected things we learned

Time Line

Here's a rough time line of how the engine progressed.  By no means was this worked full time, we're not in a hurry.  I live 2000 miles from my parents (where the truck is), so I can't just go home on the drop of a hat.

Thanks

I really have to thank my Dad.  He did almost all the planning, parts ordering, followed the machining, did most of the porting and relieving, made the engine stand and breather, machined the flywheel, and on and on.  Thanks Dad!

Car Parts Machine & Auto Supply (now Stafford Performance) in El Paso, Texas (on Texas Street), in particular Joe Hicks who gave us a lot of technical advice.

Patrick's Antique Cars & Trucks in Casa Grande, AZ, in particular Patrick Dykes from whom we ordered a lot of parts and got valuable advice.


If you're rebuilding a flathead and need more info, send me an email.



Last Updated: Monday January 02, 2012 Valid HTML 4.01 Transitional Powered by Powered by Sun