The Stovebolt Site Forums The Shop Area Engine Shop "rebuilt" engine, super-low compression?

I came across two 250ci Chevy engines to replace the very sick one in my 46 2-ton. One is a stocker out of a 1971 Nova. The other, a 1978 model (non-integrated head) was supposedly rebuilt with new valves, an RV cam and a 4-barrel intake. I did a compression test on both engines after getting them home. While the stocker tested great, the best hole in the "rebuilt" one only tested 30 PSI! I don't think that's enough compression to fire ether, let alone gas.

I pulled the entire engine down today. I can plainly see the fresh cross-hatch markings of the cylinder hone in the bores... and a lot of carbon on the piston crowns. Cylinder bores and piston skirts all measure what they are supposed to (although the piston ring gaps weren't alternated like they should be). I thought maybe someone mistakenly used some 230ci components, but the block, head and crank casting numbers all say it's a 250.

I'm not sure where else to look. I figure the excessive black carbon on the piston crowns is resultant of a 4-barrel carb mixed with the low compression. Oh, and the head gasket I removed doesn't look like it ever sealed. Thing looks brand new, other than the carbon sticking to it. Even the impregnated "glue" around the coolant holes is intact. No signs of a gasket breach anywhere.

Any ideas???

If it still shows cross hatch marks, its a low miler. If you have carbon on top of the pistons, you probably have carbon in the ring lands and the rings are stuck. Since you have it apart, I would put on try a set of cast iron rings. A 250 can easily be over carbureted with a 4 barrel which will cause it to accelerate like crap and build up carbon from too much fuel. You don't need a 4 barrel on a 1946 2 ton. With the correct carb, it will probably run good.

While you're in there swapping rings, install a set of 307 V8 pistons. They're flattops instead of the dish-top 250 pistons, and they're a drop-in swap, same bore size, wrist pin, compression height, and weight as a 250 piston. The only difference is that the 307 doesn't have the 1/4" dish in the top of the crown. I've built several of those engines that way and the power boost is noticeable. They still run well on regular gas, also.
Jerry

Thanks guys. I wiped down the bores good with some carb cleaner and the cross hatch isn't near as prominent as it appeared when slathered in oil. And you're right, the ring lands are all gooed up with carbon. I'm thinking new cast rings and a swipe with a de-glazing stone will work wonders.

I've already installed the stock 250 from the 1971 Nova in the '46. What a good-running little engine! And as near as I can tell, quite a bit stronger than the 105hp, 1974 smog motor it replaced.

Since all the slugs are out of this 2nd engine I think I very well may go the 307 piston route. Who knows about the "RV cam" that is claimed to be in there now. I'll pull it out and see if I can find any grind numbers to be sure. If not, I'll give Comp Cams a call and see what they have. Figure 10:1 compression, a slightly lumpy cam, a little 390 Holly on the Offenhauser 4-barrel intake and a set of headers or the 292 exhaust manifold that came with the engine and I'd have a sweet little street rod motor! Well, once we get some squeeze back in the cylinders anyway. LOL

If you cam it up a little, also mill .050" off the head before you reassemble the engine. Just watch the piston-to-valve clearance. The one I built that way didn't have any interference problems, but it had a pretty mild cam. One of my auto mechanics students assembled it for his 68 Chevy Nova. That one could hold its own with a 350 Pontiac in a Firebird that another student drove- - - -they would race 20-something miles to school every morning, and neither one could outrun the other. The Tennessee Highway Patrol finally put a stop to those shenanigans!
Jerry

Yeah I have a stock 230, and it runs great.....

John

194's and 230's have a nasty habit of getting cracks into the water jacket alongside the lifter bores that allow an oil/coolant intermix. Pressure test or magnaflux those blocks before you invest in any machine work.
Jerry

The 230 has been in my 1970 Nova since it was new, never been a part for a rebuild yet. Has 46,000 miles on it.
I always thought the 230 and 250 block were the same other than stroke, the 250 having a longer stroke?

John

Yes, the crankshaft is the only difference, along with a different pin height on the 250 pistons, IMHO. 230's and 250's both run connecting rod #3788435 with a 5.7" center-to-center length. I believe the block-cracking problems were limited to the early 230's, at least according to the block specialist at the machine shop where I ran the rod reconditioning department.
Jerry