The Stovebolt Site Forums The Shop Area Engine Shop General Overhaul questions

Looking at putting a mid 50's 235 (full oil press, insert bearings) through a rebuild. Its not the original engine. Truck is a 43 1.5 ton G506. Its running now, and I have yet to tear into it... so still in the planning stage:

Would like to bump the compression up to about 8:1. What would be the least expensive way to do it? Not looking for hi-perf, just 'better'. I am not sure if any of the ideas I have are even possible:

New pistons (cost?)
New rods (cost?)
Mill the block down? (seems the least expensive)
New cam? (would a new cam at low compression be 'worth it'?)

I have decided that if the bore is more than .040 over I will sleeve back to standard... and if so I will live with the lower compression/performance UNLESS there is something that is very economical to do.

Last question:

It looks that a full rebuild kit is about $1k, and machining could run another $1k. At $2k is a crate motor (again, not hi-perf) a better deal?

Thanks for any insight!

There is nothing wrong or bad about having the engine up to .060 over.You don't need to sleeve it,no gain.

Don't need new rods unless you're old ones are bad.

A good rebuild will cost about 3000.00 give or take alittle depending on what you find on the inside after you take it apart.


Pete

you can take these to .060 over very easily. I wouldn't recommend sleeving an engine just as a matter of course. the only things on your list that would raise compression is new pistons, if you use ross or venolia they will cost $800 a set. Although there is some head combination that already gives you something like 8.5-1 (several people will chime in here) with stock pistons. Milling the head is the next easiest, but not so cheapest, I wish I had milled mine, I may still do it. New rods won't change compression, and the old ones are fine, if you want a stronger rod I think a 270 gmc rod will fit provided you cut the big end down to fit the crankshaft, (if no one here knows if these rods clear the cam, I can find out easily enough probably) however they are longer and will require custom compression height pistons, which ross at least will adjust for no charge. Milling the block will only make your pistons stick out the top of the block requiring once again, custom piston compression height. I got an RV cam and it seems to work fine, you have to be careful with RV cams and compression you can't run the cam with too much compression, but here again I don't know the exact numbers, but some one will jump in I am sure. One thing that will improve compression is over boring. You can probably expect to spend considerably more than $2000, more like $3000, you will probably have a hard time finding a machine shop that knows anything about these engines. I just finished one, it took two years and about 10 trips to different machine shops, The cam grinders lost my cam, I never did get it back. the list goes on, but it is great.

Don't mill the block other than ensuring it is flat. If you want to raise compression, the old school way was to mill the head .125". This requires putting a .125" spacer under the rocker shaft supports.

This requires putting a .125" spacer under the rocker shaft supports

Assuming the intake valve seat is also moved by .125", this cures the intake rocker geometry.
Unfortunately, since the exhaust valve seat didn't move the exhaust rocker geometry is now wrong.

Over boring the engine will raise the compression.

If your block will accept the 848 head it will raise the compression to 8.5 to 1.

Due to the age of the motor my guess is that its already been bored to .060, and may have heavier that average wear. Unless the innards are pretty much pristine I'd rather spend the money now and go back to standard rebuild than wait for wear to take it past a rebuild-able state. Just my preference.

It also seems that 'metal shim' type head gaskets are an option instead of the thick metal/fiber stock type?

Again, not knowing any better it would seem that the thinner head gasket and taking a moderate cut off the head (say .020 to .030) would be a winner with no real downsides or a super high cost.

Is there a cross reference of 50's heads and the cc's online?

Of the two blocks I have, one block number is 3692713 and this is a babbit/dipper motor (If I did the lookup right) and not really an option (but it does run).

The second block number is 3764476 and this is the most likely candidate if not cracked. I have another motor but not sure of the numbers... most likely a late 50s.

go back to standard

Sleeves don't bring the block "back to standard", they bring it up to way, way oversize.

I'm seeing different model years for the 3764476.

One site is listing 56-62
Another site is listing 58-62
On site said 52-62?

Only thing that seems consistent is the 62 as the last year.

Huh? You bore the cylinder to accept the sleeve, press the sleeve in, bore to dimension. Is this not correct?

