LA 360 piston ID

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lawnmower man

lawnmower man

lmm
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Picked up this 360 LA short block, would like to ID the piston. Anyone have any possible ideas. No markings, it is .030” over. It is factory 3.58” stroke. It is approximately .070” above the deck. Unaware if it’s been decked. Any help is appreciated. Cheers.

FF438DF6-CA3A-4EEF-933A-81F7B08D6E12.jpeg
 
I guess that's one way to get some quench out of an open chamber... It looks like someone took some stock bore forged 340 pistons and milled about .050 off the tops, but that would have a stock bore size of 4.04. I would put in a stock oversize piston and rod to see where it comes in at to get some idea of how much may have been taken off the decks. Can you get a picture of the piston from across the deck towards the valley so whe can see what the deck height looks like from the side? It looks like that the outside edge at the spark plug has been relieved to even it out with the deck, possibly for plug clearance.
 
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The bottom looks like the base of a dome that was milled down and it reminds me of the 11.5-1 domed pistons that Direct Connection use to sell. But I can't place the dimple in the middle, unless it is an OEM hi-compression 340 piston.

Even the "Hoover's Mover" (Tom Hoover- father of the Hemi) 360 from the '70's had to have its OEM 340 low compression pistons milled top and bottom to make them work. This was before aftermarket performance pistons for the 360 were readily available. I did build a couple of 360's using DC's Nascar 355 pistons which were made for something like a 3.454" stroke crank. Worked out pretty well with higher compression & lighter weight without compromising strength.

Can you get any pics showing the bottom of the pistons to see if the wrist pin bosses were milled for counterweight clearance and might be able to tell if it's cast of forged?
 
I posted that because at a glance thats a 68-71 style 340 looking setup, fly cut pistons protruding past the deck up into the combustion chamber. If it is a 360, someone put that 340 style piston in to up compression. Its possible they are custom cut pistons etc. done in a machine shop or an old aftermarket set.
 
That may be what you want a bit higher compression it certainly looks like that was what someone was going for. I seem to recall stock LA 360s had dished pistons and were low compression.
 
With the long skirt I would say it started life as a 340 piston with a offset ground crank to raise it up more
 
Have you measured the bore? Only .070" above the deck will not be enough to hit a stock open chamber head. They are generally in the .090" range deep on the side opposite the spark plug. If it's a 4.04 bore, somebody built a Hoover motor.
 
RustyRatRod said:
Have you measured the bore? Only .070" above the deck will not be enough to hit a stock open chamber head. They are generally in the .090" range deep on the side opposite the spark plug. If it's a 4.04 bore, somebody built a Hoover motor.
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Factory crank according to casting number, 645 stock rod, 4.030” bore.
 
teringer said:
With the long skirt I would say it started life as a 340 piston with a offset ground crank to raise it up mor
Dartswinger70 said:
I posted that because at a glance thats a 68-71 style 340 looking setup, fly cut pistons protruding past the deck up into the combustion chamber. If it is a 360, someone put that 340 style piston in to up compression. Its possible they are custom cut pistons etc. done in a machine shop or an old aftermarket set.
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i Think you’re probably right. Looks like there’s some machining that took place. Look at the small chamfer on the pin boss. The area above it looks very true and smooth.

FD07F678-8880-4173-A494-20F895B8D8B6.jpeg
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RustyRatRod said:
Have you measured the bore? Only .070" above the deck will not be enough to hit a stock open chamber head. They are generally in the .090" range deep on the side opposite the spark plug. If it's a 4.04 bore, somebody built a Hoover motor.
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This^^^^. Don’t measure the dome. That’s screwing you up.

Only measure the quench pad of the piston, which is ABOVE the valve notches.

The you’ll know how far the piston is out of the deck. I’d be hoping like hell that piston is .070 out of the hole. That’s where it should be for an open chambered head.
 
They look to me like TRW pistons. I do remember they had the hole in the middle from when the were machined. Plus the marking underneath. I think someone did some fancy machine work.
 
lawnmower man said:
There is no dome. It is essentially a flat top with valve reliefs
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Look around the bottom of the piston. You can see a rolled edge.

Your pistons look like an older TRW domed forging that’s had the domes milled down. Thats why I said just measure the piston above the valve notches. That’s the part that matters.
 
It's a TRW or Speedpro that was made to fit. TRW got bought by Speedpro. Here is one before it was milled down....

Speed-Pro.JPG
 
Most likely hypereutectic piston. Better than cast but not as strong as forged. They are good pistons. Just heavy. The plus is you already have them and the block they fit in. I'd say you scored!
 
They look like they have extrusion marks from forging inside of the piston. A good set of well massaged 974 heads or some open chamber W2 heads could make a ripper!
 
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