Board index Flathead Power-Technical Questions, Answers, and Suggestions K-Models KR/K piston

KR/K piston

Post Sun Feb 23, 2014 5:50 pm

Posts: 420
Location: Bergen, Norway
In Gordon Jennings article from June 65 on Jerry Branch and KR tuning, there are pictures of original and modified pistons.
Both the original and modified piston has a narrower diameter at the dome going half way down to the top ring. I have no KR piston to compare with, but Jerry states in the interview he is using standerrd K pistons as they are lighter than the KR pistons. All K/KH pistons I have seen are only aftermarket stuff and they are all straight with no smaller diameter at the top.
Anyone know the dimension of the reduced diameter?

Link below is to Beauty of Speed webpage where this picure can be seen ... nch_03.gif


Post Sun Feb 23, 2014 6:05 pm

Posts: 1727
Location: Interlaken, NY USA

Gus, I think that the pistons only appear to be smaller at the top due to some photo graphic illusion. However, some cutback of the crown might be useful if 45 pistons have the same tendency to expand and become scored as I've seen 80 pistons do. You might cause some problems with the flame front from the pocket of trapped gas that you'd create. Interesting idea.

Post Mon Feb 24, 2014 8:48 am

Posts: 420
Location: Bergen, Norway
I have the Cycle world booklet and have studied the text and pictures many times. I'm pretty sure the top diameter is smaller.
Have gradually upgraded the old 45. Next up is new stroker pistons. I will have them made with a pop up. Plan is to copy the bevelled piston. Head is already welded with the shelf. Plan to do the 3/16 pop up to increase flow. Basically copy the setup as by Branch.
I have no literature describing the cutback and the reason for such, but I see an advantage exactly as you describe. More or less all 45 I have touched are scoured where the dome scratch the cylinder wall due to expansion/ carbon build up.
If it was for the carbon only, there should be no reason to give room for the build-up, as the long time solution is to have right air/gas mixture.
Would be interesting to know if there is another theory than expansion and or carbon build up that would be cleaned up by regular tear down?
Maybe to ensure balanced pressure build up after ingnition?

Post Wed Feb 26, 2014 8:11 am

Posts: 598
Location: Crewe, Great Britain

There's no way you're gonna get some carbon built up on the side of the pistons that faces the flame front...

It might get scored a bit, no wonder, it's directly in the flame front path, but nought to worry about...

The Jerry Branch (met him last year on the Flats, what a guy...) KR pistons were just relieved to expose less than a vertical wall to the flame front.

Can't really see that as an improvement (loss of CR, might not work too well with the head shelf angle...) and he was the only KR guy to do that for a while. Pretty sure he did not do it all the time.

That's what my KHK pistons look like on methanol and 12 psi boost...

Flame facing side:


Other side:



Post Fri Feb 28, 2014 8:10 pm

Posts: 333
Location: north central Ma.
I checked my NOS 20 over pistons on my shelf and there's no change in diameter our shape.
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