10 replies to this topic

[Keith Novak]

The GCR lists the new rule as: The cylinders may be bored .010” over...

The Mazda parts catalog lists the OS rings as 0.25 and 0.5. Is Mazda listing the oversize in hundredths so the rings are either 1/4 of .01 over or 1/2 of .01 over making the 2nd OS pistons the ones allowed provided you meet the correct weight?

[dstevens]

The MSM listing is in millimeters. A .010" is about .254 mm. A .50 mm is about a .020". The oversized pistons will meet weight because they will be slightly larger than the stock pistons.

[Keith Novak]

Aha...mm makes sense. Thanks!

I wasn't referring to the piston weight though. OS rings with stock pistons. Pretty sure that's the only way it's legal The OS rule includes my car has to go from 2350 to 2365 with an alternate bore.

[dstevens]

If you are going even 10 thou over you're going to want to run over pistons. While the rules don't explicitly state over pistons, it's standard procedure to oversize on a bore, even at 10 thou. That's why for your 95 they make a BPY1-11-SB0A.

[Keith Novak]

Aha again. Never done OS rings before and it shows.

[davew]

I actually did the math a while back. According to the numbers listed in the GCR, 0.006" overbore requires the extra 15 pounds. 0.015" overbore is the maximum limit. So there is a built in 0.005" wear/fudge factor.

Dave

[dstevens]

That's good info. Here's a shot from when the block was honed last year (hard to believe it's been a year...)



bore by Dave Stevens, on Flickr

According to the FSM the bore is supposed to be 78 mm, I'd reckon for that measurement I didn't hit the apexes of the cylinder. I did use a telescoping tee to check the relationship within the depth of the bore. Plus I knew when I sent it to my guy, he'd measure with better accuracy. Dan at Fultz Machine rocks. For the money, the Mitutoyo caliper is a great value and much more accurate than an HF.

[Justin Baltrucki]

3 prong bore mic's are pretty good if you can find someone with th right size. 2 point mics are tough to get just right

[dstevens]

Indeed. Shortly after that pic on the advice of my machinist I got a set of telescoping gauges. Much better that a caliper alone. I use the measurements to get a rough idea and if there is a question I send it off to the pros.

[Protech Racing]

Standard Piston in the bore with one piece of shiny newpaper will give .0025-003 PTW value, about the largest number that wont have to add 15#.
You do need to have it honed to that size, as it will never wear that much.

[Jim Drago]

Standard Piston in the bore with one piece of shiny newpaper will give .0025-003 PTW value, about the largest number that wont have to add 15#.
You do need to have it honed to that size, as it will never wear that much.

It wont?