Piston to valve clearance wisdom

Hello,

I have a freshly decked head that i am trying to check piston to valve clearance.

Using helper springs and some modeling clay, I was able to get a sample to measure but dont have a spec.

Engine is Mazda Skyactiv 2.5L, high compression engine means extra concern here.

Questions:

1. How would I compensate for VVT for this test?

2. The service manual doesn't have a piston to valve clearance requirement. How do I know what is acceptable?

Thanks,

Phil.

Generally 0.080in intake and 0.100in exhaust, but race engines might reduce this to 0.060in or less. With a Variable Valve Timing, you would typically check at both ends of the adjustment range. You will probably need to temporarily pin or block the adjusting mechanism to hold that position.

BTW there is a module specifically on this topic in the Engine Building Fundamentals course.

Hey David, appreciate the quick response.

Regarding the cam phasers, Is it safe to assume that the mechanical limit of the phasers is the maximum cam timing the engine will command?

The exhaust VVT is hydraulic, the Intake side is electric.

Yes, there has to be a mechanical stop -- so you don't rely on software to prevent a piston to valve interference. If you find your new decked block has insufficient clearance in the maximum position (advanced for intake, retarded for exhaust), you could modify the phaser to prevent that. Then tune the ECU to only use the available range.

Update:

I was able to turn both cam phasers and run trials at their extreme positions.

The lowest reading I found for Piston to valve clay measurements:

Intake: 0.12"

Exhaust: 0.07"

Since I cannot hold the exhaust cam phaser springs in the unlocked position, I compensated for the rotation when timing the engine by retarding the cam position by 3 teeth (visible maximum cam angle). The problem is that the phaser rotation is not quite 3 teeth, so I cannot get a perfect measurement.

This leaves me in a bind.

This is a high compression engine, designed for daily driving. You mentioned race engines will accept a tighter tolerance, but shouldn't it be the opposite considering heat and expansion?

Any advice is appreciated.

You could have the exhaust valve pocket machined an extra 0.020 to 0.030 deep (you will lower your compression ratio less by doing this vs adding head gasket thickness). What happens if you use 2 teeth for your test, does that increase the clearance enough.? That might give you confidence to limit the exhaust cam timing.

BTW - that piston looks a bit beat up -- what have you been feeding your engine?

The reason we accept smaller clearances with race engines, is we accept the risk that we may need to rebuild / repair it with a big overrev. I ran air-cooled Porsche 2.8L 911 engines with 0.040 clearance (12.5:1 compression ratio), we had proper valve springs to avoid valve float on a minor over-rev. Engines were rebuilt probably every 100 - 200 hours.

The origin of this clearance check is due to machining of the head deck surface. Service manual states there is no machining allowed. Deck measurement before machining was 0.008" out of true, machinist seemed competent, but did not record how much material they removed from the surface.

I do not know how much 0.008" off the deck can affect exhaust valve to piston clearance, and I dont know what the unmachined clearance was either. Considering 13:1+ compression here, I imagine it was tight from factory.

This will be running OEM ecu and wiring, on a daily driver SUV, I will not be managing cam timing.

Really this is an educational experience and potentially a spare engine if I find a solution to this concern.

Unfortunately I have no history on what went through this engine, 3/4 pistons have experienced a sprinkle of "blunt force trauma" from some tough particulate. Interestingly enough, the cylinder walls were undamaged, but I believe they also got 5 valves in the process (bent).

The head itself also has witness marks of the impact(s):

[img="blob:https://www.hpacademy.com/c26956c2-e78b-47a0-ae69-72a98f1b2d4f" alt="" alt="" ]

I would think you will be fine, if less than 0.010 was removed. Good on you for checking however..

David, I appreciate your input.

I did some logging on this car and was able to determine the max angle the exhaust cam turns, math comes to ~2.7 teeth.

Re-doing the clearance check with only 2 teeth (instead of 3), I measured 0.12" P to V clearance on the exhaust.

Obviously cam profiles make it non-linear, but at least I now know the actual clearance is likely closer to the 0.10" and puts my worries at ease.

Thanks again for your assistance!

Excellent. Good on you for continuing to explore and seek more understanding!