I believe what panic means (correctly, by the way) is when you bore the block for sleeves, you will have to bore the cylinders out much further for sleeves, than you would for just a normal rebuild

Ok. I've never had it done (but I do have a 42 Go Devil at a very lose .060 over) so I did look into at last year.

It seems pretty cheap... $60-80 per hole... well for only 4.

I'm still planning so pardon all the questions.

By all means, ask away

mostly I think you should just forget the sleeving thing. These engines aren't that hard to come by. And .060 over is just fine, you can go .080 if you really get down to it. People don't tend to just sleeve engines as a matter of course, maybe one cylinder if it gets damaged, but not the whole engine. You can probably get away with getting the head that fella mentioned to get extra compression, along with an overbore, should set you up fine for compression.

So in the aftermarket world, are there "new" rods for the 235?

I'm a bit lost here. How does an overbore raise compression? Maybe I don't understand the fundamentals of compression. I would have thought that an overbore would increase the volume in the cylinder and therefore lower the pressure of the gas when compressed the same amount during the piston stroke. Am I out in left field here??

I know the question may not be relevant to this discussion, but I never pass up a learning opportunity.

The overbore raises compression, because the combustion chamber (in the head) is the same volume it was, although the cylinders have a larger swept volume. That's my understanding, perhaps I'm wrong? It wouldn't be the first time!

Mr Lang, you are correct...More area in the cylinder bore compressesd into the same head space increases the compression pressure (ratio).

Mike B

Aha!! Thanks.

So, for the folks that have done this before:

Say my pistons are at .040 over, and when the shop cleans and mic's everything the wear is enough to require going to .050 (the next step up).

Since that would require new pistons, rings and bore/hone the cylinders.. my searching is coming up with about $100 per piston (rings, piston) and whatever it costs the shop to do the wrist pins.

That would be about $600 to go to the next size.

There is no TECHNICAL reason, if its cheaper, to sleeve the cylinders and reuse the pistons. If it saves $200-300 vs going to the next size.

no really, give up on sleeving the block, it just isn't done on these engines, you can get standard pistons for about $250-$400 a set. with rings. Sleeving puts quite a bit of stress on the block if you were to do all 6 of them, if you sleeve one cylinder, you have to hone the cylinders on either side because pressing the sleeve in or however they do it distorts the adjoining cylinders. There is nothing the machine shop needs to do with the wrist pins, you can easily do that yourself, once you see them you will understand. Just dismiss that sleeving idea...

I am trying to keep the costs down.

It looks like new pistons are about $30 each. I guess the ones I was looking at $800 for a set here not standard ones?

For $300 to re-piston/ring then yeah, sleeving makes no sense.

As for the stress thing.... its that really an issue? I've talked to many jeep owners and they all seem to have, or know of, someone running with an overhauled engine 'back to standard' (aka sleeved).

If you really want to keep costs down, then bore the block enough to clean it up, and look/wait for some NOS pistons and rings to show up on eBay. You can get some great deals that way. Conversely, you can figure out the minimum oversize you need, and buy pistons and rings at least that big or larger, and then bore to suit. Your machinist will want the pistons before they bore anyway I would think.

Also, depending on what you find when you take the engine apart, you might be able to just re-ring the engine, put some new bearing shells in, and not have to do any machine shop work.

If the engine runs now, has acceptable compression, and doesn't smoke much, then I wouldn't get too worked up about rebuilding it.

Scrounging for parts is not a new thing for me

My last concern is that I would like my truck (43 Chevy G506 1.5 ton 4x4, 8000lbs) to be a daily driver. I live in PA and there are some pretty good hills to climb.

The 235 that I am planning to put in it is not the stock engine. The original 235 made 91 HP 182ft/lbs torque. The late 50's 235 I do have is 145 HP 215ft/lbs torque(I think). Will that be enough to get me up the hills in something other than 2nd gear? Other than my 4 hounds and an occasional chord of wood, it will be just a driver.

Originally I thought I would need to bump up the HP (cheaply). But does that sound like 'enough'? Should a start looking for one of the 261's? OR would that be to big? I have a 270 I can rebuild, but don't know it if will fit.