How much can I mill a Willys banger?

How much can you mill a stock circa-1940s flathead Willys (Jeep) four-cylinder cylinder head? Not looking to make a race motor out of it, would just like a modest increase in the CR - I am still planning on using regular gas, and the rest of the motor is stock.

This is the honest-to-god military version of that motor, I think they called them GPWs.

I have the head off to change the head gasket, so figured maybe I can shave it a bit while I am at it.

 

bttt

 

I wouldn't mill it much. A machine shop can probably tell you more but as you decrease the compressed volume, you also decrease the area for the explosion to move to the piston.

 

Yes, I'm thinking of a cut around .020 or .030. The rest of the motor looks perfect. Not even a ridge in the cylinders. But stock it is very low compression, so any boost would be good.

 

You really can't get much by cutting the head on a go-devil engine. I would mill it just to true it up and resurface it. Engine man is right in that you might make things worse. Those engine like low compression. I've rebuilt a number of them but I've never measured valve clearance.

put a flathead in it..........

 

LOL wish I could. It's the motor on my generator. An old Army unit.

This is an 1800 rpm application, so there is no chance of floating a valve. I will measure the valve clearance just for grins but I am sure that something in the .020 to .030 range could be accommodated. Is this what you are thinking, or is it more?

 

Check valve clearence and allow a min. of 040-.050. Most flatheads could live happly with .060-080 haircut.

 

Sure, that much won't hurt anything. Does that block have a serial # on it? Probably pretty early.

 

If your plans are to keep it to run the generator consider the profit/loss available. If you cut it and get a little more compression what have you really gained? If after it's cut it starts to run hot where you gonna find another generator engine? I think that the head would prove to be more difficult to uncut than leave alone.
You may consider this advise to be worth what you paid for it.

 

Here's what I might gain: a little better fuel consumption. This unit runs almost 2 gallons per hour at full load, and that gets expensive at $4 a gallon.

A tiny bit higher volumetric efficiency would let it run a tiny bit cooler, since the same work would be accomplished by burning a tiny bit less gas.

These units are designed for jungle operation, so the cooling system is huge, anyway.

I understand your concerns, and appreciate them. My real question is 'how much can I cut this head', though, not 'should I'. And if I mess up the head, it is not hard to get a replacement. But with a tiny cut it should be OK.

 

I see what you're trying to do. Today's gas provides much less power so you might need a little boost for the generator to put out as much power as it can. If you take too much off, you can double the head gaskets and I think there are still steel shim head gaskets available in various thicknesses. These engines don't like to rev above 2000 rpm without losing the bottom end. If it's governed at 1800, you should be ok.

Drip some spots of wax on the highest point of the valves, bolt the head down without a gasket and slowly turn the engine over by hand. The wax will crush if it touches the head and the thickness can be measured.

 

Good idea, I will measure the valve clearance tomorrow. I suspect that there is plenty of room, but we'll see.

This motor is indeed governed to run at 1800 and there is a separate overspeed governor that kicks in at 1900 rpm in the case of a runaway.

 

if that is a GPW marked head / block pull it and sell it to a jeep guy. you might be amazed what those little 'F' marks are worth to those guys.

then stuff wad of cash in your pocket and replace it with a CJ-2A motor.

 

Good thought, but this is an all-original unit with all of the side panels, etc. and (except for the blown head gasket) it works fine (just very thirsty).

To tell you the truth, I have thought about a motor swap, but the best would be a small diesel. Then after all that work, I still have a 70 year old armature and field windings and wiring and so on and so forth. Best to keep this unit intact, and if I ever want to bite the bullet, I think I would buy a new diesel genset and then just keep this one as a spare.

 

That's good to know. The head surface looks pretty thick. Thanks.

 

It is. The build date on the genset is 1942.

 

.020 or .030 will not cause any interference, if nobody else has milled the head in the last 60 years. Todays gas is much better than Army gas in '42 and a little more compression won't hurt. Any help is kind of theoretical.All the guys who voted for a 2.3 Pinto or an Onan are on the right track.

 

Thanks for the info. This is the first time the head has been off. The unit has realy low hours - less than 400 - and most of that is mine. The cylinders do not even have a ridge.

I agree in principal with the concept of an engine swap, but due to the aforesaid issues - namely, that I would still have a seventy-year-old everything else, it makes more sense to me to keep this unit original. I can always buy another, new unit and keep this as a spare.

I also agree that any upside is going to be minimal, but at least it is in the right direction.

 

Today I measured the valve clearance from maximum lift to the head. Without a gasket, there is a solid 1/8" of clearance. This would seem to confirm the statements that a .020 to .030 trim would not cause a problem, and that I could go as high as .060 to .080. While I am still undecided as to what I'll do, at least now we have matched the guidelines to the actual lift/clearance in this engine.

 

check the top side real good for cracks...

 

Yes - hopefully it is OK. The motor overheated when the fan belt broke. It ran fine afterwards but slowly a head gasket leak occurred between three and four (confirmed with a compression test and by eyeballing the marks on the gasket, block and head). It's a lot of cast iron for 35 hp, so (knock wood) there are no cracks.

 

if it cooked good..... look good between 3 & 4 on the top side.... been there done that

 

I'm old enough to take BTDT seriously. I just looked at the head closely, s****ing the area with a razor blade and with an intense light, and happily there are no cracks.

Thanks for the head's up on this.

 

Being military spec, I would guess the original cr wasn't much above 6 to 1, built to run on anything availabel north of kerosene in octain rating. So your combustion chamber is probably pretty deep. I would think you can get away with up to .040 / .050 off with out worrying about valve clearence. Whatever you decide to do, when you get it done, crank in a few (4 to 8) degrees of initial timing to take advantage of the extra squeez.

 

That's a good point. I think the regular timing on this engine is something like 9 degrees, period. Not even sure if there is any cent advance built into the dizzy - for sure there is no vacuum advance..

 

yea the timing is probably set for the governors range, with no thought to idle or high speed operation. No centrifugal??? I have seen stationaries with no vacuum advace because of the narrow operating range, but I would think there might be some weights in there to work with the governor's needs, off idle, but maybe not.

 

Things that make you go "Hmmmm?"

Milling the head won't change the area of the piston surface. A higher pressure compressed charge, due to the smaller chamber volume, will result in a higher combustion pressure. That higher combustion pressure, pushing on the same piston surface area, is going to result in increased power.

 

It's a good question and I am not sure as I have never taken the dizzy apart. It is one of those military sealed (waterproof) distributors. The cap, rotor, coil, etc. are all inside a waterproof housing and of course it uses the shielded ignition cables.

This motor never idles. As soon as you crank it it revs to 1800 rpm. You can see the governor holding the throttle wide open when the motor is off. Also, the motor never sees more than 1900 rpm as there is a separate governor (a second, independent throttle plate underneath the carb throttle plate) that kicks in as soon as the rpms rise.

So it is possible that this motor just runs with one timing setting. I will have to find out.

 

Thank you.

 

I picked up the head from the machine shop today. I decided to cut it .040. Now I have to find time to put it back together and figure out how to time it.