GM P/N 3966921 Service 70 LT1 Short Block

[Jephrey S.]

I am looking at (over duh innernetz on e-scam) an assembled 3970010 block with 186 heads installed. The owner says he bought the block new from the GM dealer in 74 as P/N 3966921 (with receipt.) It is advertised as having a Z28 hydraulic cam. He is asking $3K.

What innards came with the P/N 3966921 service replacement block assembly? Not by part number, but what parts would have been included? Like block, crank, pistons, rods, bearings, not the camshaft, etc.

And, how much of those internals are now shot after sitting in someone's garage for something less than 30 years? He said that he installed it in a 65 corvette in 1976, but the engine has very few miles on it. The block does still have the C2 valve covers installed, so that part fits.

Getting a replacement LT1 cam is not a problem, finding replacement solid lifters may be more of a challenge but should be doable. Driving to Chicago to pick it up is going to be the real long pole.

I'm looking for some sort of reasonable expectation that I can use most of this before I put effort into finding someone to go pick this up and stick it on a plane for me. What other questions can I ask the seller?

[Paul D.]

Re: GM P/N 3966921 Service 70 LT1 Short Block

Jephrey, You may want to wait for Joe Lucia to confirm this, but according to my March of '83 revision of the '53 - '82 parts, book that short block would include " Block, connecting rod/piston assembly camshaft and related parts. Cranksaft, bearings, and rear seal. " Part # 3966921 shows as '70 only (350) W/ SP. H/PER. The same part # appears in a old Pass. car catalog I have also as it would have been used in the Z/28. Not sure about the heads, but that short block would be about the pinnacle of small block HP. Highest comp. ratio and solid lifters. And please, no 327 VS 350 high performance arguments. Chip.

[Duke W.]

Re: GM P/N 3966921 Service 70 LT1 Short Block

3966921 is a '70 LT-1 "short block". It should include the complete rotating/reciprocating assembly, timing gears, chain, and the 3972178 mechanical lifer camshaft - what I refer to as the "LT-1 cam". I don't think lifters were included, but I'm not sure.

Current replacement lifters are of the big block "piddle valve" type. I know of no source for original "edge orifice" type that were OE with all small block mechanical lifter cams. The "piddle valve" will work in a small block. They increase overhead oiling by about 20 percent, which was necessary for the big block valve train.

Heads are not included in the above.

This short block assembly was unique to the '70 model year due to the drop in compression ratio in '71, which used different pistons.

No 350 block can be used with the early "Corvette" aluminum valve covers unless you engineer a proper PCV system. Prior to '68 all SBs had a provision for crankcase ventilation at the rear of the block next to the distributor opening. Beginning in '68 this feature was eliminated, and the PCV system inlet and outlet were provided in the valve covers.

As far as condition is concerned, who knows. If the crankshaft can be rotated it might be okay, but it would be better to remove the heads and inspect the cylinder walls.

Duke

[Richard K.]

Re: GM P/N 3966921 Service 70 LT1 Short Block

We need to confirm the part number, with a hydro cam installed by the factory-thats important, that motor could really be a an L-82 with 9 to 1 compression, or better, an 11 to 1, 350/350 service short block. In reality, I bet the casting date on that short block is 73 or later.

[Joe L.]

Re: GM P/N 3966921 Service 70 LT1 Short Block

The GM #3966921 short block (partial engine) was, indeed, the 1970 LT-1. Of course, we have no way of knowing that's what the short block actually is because it's, apparently, now out of the crate. If it is what's claimed and it was purchased in 1974 it will almost certainly have a 1973 or 1974 casting date. I purchased a new 300 hp short block, GM #3970655, in 1975 and it had a 1975 casting date.

The partial engine assembly included:

block with 4 bolt mains

forged aluminum pistons (11:1 CR)

forged steel crank

"pink", forged steel connecting rods

solid lifter camshaft (LT-1/Z-28)

timing set

all engine bearings

rear main seal

Everything else has to be added.

[R N.]

Re: GM P/N 3966921 Service 70 LT1 Short Block

My '70 Z28 was previously owned by a GM dealer who purchase the LT-1 short block and installed it in the mid-80's. The casting number on the block ends with 379 and the casing date is '85. It too had the pink rods and solid liffer cam.

[Joe L.]

Re: GM P/N 3966921 Service 70 LT1 Short Block

My '70 Z28 was previously owned by a GM dealer who purchase the LT-1 short block and installed it in the mid-80's. The casting number on the block ends with 379 and the casing date is '85. It too had the pink rods and solid liffer cam.

Kurt-----


By that time, the 3970010 would have been long out-of-production. So, another block casting, including the GM #14016379, would have been used for the LT-1 block assembly.

[Jephrey S.]

Re: GM P/N 3966921 Service 70 LT1 Short Block

74 castng date, no biggie, but "hasn't been rotated in some time." So much for that.

BUT, how much would this block assembly weigh (getting shipping estimates)? I figure the entire engine around 600# complete, the bare block at around 200#, but can't decide where to take my swag at for a short block assembly.