RichBad

Piston compression height - 1928 4

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Hi,

 

I'm in the middle of having some engine work done (re-bore etc) and the engine builder just asked what the correct piston compression height should be as all the ones he can find listed seem to be too high.

 

It's a 1927/8 Series 128 (fast four engine) but I believe the pistons are the same on all the 4s?

 

All the listed pistons have a compression height of ~ 2 3/16" but my engine builder thinks that they should be 2" otherwise they protrude from the block too much and the top ring is too close to the top of the bore.

 

I can see a few possible reasons and wanted to check with others here:

1. The 2 3/16" compression height is correct and the protrusion is normal (the old pistons were the same and it had clearly been running like this for quite some time)

2. The block has been decked multiple times (it didn't look like it but anythings possible on something this old!

3. The rods are too long (they looked like the correct and original ones).

 

Is 2 3/16" correct and should they be flush with the deck or not?

Thanks,

Richard

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Hi Rich: This response is 2nd hand from a reply to one of my queries.

The reason there are no ridges at the top of the cylinder walls is because the top piston ring goes all the way to the top of the cylinder. That would mean that the top of the piston does extend into the area above the top of the block. 

I am sure others with more empirical data will chime in. 

Paul

 

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Yes Richard the  the piston comes about 1/8 above the top of the block.

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d t

 

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Great - thanks!!  Tried calling you Bob but couldn't get through - howe all is OK:)

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