We all know that a set of pistons will take a big chunk out of our pockets and then we have to sometimes make more room for the valves because of a big cam were installing. Over the years i have been milling my own pistons for various reasons (saving pocket money) (fitting my own combos) (loosing piston weight) . One of the common questions is how deep can you fly cut a piston for valve clearances. I created a system that can tell you that or atleast see what you have in thickness to make your desision on this subject. Because of all the diff. power adders Min. thickness under the valve pockets will change in your appl. For me its N/a motor high rpm,s and as light as possible without breaking. I have talked with many piston manufacture engineers over the years on this subject. Short of ordering custom pistons God Bless ya if you have that kind of money you can find out the anserwer yourself and either do it yourself or tell your machinst how far to go down safely.
You will need a good magnet one and a 3 or 4 inch piece of threaded rod 1/4-20. This length of threaded rod needs to be long enough so that piston doesnt hit magnet when moved around. Grinding a point on one end and attaching the magnet to the other end it will be used to probe the underside of the piston, ie walking it around the underside of the piston. The magnet probe will be set onto a piece of flat steel about 10" x 10". You can buy a platform known as a height indicator stand a heavy flat steel platform with a vertical tube were a height indicator attachs to but this is a make your own deal for low buget engine builders trying to get ahead. Hopefully you already have a one (1") inch range dial indicator with stand and you can used that here in this set up. If you dont have one there cheap now and items from china work well it doesnt have to be a top notch unit. Normally they come with a Magnet and adjustable arms. A long reach rod is needed and a ball tip works the best at the end of the indicator. You then set up the indicator to rest on the tip of the point of threaded rod and zero the dial. This is done on the same steel plate. You should now be able to raise shaft of indicator up and down and rest on point and still maintaine zero.
You now raise the idicator shaft and place the piston between the point and indicator and you will get a direct readout of how thick diff. areas are of the piston. Moving the piston around will give you a map that you can create of how much material is under the valve pockets. I have milled pockets for valve clearance to within .090 of deck thickness only in small areas the size of a .100 circle the area around it generally have alot more meat in the area so that doesnt seem to hurt strength but this area forces you to stop and go no deeper. A min. thickness of .120 is more enjoyable for me but .090 is pushing it but without any problems but not this is for a N/A engine only no nitros or boost, and this is in making a 900 plus motor at 9500 rpms. I do hope you have good springs another subject. The exhaust valve pocket is the most critical pocket to be careful of and always the first to get hit by a valve, this is the area i mentioned of .100 circle with a min. of .120 of thickness needed. The rest of the pocket will have plenty of material around it usally .190 to .240 of meat left or more but that small area in the pocket determins how far you can go to gain max. valve clearance on the exhaust side. Most Piston people i have talked with agree with my out come. Some are surprised. Just be careful on the intake side so you dont cut into the top ring land leave yourself some meat there or youll have a top ring gas port but piston will be ok if you do but dont.
I have milled as much as .100 off the piston deck to spill off weight and get a taller domes for more compression so buying someones pistons that are to tall (deck height) cheap can be your combination when finished or something you had extra and can be a new build up for a diff. stroke combination. Ie: a 1.385 compression height piston can be safely milled and used in a 1.250 compression height setup. Its just alot of work but your not paying the big money and your time isnt 50.00 ahour either. If your doing Nitros this puts top ring closer to combustion so be careful nitros pistons like the ring further down out of the heat. So this article is ment to get your brains jump started and expand your knowledge and thinking. How close do you want to run your valve to piston clearances is yet another subject. GOOD LUCK
P.S if Pics are needed let me know i will post em.

Douglas

This is the system that allows you to measure piston thickness. Notice home make magnetic base with 1/4-20 threaded rod with a ground point

Very cool star.

I am not sure if I will ever have the talent to do this but I certainly may try one day.
And for those that have the talent, this information is priceless.
Thank you for spreading your knowledge and experience with the rest of us, and that includes everyone that contributes to the tech side.
It really helps!!!