pistons below deck heigth.

[cuzncletus]

Turning my old 59A over with the heads off to find TDC, I noticed the pistons do not come up to the top of the deck. They appear to be about 30-40 thousandths from the deck at the top of their stroke. The center of the piston (slight dome, appear stock) barely comes out of the hole. This is all by eyeball, no flat object across the top of the bore or micrometer. I know the motor was rebuilt in the 70's and has very few miles since. It ran well when I pulled it but I never really tested it for power.

Is this normal for a flathead?

[Mart]

It might have Merc pistons? If so, they'd be .125 down the bore. Bit more than you have said, but to be honest, without some measurements, we are all guessing.

Mart.

[51 MERC-CT]

Somebody told me that those are super rare pistons that allowed for future shaving of the deck.

[FL&WVMIKE]

Many 'rebuilder' pistons are low like that. They allow for engines than need to have their decks shaved like that. Some can be 0.030" to 0.040" down in the hole. If the piston is further than that, (roughly a spark plug gap) then it's possible that there is a mismatch of parts.

Better measurements might help.

Good Luck!

[Brian]

I found that hypereutectic pistons sold by Speedway a while back came up about .030" short as you describe. Maybe it has those?

[JSeery]

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Flathead blocks have thin decks to begin with, cutting them these amounts wouldn't be that good of an idea!

[JWL]

If the bores are oversize the typical procedure during manufacturing was to "de-compress" (Sealed Power Term) by machining the dome slightly to maintain stock CR. Overall a bad idea that was done with good intensions.

[Seth Swoboda]

Originally Posted by JWL

If the bores are oversize the typical procedure during manufacturing was to "de-compress" (Sealed Power Term) by machining the dome slightly to maintain stock CR. Overall a bad idea that was done with good intensions.

JWL, can you further explain what the purpose of the "dome" piston was?

[Jason in TX]

I found the same thing on a 1934 engine with .060 aluminum pistons in it. they fell about .030 below the deck. When I took the head off and saw the first piston come to the top, I thought, "Oh no! Bent rod!" Then I noticed that all 8 pistons were exactly the same. About .030 below the deck. I then thought. "No way. Not 8 bent rods." haha

[Ronnie]

Here is a read on destroked pistons common for piston makers.

https://www.google.ca/search?q=de+st...troked+pistons

R

[cuzncletus]

Ronnie; Interesting bit of info in that article, something I'd never seen before.

www.enginebuildermag.com/2013/06/destroking-or-decompressing-pistons/

I remember some smog motors from the late 70's that had pistons with negative deck height, 400 Fords I think and also some Mopars. They were pigs. That said, I'm not going to tear down a good rebuild to correct a minor problem. I'm buttoning her back up and going.

[Mike51Merc]

I bought a "freshly rebuilt, never run" SBF 302 (1975 block). Before running it I found the pistons were .054" below deck. Turns out that the 1975 block had a higher deck to begin with and the rebuilder put in "rebuilder" pistons with .020" lower compression height.

This won't help your flathead issue, but it confirms the low compression height piston theory.

[scooder]

I've found most stock Ford and there contemporary replacement stock pistons come in at about 0.030" below deck at their outside edged. Not an ideal situation, but in my experience very common.
Martin.