Thanks  Pistons are .060 over 292 so 3.81?  Engine is still together since 1965. Has very few miles (1/4 at a time) but have not bore gauged cyl bore. Did pull cam out, was still perfect .290 lift S/C cam I bought in 65 over counter  ford dealer for less than $20.

COS, are your Jahns pistons 3.875"? If so have you found rings for them? 
Thanks

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Thanks  Gaga  Exactly info I need.  Those lumps look big, will do math and see how much it  rises comp. ratio with g-head 69 cc (?). As mention above over 11 to 1 I suspect.. I cc some 471 heads i had redone for SC engine, they came out 78cc with .015  surface clean up. Guessing those  pistons must weigh 800 grams.

cos,
 I ran those Jahns  pistons too. Had the same experience and retired that engine. 3 7/8" bore.
I made a wax cast of the dome on my pistons and cc'd it. the result was 17 cc.
 Jahns also had "1/4 inch deflector" piston as they called them.
Hope this helps.



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New rings would require a fresh finish on the bores even if nothing but a glaze breaker or a ball hone is run through them.  Assuming the original rings are not worn, then I see no issue in reusing the same rings in the same bores.  Because the second ring is a tapered design, looking at the amount of wear on that particular ring is always a good indicator of how much wear is on the rings.

Lorena, Texas (South of Waco)

If Cos doesn't have an issue with the bores, the cheapest route would be to cut down those domes. Just keep track of which piston belongs in which bore.  Would balancing become an issue (just thought of that)?

Ted, would you re-ring the pistons without touching the bores?

I am only familiar with the Jahns cast domed pistons having a 10cc dome.  While I currently do not have any Jahns pistons sitting here to look at, I do have some older JE and Mickey Thompson cast pistons that also have the 10cc domes.  Even some of the older SBC cast pistons used a 10cc dome so that must have been an industry standard for a while.  Both the JE and M/T pistons are cast in a hollow dome configuration, so completely removing the domes on those would be an issue.  But if memory serves me correctly, the older Jahns pistons can be cut to a smaller dome and/or a flattop configuration as they were cast with a solid dome.

Lorena, Texas (South of Waco)

It might take a while to find a set of 471 heads that would be a good candidate for a rebuild. The cost of the heads + cost of machine work to put them into service won't be cheap. Personally, I prefer flat-top pistons and smaller cc heads.

Would it be possible/practical to cut down the domes of the Jahn's pistons to lower the compression? I have no idea how much meat can be removed without ruining them. 

Thanks for replies. everyone. Ted answer my question as usual.  Ted do you remember if Jahns made   more than one  dome height, these pistons  dome is about 1/2" highest  point   ( half moon shape) and tapers down to  noting  over pins. Thanks again.

Many of those early Jahns cast/domed pistons had a 10cc dome.  A std bore 312 with the stock flattop pistons 0.024” in the hole, composition head gaskets, and 69cc heads will have the static compression ratio (SCR) at 8.66:1.  Using a 10cc dome on the pistons in place of the flattops will raise the compression ratio to 9.70:1 SCR.

The advertised combustion chamber volumes for the G heads is 69cc.  The combustion chamber volumes for the ‘113’ heads is 72cc while the combustion chamber volumes for the ‘471’ heads is 82cc.  Back in the day it was a common practice to use the 471 heads with the 10cc domed pistons which essentially kept the compression ratio the same as factory delivered.

Lorena, Texas (South of Waco)