Oversized valves and valve reliefs
 

Is it necessary to cut the valve reliefs in the pistons wider when changing to +1 valves?

From my experience most have more then .5mm, .020” clearance to accommodate a 1mm oversized valve. In fact most have a generous amount of clearance. It really depends on the piston manufacture and application. For example less generous on max effort high comp pistons.

Is your concern if it will become an interference motor or for just normal timed operation? 99% of the engines that I have built the valve relief clearance was a necessity for normal operation because of them being high compression with high lift cams. Non-interference engines are definitely not the norm. I will be mocking up my new eng later this week with st 9.5:1 pistons and 1mm oversized exhaust valves. I will follow up with the results.

Yes, contact is the concern. It will be a larger cammed 99 head, low compression pistons.

I just clayed my pistons and there is no contact between the pistons and the valves so it is still a non-interference engine. I'm running 1mm over size exhaust, std intake with the msm cam and ST 9.5 pistons. It is close and 2mm oversize with larger cams might be another story. There is still a chance of bending valves though because the valves will now hit each other so in another sense it is an interference engine. If the belt breaks it will most likely bend the valves. You will have to be careful turning the cams independently when assembling the head and stetting the initial valve lash.

The reason I asked what your concern was is because 90%+ of all automotive engines are an interference design. It is always a performance loss or sacrifice to make an engine a non-interference type. I would never consider having a custom piston made for a race application to accommodate that scenario.

I can only offer the one data point I have small experience with...

My BP4W +1 valved head clears with unmodified Supertech 8.6:1 83.5mm pistons and a Mazda 5W cam. This is probably less cam than you are running but its a data point. I had originally specc'd +2 valves and had to back out because the vendor "could not confirm that they would clear" and "check with your professional engine builder"

"Verify fitment" is my concern, lol.

I think it’s very unlikely today’s pistons will not clear. +1 mm valves.

My older JE pistons would not clear with a high lift cam, but I was able to clearance the pistons without any specialized tools. It’s also a simple (a little time consuming) task to check the clearance with clay.

When you say fitment do you mean can the valve seats be opened up enough to accommodate the larger valves? If so yes. If your question will it become an interference engine then your answer is it depends on how big of a cam and how much oversize on the valves. 1mm over and oe cams will still maintain the non-interference scenario. It does not matter either way because most engines (90%+) are of the interference type. The valve reliefs are part of the HP pistons design to allow for larger cams and valves not to maintain the non-interference capability. The reliefs are needed in that case to allow the engine to run while cam timing is maintained. I can guarantee you any over size valves will hit each other if both intake and exhaust valves are open at the same time. Its important that you understand that.

I am certain OP knows that. When you start playing with big cams, you never just willy nilly remove the cam belt and start spinning the motor. I had to learn that discipline myself, and it’s easy to make a mistake.

Definitely with larger cams but even stock cams with 1mm oversized valves will clash.