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[btceng]

I assume that I have .040 over pistons here. Any idea of the compression these would be? It's a 72 model 340. First time engine build and I want to use this as a starting point but may be wasting time and money.

 

[Juan Veldez]

Looks like a flat top 8:1 piston due to the four reliefs as used in a 273.

 

[DetMatt1]

Get yourself a bore gauge and take some measurements so you can compare them to the original diameter to be sure. I don't know what else the 040 would stand for but it can't hurt to doublecheck.

 

[challenger6pak]

Looks like a .040 oversize replacement piston. Compression should be the same as stock.

 

[Juan Veldez]

Verify, but I think a 1972 340 came with 8.5:1 flat top pistons, but they only had 2 valve reliefs. The one shown above has 4, so this might not be "stock" compression. Do "replacement" pistons add 2 more reliefs? Anyone an expert out there and shed more light on this?

 

[btceng]

Thanks for the input. I've been trying to find someone with a bore gauge but no luck yet. The bore measures 4.077 with my digital caliper, but that's at the top of the cylinder. I think it's obviously bored. Was thinking about replacing the rings and berry honing the walls and see what happens. There was very little wear on the camshaft and tappets. The oil pump looks like it sucked up oily dirt off the shop floor. the pickup tube is full of grime. There was a small hole in the pan but I'm sure it was done by a concrete floor over the past many years of storage.

 

[Chryco Psycho]

4.04 is stock bore so 4.77 would be .040 over , put a flat edge across the cylinder & use a feeler guage to find out how far down the hole the pist is @ TDC , looks like it is .035 below or so so maybe mid 9:1

 

[btceng]

Rings are 5/64, right?

 

[moparguy]

Yeah those are 40 overs and I don't think that they should change compression.

 

[moparleo]

cubic inch and c.r. chart

Here you guys go for your scrap books.​

 

[btceng]

Thanks for that. It clears up a few questions . I'll have to measure the deck height and do some figuring.

 

[Rocky_JS]

look inside the piston skirt area and see if there are any numbers in there. 4 reliefs probably cast, universal (used on either bank)

 

[btceng]

I can't make out any numbers in there. I'm trying to get it to the machine shop for a professional opinion. I hope they will be honest and not try to push me into a total rework. I do understand that they must be confident enough to stand behind their opinion, though.

 

[mopar410]

Like rocky js said the reliefs are so it can be put on either side and the 040 is .040 over bore.miWith it mic'ing at 4.077 the shop probably bored each cylinder with the piston in hand to match them.