I just picked up a set of Jahns pistons from a friend. He tells me that they are 11:1 compression ratio, but he wasen't sure. I have used Jahns pistons in the past and two things were different then. #1 the top of the old Jahns' weren't machined as much as these and #2 the dome, as I remember it in the 60s, was a lot larger and they were only 8.5:1 compression ratio. Plus the compression ratio was cast in the inside, these aren't. My question is, is there any way to tell what compression ratio these are.
I want to be able to use these in my daily driver.

The type of engine you are working on would be helpful.The bore size would help to,as would any part numbers that are on them.The more information,the easier for us to help you.
EvilDr235

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Two types of people drive old cars.Rich people because they want to and poor people because they have to.
Remember EvilDr235 has a cure for Your sixness.

If there's a part number on them, give Jahn's a call.

Otherwise, one way to find out the difference between the two pistons would be to get a chunk of modeling clay, push both pistons into it up to a common point, like the top piston ring groove, then measure the volume just like you would for a head chamber, and note the difference between them.

The pistons have Jahns cast on the inside but no numbers are visible anywhere. These are for a 235 +.030. I will look again for numbers.

OK! No numbers.
The piston, I was told, is 235+.030
I measured the piston right below the bottom rings and parallel to the piston pin and the dimension is 3.566.
The popup is .266
Is Jahns still in business?
If so does anyone know an E-Mail?, A phone number? or any other means of contact?
Where is the right place to measure a piston to get the diameter?
Doesn"t a stock 235 = 3.5625? = 3-9/16"

Dear Brad;

I'm prety sure that Jahns only made 8:1 sets for the 235" engine. That was a lot in the l950s as pump gas was only 90 octane on a good day.

The 11:1 sets were for the "full race" GMC.

Also; Your right the stock bore on a 235" is 3 9/16.

Good luck, John M., #3370.

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