The Stovebolt Site Forums The Shop Area Engine Shop Engine replacement:

If this question has appeared before, I apologize but I have a dilemma about my '54 GMC 350 dump/flatbed. It has its original 270 in it, which I overhauled in 2002 - rings/bearings & valves. Afterwards it had some poor compression in 5 & 6, but ran pretty well nonetheless. This summer I noticed that it was using oil and that the oil on the dipstick was creamy brown. I milled the head and replaced the head gasket, but that wasn't the problem - with the compression issue in 5 & 6, I figured I probably had either a cracked head (which was OK with the original rebuild) or a cracked block. So I clenched my teeth and kept driving it replacing oil frquently.

Last week, while hauling a load of gravel, oil pressure plummeted and fluids went to zero very quickly - engine overheated and I shut her down before anything siezed. But I think there is a cast iron problem, either block or head on this engine, and my question to you all is this: do I stay stock and rebuild the 270, knowing that it may take sleeves and a LOT of expensive machine work, or knowing that this is a working truck that hauls a lot of heavy loads, do all the fabricating of engine mounts and bellhousing to put a 350 in it? (I know, I'm on a stovebolt forum, but just listening to opinions at this point.)

I do have another 270 long block that a kind member of this forum donated to me a few years back - haven't yet torn into it to know if it works at all. I do know that this engine works at nearly the limits of its power all the time, and while I don't have to get anywhere fast, I just want to know that, if I rebuild it, I am not going to push a 50 year old casting into failure again. Appreciate your thoughts.

I'd suggest looking for a 302 GMC, which would be a blt in power upgrade.

If you have the extra block & it would require no additional fabrication to your truck. I'd say stay stock. Since you will be doing a complete rebuild anyway - see if there are any power options like a bigger carb or intake/exhaust manifold.

Besides - you said yourself your in no hurry.

350's are fine - but for my money it seems like a lot less work to just replace the 270.

I'm a big fan of V-8's but I agree, you have another 270 short block, I'd at least investigate that before replacing the six with a 350.

Or alternatively I could clear some shop space for you by driving to Eugene to have lunch and swing by your place to collect both 270's. I'd hate for you to be stuck with motors you don't want. I'd even bring a core 350 with me if you asked.

And yes I do have a 270 and love it.

Your first step is to tear it down and see what happened. My guess is that you got too much anti-freeze in the oil and that caused bearing failure, but that is just a guess. The fact that 5&6 never had much compression could be that the rings never seated or the valve seats were too wide. On a re-ring without boring, you want to use nothing but cast iron rings. The milky oil is a sign of coolant. Quite a few blocks cracked behind the push rods and leaked coolant into the oil. If you have another engine, you may not need to spend a whole lot. I drove 270's when they were new and they were built to work all the time. A 302 is noticeably more power and a lot of your 270 stuff will work on a 302. A 350 is doable, but it requires making room for it and a lot of other things.

All good stuff, guys, thanks. I'm pretty sure the bloack was bad from the git-go, (in the wall between 5 & 6?) but the old engine has significant bearing issues now so -

Here's where I've landed: I have located a 302 (out of a military 6 x 6) which ostensibly was running. I've decided to tear into it and use it as the replcaement; it'll have a little extra torque. Will do it right this time with the machine work and will take the added steps of putting some additional carburetion (two 2's? 4-barrel? any suggestions?) and cast iron headers. I'll keep you posted.

An extra-capacity single carburetor like a Rochester 2GC will involve a lot less fabrication, and probably be less of a headache to tune than any dual-carb setup. The small-bore 2GC like the 265/283 V-8's ran will give you a boost in CFM capacity over the original carb without creating the torque-killing over-carburetion problems that almost always crop up with duals, and adapting a small-volume 4-barrel to the 302 might be a hassle!
Jerry

OK, so here's where I am: I found TWO GMC 302s out of military vehicles - M211s, I think. Both are unrestored, but the manifolds are intact and I am hoping to cobble enough stuff off my 270 to make it drop into my truck. ($600 for the pair.)

I am guessing that I will need to use the bellhousing and flywheel off the 270, since the mil 302s were mated to hydramatics. I've pretty much decided to just leave the Holley/Weber 2 barrel to the stock 302 mani unless there is a better option that any of you can suggest.

The mil 302 will need an electric fuel pump, which should be no big deal.

Also looks like the front engine mounts are very different on the mil 302 than the civ 270 - anything else I'll need to worry about?

The 54 GMC front motor mount will work on the 302. You will want to use the vibration damper from your 270 because the military used a 2 groove 7/8 pulley. Once you have the pulley off, you remove the timing cover you can see how it is attached. You will have to remove the cam which is no big deal. Just make sure you put the lifters in the order they go. They have worn in on the cam and need to go back in the same hole. You will want to use the 270 oil pan and oil pump because the government pan is too deep. The engines were mounted in front with the bosses in front that are threaded. You can fabricate a generator mount to fit the one on the driver side. There is a hump on the back of the block where the starter goes. You will have to grind it off to get a starter to fit. You will want to use the 270 distributor. The cam has a lobe for a fuel pump and the block has a boss where the fuel pump goes. It is possible to use a fuel pump gasket as a template and drill and die grind out a hole and drill and tap mounting bolt holes, but an electric fuel pump is the easy way. I have never known anyone to use the government carb system. The manifold is the same as the civilian that uses the Stromberg WW 2 barrel which is a very good carburetor. They were used by GMC and a lot of Chrysler products of the same vintage and parts are readily available. I just take the gvt carbs and governor of and throw them in the dumpster. The clutch housing, flywheel, clutch and all that will work from your 270. Don't forget to put a pilot bearing in the crank.

Well, here's were we are: of the two 302's I procured, I seem to have enough parts to get a good running 302. One engine has a pristene crankshaft but a worn cam. It has been bored out 0.040 and the oversize pistons are good. The head from the other engine is OK, and am set to begin cleaning out the oil journals and such. Will replace the cam. All the return springs were shot and will be replaced. Lifters are OK. Needs all new valves. There will be routine machine work to do, and a new oil pump.

I have access to a Stromberg WW carb; also a Rochester 2G - any preference or recommendations?

It is not wise to use old worn lifters on a new cam. Best to replace them as a set.