So I had the oil pan down last weekend to swap the rear main seal. I could see the bottom of the pistons and read what looks like "157-99" on each of them. I did not build this engine and assumed they were Keith Black Hypereutectic pistons. The last rebuild was around the late 80s/early 90s. I did a google search and it came back with one hit for that number and they claimed they were 12.5:1 race pistons. Pretty certain this motor is not 12.5 compression... The Keith Black catalog I found on-line did not have that part number anywhere close to the hemi piston numbers. Anyone have an idea of what brand they might be?

68jim

From the underside, probably looking at a raw forging/raw casting number that is then machined into several different finished parts, given different part numbers, and then
put into a package of 8 with wrist pins and so on and assigned different part numbers again.

I'm sorry that's not helpful at all but it may just be an internal ID number for the piston mfg, and no good to you and me.

Yeah, likely a casting/forging #.
Whatever they are, how does it run on pump 92 ?
The Hemi chamber is inherently octane-friendly compared to a wedge chamber; I ran 10.8:1 forever on pump gas, 36 mech + vacuum, no problem, though that one had a fairly large old-school cam.
Hemis like CR & fuel, basically.
I've never understood why some guys would run 9.0:1 in them, unless they wanted to run 87 octane.

I would never assume that. I don't even know if those existed back then. My guess is that is an old TRW forging number. And could very well be 12.5 since that was a common race cr. If you take the spark plugs out and look inside 12.5's come way up there.

I have an example of a piston with that casting number. It is a HC racing piston with Dykes top ring and 1.094 pin. But it CD is shorter than the normal SS piston. So depending on your block deck height, and rod used they could have attained a lower CR.

Agreed, a 12.5 piston ( shown ) will have a sharp dome on it compared to say a 10.5 piston

This is approx a 10.1

however, [and i know virtually nothing about a hemi] without knowing the head combustion chamber volume, would those pistons really be 12.5 : 1, say, on a stock head that never had work done to it ?
please educate me some on gen 2 hemi's.

Same as other engine.

Final compression depends on cyl head cc. Head Gasket thickness. Etc etc

Thanks everyone for the information. Looks like I will break out the borescope tonight and take a look through the spark plug hole. It runs good but I also just use the ethanol-free 92 and maybe a little blend of 110 race fuel (20-25%ish) Have not heard it ping - even without the race blend. Previous owner also told me it had a Kellog crank but this weekend I saw a part number stamped on a counterweight vertically (2780533), that crosses over to a stock hemi crankshaft. I buzz this engine to 6K pretty much every time I take it out...

I'll let you know what I find.

68jim

Who built the motor jim?

If it's a 12.5 you won't need a borescope. It will be very close. They made a 12.5 Hi-top that nearly hits the plug.

as i surmised, however, as has been, already stated, [i now know ?] the pistons come closer to the plug than the valves ? [depending on cam lift and duration specs ?]

PM sent

Hello Jim,

My buddy has a bore scope. If we pulled out a spark plug we could get an idea of the compression ratio of the pistons. I have found that a Hemi piston with a "sharp" dome is typically 11-12.5 ratio. I piston with a "flatter" dome is 10.5 or less.

Shoot me a text if you want us to swing over.

Stan

I was too slow typing! It looks like some other folks are thinking the same thing I am thinking!

Stan

It is the lower compression style. I can see two partial valve reliefs and the wider flat top. I guess the brand really does not matter at this point, but it has been a good exercise so far. I can understand about the numbers on the bottom being a batch number or maybe raw casting number to ID the item prior to machining. I would have thought that a 12.5 piston vs a 10.25 might have used a different casting rather than just machine off a section off the top. That would mean a real thick section on the 12.5 version unless also machined from the bottom??? Anyway, I'm sure I will still drive it as before. That motor makes such good music at 6000 rpms...

Were they nascar rods? Here is a picture of 2 HC pistons. The 159-99 on the left. The other is a 2531506 o/s piston. Like I said these have 1.094 pins. Different CD. Someone could have milled the pistons also as they blue printed the motor. Especially if it was decked.

I have no information if the block was decked or not. I did not look for part numbers on the connecting rods. About the only thing I noticed was that all the rods were slightly hitting the windage tray judging by the multiple patterns of dimples and scratches. I kind of figured that the tray may have been flattened out ever so slightly somewhere along the way prior to installation. Looking at the tray on edge, it appears very "square" and not bent anywhere. It got replaced with a newer style plastic windage tray that has the gasket molded in. Way much easier to install than trying to fight 2 gaskets with RTV, tray and oil pan. Not sure if I would have heard any contact since it would have probably been masked by the mechanical lifters. I don't think I will hear anything if it contacts the plastic. I suppose the fix for that would be to use the tray designed for the 4.15" stroker crank???

Have not taken the car out for a drive yet so will hold out on any endorsements...

68jim

Nascar rods have the larger rod bolts. Beside the thicker beam.