The Stovebolt Site Forums The Shop Area Engine Shop GMC 270 Engine Rebuild

Not sure if this belongs in the Project Journals, since is only an engine. I'll let the admins decide and move if appropriate.

As I mentioned in a previous post, I recently purchased a 1956 270 GMC. The goal is to build up a spare engine for my 1944 GMC CCKW.

As also mentioned previously, the engine came with an automatic trans, along with about 3/4 of an additional engine in spare parts: (block, crank, head, assorted small parts). The spare block, unfortunately, has a crack in one of the cylinders. In light of this, I decided to set it aside for now, and make the complete engine the target of the rebuild project.


Disassembly

After separating the engine and trans, the next step was to get the engine on a stand and begin disassembly and inspection. The engine looks essentially unmolested, except for the addition of a Delco SI alternator in place of the stock generator. The engine turns over by hand, with compression on most cylinders - except for #1 and #5.

A look under the valve cover proved unsurprising - given the decades of unfiltered air that has been pulled through the breather slits.

Rocker assembly, side cover, pushrods and lifters came out next. The thick layer of crud aside, they all looked in surprisingly good condition. Most of the lifters actually looked in very good condition, with some minor scoring on a couple of them. #1-E lifter came apart while removing it. The top of the "milk can" popped off. Not sure how these are assembled. I have no experience with these as the military version uses the one-piece design. The cam appears to be cast iron and is also in very nice condition.

Removing the head revealed some rust in cylinders #1 and #5. Looks like the engine had been sitting for a while at some point in its life. This also revealed the first signs of possible human intrusion, as the head gasket was a McCord, and looked almost like a newer MLS design. There was also something that looked like silver paint on the deck. I don't believe either of these to be stock, so it appears that at least the head has been off at some point in the past.

Oil pan came off, along with the oil pump. The oil pump has this weird looking moveable pickup. Not sure what the idea is behind this design - maybe to keep it away from any sludge in the bottom of the pan ? It is totally different than the military version. A quick inspection of the insides showed no serious wear or scoring.

There was also a fairly hefty ridge on several of the cylinders. After trimming it off, the pistons and rods came out easily. Rings on #1 and # 5 were rusty and stuck. #6 had a broken ring.

Measuring the bores showed some taper (as expected) and some pitting in two cylinders. Most wear was in the range of 0.004" to 0.006" above spec. (3.7812"), with #6 showing the largest deviation at +0.0092" at the top of the cylinder.


Bearings were very worn (tons of bearing material in the pan) but the crank looked very good. No rust of scoring. Pulling the crank was a bigger surprise. For as worn as the bearings were, the surface of the journals was really nice. Measuring them showed that they are all dimensionally correct, with very little wear. Am thinking that just a polish, and maybe a cleanup of the seal area is all that is needed.

Decisions, Decisions..

So now I come to the proverbial fork(s) in the road, that will drive several future decisions on my part:

1) Pistons.

I have a set of NOS 0.020" military (domed) pistons. They have some minor corrosion from storage, but I think they would be OK to use. I also have a spare large chamber head to use them with. However, using the flat top pistons with their higher compression ratio and lighter design (yielding a higher redline and about 30 more HP) is also an attractive solution. Extra HP (ANY extra HP) is always welcome on a 10K lb. underpowered vehicle.

It sounds, however, that these (flat top) pistons are very uncommon, and would have to be custom made?

BTW, would using the heavier domed pistons require a rebalance of the crank?

Comments welcome...

2) Crankshaft.

Both cranks have the newer 6 bolt flange. My understanding is that this necessitates the use of the 11" flywheel and clutch, which typically have the 12 volt ring gear. The ring gear could probably be swapped out ( I think ) but I'm still left with the non-OEM clutch, which may possibly present other yet-unforeseen compatibility issues on installation.

In contrast, the 4-bolt crank fits the stock 11.5" flywheel and clutch.

Again, comments are welcome...



Next on the list is block cleaning, close inspection, and pressure check.

Best Regards...

I'm waiting on delivery of a set of GMC 270 pistons and connecting rods from a recent Ebay purchase, just to get an extra set of connecting rods. I have no use for the pistons at the moment, since the engine I'm planning to build will have an altered stroke, and it will use flanged slip fit cylinder sleeves and pistons for a Japanese Diesel. They are the domed style. If they might be useful for your buildup, please PM me. Piston weight does not affect the crankshaft balance on inline six engines, since the 120 degree separation of the rod journals makes for an inherently balanced rotating assembly, assuming there's not a big difference in weight between cylinders.

Thx, Jerry.

PM sent.