Can I run this on my engine?

If that is 1.95mm on the inlet and 2,54mm on the exhaust then you are ok if those readings were taken with all adjustments and timing in spec.
The recommended absolute minimum is 1.3mm inlet and 1.9mm exhaust

Top Dead Center Overlap is generally NOT the closest point of intake valve to piston contact. That's why I don't use clay for checking that particular clearance issue.

Well, I laid the clay hamburg on the #4 piston at TDC, carefully placed (and torqued) the head down on the old head gaskets, set both cams with the intake at the 44th pin, grabbed my breaker-bar with the 17mm socket, turned the crank two revolutions...and stopped. :huh:

If there is more involved, then I'm in over my head...
I'm planning to install the exact same OEM pistons, except the replacement set is brand new.

Should I jig up a couple fixtures and machine the valve pockets 1 mm or so deeper, and/or relocate the curved-cut closer to the outer piston edge? I'm a cabinet maker and woodworker by trade, so the process wouldn't take too-much effort to duplicate them... :whistle:

Better safe now than sorry later, when the rpms peak in the Summer.

I presumed you had rotated the engine through several full cycles before taking the measurements ( and not just TDC ) which I appear to have been correct in that presumption.
The valves will have run through their closest pass to the pistons and as long as you are going to use the same thickness gaskets or thicker then all looks good.
I would repeat the check with the piston crown and valves free of carbon to get the most accurate reading.
As for machining the valve pockets.
If you are using STD oem cast pistons I would be wary of the crown thickness.
After market forged pistons usually start off as a blank and the crown then machined to provide the required compression ratio etc so generally have enough thickness on them for further machining.
The std cast pistons however are built down to a price and for a specific application so may not have enough material on them for further significant modification.

I can't speak specifically for the stock J piston BUT I can tell you with absolute first hand experience that the OEM KZ900 and 84 GPz11 pistons can be fly cut for adequate valve relieve without issue.

The KZ900 scenario was to use those same GPz cams with stock pistons.
The GPz scenarion was to use Web 109 Grind with stock GPz Pistons.

Clay mock up is only useful to a point. When you want an accurate picture of the relationship between valve to piston clearances at specific instances, the best way is a dial indicator and degree wheel. Set the head up with light springs.

It's possible to accelerate a valve into the piston crown just after ATDC. The cam lobe profile has everything to do with that scenario. Obviously that varies from grind to grind. Every build has to be taken for the components being used and all aspects of modifications have to be considered and accounted for.

Here's the danger in giving someone in your situation a thumbs up that all is well...bolt it together and run it.

You say you had the cams timed correctly. Perhaps you did, perhaps you did not. Was the cam cover installed? Was the chain tension correct? NOBODY on the Internet knows for certain. That's not to slam you...but it is a consideration that can't be ignored. Do you have any idea of how many guys get their hands into these engines and then totally screw up the cam timing?

IF you have everything timed correctly and if you don't do anything like swapping the cam sprockets and if you assemble the top end correctly during final assembly....then the indications are that you have adequate piston to valve clearance.

I removed the head after two complete revolutions of the crank. :whistle:
Quite a difference in lobe shape from standard to GPZ. My guess provides longer/smoother contact with the valve shim.


Machine the piston valve pockets, at least minimally?
What effect on compression, or air-fuel flow in/out, etc.? Seems the valve pocket size shown in pictures of aftermarket (Wiseco) pistons are massive, by comparison.

You guy's are totally Great!!!

Depending on application the Wiseco pistons take into account the use of high lift cams hence the deeper valve pockets to allow adequate clearance.
The clearance you have stated if correct is around 0.5mm wider that the absolute minimum so in theory should be enough.
If the piston crown thickness allows some machining then i'd go 0,5 to 1mm which would give a comfortable amount of clearance but you may not need it.
With the Bathtub head having a slightly smaller combustion chamber volume and therefore slightly a higher comp over the 'J' mean some removal of material from the valve pockets won't result in a significant drop and you will most likely end up somewhere around the 'J's std comp.

Looks like you may need some rings.
Steve

swest wrote: Looks like you may need some rings.
Steve

...next week is assembly time!