Don Terrill’s Speed-Talk

Mocking up for piston to valve clearance on a new combination. For various reasons, the 3.275" stroke crankshaft I will be using will not available for some time. I have a 3.1" stroke crank I can use. The question is will the longer stroke crank change the p/v clearance appreciably? Both will end up with the piston 0.015" in the hole.
Thanks,
Don

No. As long as you can simulate the minimum clearance crank angles (usually ~ 5° before and after TDC for the exhaust and intake respectively), any difference would be in about the fifth decimal.

MadBill wrote: ↑Sun Apr 01, 2018 10:54 pm No. As long as you can simulate the minimum clearance crank angles (usually ~ 5° before and after TDC for the exhaust and intake respectively), any difference would be in about the fifth decimal.

Thanks Bill, I didn't think that small of a stroke change would mean much, but it's always good to hear from one with much more experience than I have.

Mock it up as you were going to with the 3.100" stroke crank and take .087" off your observed clearances ,as the pistons are presumably already machined with valve radial clearance that the engine will see @ -.015" deck or whatever proposed deck ,the angular clearance error will be minimal.

If the radial clearances are not known ,and part of your mock up is to confirm that and other all physical clearances maybe it would be better to check with what crank is in the final assy.

Cheers.

donclark wrote: ↑Sun Apr 01, 2018 12:47 pm... Both will end up with the piston 0.015" in the hole....

So no adjustment for piston/deck spec is required.

Are there new -.087" compression height pistons to go with the proposed 3.275" crank to give the same -.015" deck?. Or are you running a shorter rod to accommodate the stroke gain? or machining piston crowns to suit?

Cheers.

Times 2 to the above question.

The new pistons will be 0.200" shorter comp height and the rods will actually be a bit longer then the current combination. Rod/stroke ratio was 1.75, new will be 1.7

hoodeng wrote: ↑Mon Apr 02, 2018 11:46 pm Mock it up as you were going to with the 3.100" stroke crank and take .087" off your observed clearances ,as the pistons are presumably already machined with valve radial clearance that the engine will see @ -.015" deck or whatever proposed deck ,the angular clearance error will be minimal.

If the radial clearances are not known ,and part of your mock up is to confirm that and other all physical clearances maybe it would be better to check with what crank is in the final assy.

Cheers.

The radial clearance will actually change more than you might think if you're just dropping the piston .087 because the piston is travelling vertically and the valve is travelling at it's angle, presumably 23 degrees. It's the same as flat milling the head .087 because the valve gets moved further back towards the piston's valve relief and the radial clearance gets tighter. You can verify this distance by having the piston mocked up .087 in the hole, drop a centerpunch ( or an old valve ground to a point) down the guide to the piston, push the rod and piston up until it stops, remove the head and you will see the scratch on the piston indicating how much the valve centerline has moved. All this to say it's better to check with your final parts and I agree that the difference between the two strokes will be minimal.

If you have Pipemax you can have it calculate piston demand from the table and it will show you how far down the hole the piston is at each degree. rough numbers in your case are 0.002 at 10 degrees and less than 0.001 at 5degrees different with the two strokes. Rod length makes a slight difference. Mostly you can ignore the stroke difference, IMO. There are probably other online calculators that will give piston position per degree- ? Stan Weiss or Wallace formulas? I didn't look but those are likely sites.

DCal wrote: ↑Tue Apr 03, 2018 3:34 pm...
The radial clearance will actually change more than you might think if you're just dropping the piston .087...

Again, as above, there is no 0.087" involved! The OP states the piston to deck for test purposes will be 0.015", no matter which crank/piston/rod he's using.

I think getting back to square one might be handy , doing a mock up with a range of core parts that will not be used again ,ie ,pistons ,rods and crank won't give the information on clearancing that is being sought .Best wait till all the final assy parts are at hand and go from there.


There is nothing worse than having swarfe on the bed instead of still in the job.

Cheers.

donclark wrote: ↑Sun Apr 01, 2018 12:47 pm Mocking up for piston to valve clearance on a new combination. For various reasons, the 3.275" stroke crankshaft I will be using will not available for some time. I have a 3.1" stroke crank I can use. The question is will the longer stroke crank change the p/v clearance appreciably? Both will end up with the piston 0.015" in the hole.
Thanks,
Don

Finally got all the new parts and assembled this dog. P/V clearance ended up at 0.090" on the intakes and 0.130" on the exhausts, pretty much what I determined from the mock-up. Got on the track this weekend, ran back to back 5.38 @127 with 1.15 60' times, 352" SBF with C3 heads in a 3050 lb brick. Gotta love it when stuff actually works out.

Good deal! I wish I could say the same on our outing..... 13 years on a T400 is max!LOL

Mr. Clark is that a NA engine, give more details.....that is carrying the mail, video maybe , would love to see a pass. Mark H.