67 L36 Rebuild

[Duke W.]

Re: 67 L36 Rebuild

The L2268F-30 has a specified dome volume of 35.5 cc, but a couple of years ago John McRae measured a somewhat lower volume.

L-72 will only have something close to 11:1 if the deck height/clearance is nominal (9.800 deck height, .020" deck clearance) with the OE shim gasket that IIRC is .028". Larger than nominal deck height/clearance and/or a thicker gasket will reduce compression.

I agree that hypereutectic pistons are the way to go for L-36/68, and I recommend a maximum measured compression ratio of 10:1 for hydraulic lifter big blocks and 10.25 for SHP big blocks

"Managing" compression ratio has been discussed in The Corvette Restorer, my San 2012 Diego National Convention presentation and here many times including a link to an online compression ratio calculator.

Duke

[Lawrence S.]

Re: 67 L36 Rebuild

Standard Bore

Well my guess of a standard bore was wrong. It is bored .030 over. We found crank bearings dated 1970. The crankshaft was standard and just needs to be polished. The nylon timing gear set was still in the engine and the original camshaft was in the engine but the earlier cam with the groove cut in the end of the cam. "883" was stamped in the end of the cam. The pistons were completely shot. One of the pistons was broken, rings broken. On another piston the rings were broken too. I am glad I tore this engine down when I did. I sure would not want that broken piston to grenade. The pistons were the original cast style with moly rings. Heads are all in good shape. The springs had been replaced and the caps were of the aluminum vintage from the early 70's per the engine builder. There was one original retainer still on one spring. Rods were all good. I will have to bore it .040 now since that cast piston broke and pieces of the piston scratched the cylinder wall. Would really like to know why someone was in the engine in 1970 putting new bearings in the engine. I don't remember if they were rod bearings or crank bearings

[Lawrence S.]

Re: 67 L36 Rebuild

Well my guess of a standard bore was wrong. It is bored .030 over. We found crank bearings dated 1970. The crankshaft was standard and just needs to be polished. The nylon timing gear set was still in the engine and the original camshaft was in the engine but the earlier cam with the groove cut in the end of the cam. "883" was stamped in the end of the cam. The pistons were completely shot. One of the pistons was broken, rings broken. On another piston the rings were broken too. I am glad I tore this engine down when I did. I sure would not want that broken piston to grenade. The pistons were the original cast style with moly rings. Heads are all in good shape. The springs had been replaced and the caps were of the aluminum vintage from the early 70's per the engine builder. There was one original retainer still on one spring. Rods were all good. I will have to bore it .040 now since that cast piston broke and pieces of the piston scratched the cylinder wall. Would really like to know why someone was in the engine in 1970 putting new bearings in the engine. I don't remember if they were rod bearings or crank bearings

[Duke W.]

Re: 67 L36 Rebuild

It may be a good lesson for all if you can clean up the piston crowns and post some photos. I suspect they may be good examples of detonation damage.

Was the removed gasket a shim or composition type and what is the compressed thickness?

I hope you read the article in the Fall 2009 Corvette Restorer, "Compression Ratio Explained". If you don't have it, email me through the TDB and I'll send you a copy.

...also hope you measured deck clearance before you removed the pistons/rods, but if you didn't there's a recovery option.

Duke