Original '66 427/390 hp big block pistons?.....Duke/Joe

[Michael G.]

During engine tear down of my '66 427/390 hp observed part #3869968 along with GM embossed emblem on inside of piston. Everything else appears stock/original? Are these considered original stock? Thanks in advance all.

[Joe L.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

During engine tear down of my '66 427/390 hp observed part #3869968 along with GM embossed emblem on inside of piston. Everything else appears stock/original? Are these considered original stock? Thanks in advance all.

Michael------


Yes, they are original pistons. Without the stamped "A" all we could say is that they are original OR replacement pistons of the original casting number. However, with the "A" it's pretty much conclusive that they are pistons originally installed at Tonawanda Engine.

By the way, the 66-69 L-36 pistons were SIDE-SPECIFIC. Check to see if there is any difference in the casting number for the left and right side banks.

[Michael G.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

Joe,

What's the significance of the "number 10" ink stamp observed on top this particular pistion?

[Duke W.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

If that is a forty-plus year old engine, I'm surprised how clean it is inside. What is the mileage?

Had it not been for the junky OE nylon cam sprocket, it would probably run forever. GM really screwed the pooch on that one.

Duke

[Joe L.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

If that is a forty-plus year old engine, I'm surprised how clean it is inside. What is the mileage?

Had it not been for the junky OE nylon cam sprocket, it would probably run forever. GM really screwed the pooch on that one.

Duke

Duke-----


They used the aluminum/nylon cam sprockets for a long time. All Mark IV big blocks originally used this type cam sprocket. Beginning in 1966, small blocks did, too.

I removed the original small block from my 1969 at 107,000 miles because I thought that the engine was "shot". After much later dis-assembly and "autopsy", though, I found that the only real problem with the engine was the cam timing sprocket.

I later rebuilt that engine and it's "sitting quietly" now (awaiting the day I get tired of the big block).

[Michael G.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

Keep in mind much grime was wire brushed off the top of the piston however, my thoughts exactly....this engine is awfully clean. Mileage unknown, odometer stuck @ 28K. What does the "10" indicate?

Interesting observations on the crank shaft....denote sharp edges of the counter weights and welded area on last counter weight

[Joe L.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

Keep in mind much grime was wire brushed off the top of the piston however, my thoughts exactly....this engine is awfully clean. Mileage unknown, odometer stuck @ 28K. What does the "10" indicate?

Interesting observations on the crank shaft....denote sharp edges of the counter weights and welded area on last counter weight

Mike-----


I don't know what the "10" indicates. It might indicate a ".010-over" piston but that would be unusual in a production engine. It might also indicate a 0.001-over" piston. That would not be so unusual in a production engine. You could measure it to confirm or rule out either of these hypotheses.

The welding on the crankshaft looks like balance weight might have been added and welded-over. However, I've never see a GM crankshaft with this sort of treatment from the factory.

Do all of the pistons have the same casting number on the underside?

[Clem Z.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

Keep in mind much grime was wire brushed off the top of the piston however, my thoughts exactly....this engine is awfully clean. Mileage unknown, odometer stuck @ 28K. What does the "10" indicate?

Interesting observations on the crank shaft....denote sharp edges of the counter weights and welded area on last counter weight

auto machine shop balance job so i would guess the engine was not "cherry"

[Michael G.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

Hold on there Clem....why o'why would someone go to the lengths to balance the engine but leave the crappy original timing set? What may appear to be a machine shop balance job might be.....or not. I've seen the inside of numerous engines. Haven't seen a crank done like this but who knows what was going on at Tonawanda engine building plant on Monday morning or Friday afternoon in September 1965 . Hey.....pass me that bottle....LOL.

Joe...you are correct, other piston #4966988. Does that sound about right?

[Patrick H.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

I later rebuilt that engine and it's "sitting quietly" now (awaiting the day I get tired of the big block).

You would have to start using it before you could get tired of it.

[Duke W.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

"The welding on the crankshaft looks like balance weight might have been added and welded-over. However, I've never see a GM crankshaft with this sort of treatment from the factory."

My exact thought the moment I saw that. OE crank end weights were slightly "heavy" and then drilled for final dynamic balance. I don't think the plants ever added metal. Could it be possible that it's a plant repair? ...say the balance equipment went out of spec and drilled a bunch of cranks wrong.

Duke

[Joe L.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

Hold on there Clem....why o'why would someone go to the lengths to balance the engine but leave the crappy original timing set? What may appear to be a machine shop balance job might be.....or not. I've seen the inside of numerous engines. Haven't seen a crank done like this but who knows what was going on at Tonawanda engine building plant on Monday morning or Friday afternoon in September 1965 . Hey.....pass me that bottle....LOL.

Joe...you are correct, other piston #4966988. Does that sound about right?

Mike-----


4966988 is not a number I would expect. I would expect it to be a number 1-6 digits different than the other side pistons. I don't have casting numbers for these pistons and the part numbers are not sequentially related in any way to the casting numbers.

It's possible that the engine was previously rebuilt. The original pistons might have been re-used or, less likely due to the "A" ink-stamped on the bottom, new GM pistons may have been used. If they are GM SERVICE pistons then the "10" does not refer to ".010"-over" since no such oversize was ever available in SERVICE from GM.

As far as the cam sprocket goes, it's possible that a GM SERVICE piece was used for a rebuild. Folks did buy these pieces from GM or GM wouldn't have had them available in SERVICE for as long as they did. If one went down to a GM dealer and purchased a cam sprocket for a big block, the nylon/aluminum sprocket is what they would have been sold. GM dealers did not even have any other sort of cam timing sprocket available. If the work had been done by a GM dealer they would almost certainly have used all GM SERVICE parts.

[Clem Z.]

Re: Original '66 427/390 hp big block pistons?.....Duke/Joe

Hold on there Clem....why o'why would someone go to the lengths to balance the engine but leave the crappy original timing set? What may appear to be a machine shop balance job might be.....or not. I've seen the inside of numerous engines. Haven't seen a crank done like this but who knows what was going on at Tonawanda engine building plant on Monday morning or Friday afternoon in September 1965 . Hey.....pass me that bottle....LOL.

Joe...you are correct, other piston #4966988. Does that sound about right?

as cheap as those cranks were back then i can not believe that GM would do a repair if one got screwed up balancing they would just toss it in the scrap