ITM piston valve relief depth

[Derek]

Looks like I will need to cut valve reliefs in my test motor for the KN20-VCT.

 

The pistons are ITM RY6134-040 flat tops with no relief cut. I'm trying to find out how deep I can go before I compromise the integrity of the piston. ITM in California has no data. 

 

Thanks

Derek

[Chickenman]

I don't have an exact answer. But I did find this VERY detailed article by Racer Brown on building Datsun L-Series engines. It's well worth Bookmarking and reading the complete article. 

 

http://www.datsport.com/racer-brown.html

 

Chapter 13 covers piston to valve clearance. Looks like stock Flat Tops can be machined quite a bit. ITM should be no different I would think.  If you only need an extra .020" to .050" I would think it would be a non issue. 

 

Here's the important info:

 

The piston crown of a stock piston is about 5/16-inch thick near the centre of the piston and slightly thicker toward the outer edge. This means that the stock pistons can be safely machined for valve reliefs to a depth of 0.125-inch from the top surface of the piston crown IF and ONLY if there is a radius of at least 0.060-inch at the bottom of each relief. This leaves a nominal crown thickness of about 3/16-inch at the thinnest point which, of course, is at the bottom of the relief. The crown thickness increases as the relief approaches the top surface of the piston. Normally, the remaining 3/16-inch crown thickness is adequate for any application involving the use of stock pistons. It must be pointed out that the 5/16-inch total crown thickness is an average figure; some pistons may be thinner in this area than others, so it is certainly advisable to make some careful measurements before arbitrarily gouging the pistons 1/8-inch and finding out later (the hard way) that the 1/8-inch was too much.

[Derek]

Thanks for the info. I ended up going .240" at the deepest section of the intake. There was .440" of meat in that area. The valves are angled at 15 degrees so the depth lessens very quickly. The problem I'm going to have is there isn't enough piston diameter at 87mm for there to be a decent amount of material.  I imagine everyone that runs one of these heads will at least have 88MM or better bores and probably custom pistons.