It's here! The RFVC Piston Compression Height Chart!

[zrxer]

If ANYBODY WANTS A NEW IN BOX HRC XR628 PISTON SEND ME A PM

PM sent.

[elsalvadorXR6]

dang...jealous!

[valvesrule]

If ANYBODY WANTS A NEW IN BOX HRC XR628 PISTON SEND ME A PM

Maybe we could get a compession height AND dome height?

[oldxr1]

I will dig it out and attempt some accurate measurements

[oldxr1]

About the hrc 628 piston-its got a 30.25mm compression height.There is a 4.5 mm wide squish band then a 1.5 mm dome.the valve reliefs are cut very deep

Edited April 13, 2012 by oldxr1

[brianhare]

pics please ? or it never happened ?

B

[zrxer]

pics please ? or it never happened ?

B

Brian, I'm buying this piston from oldxr. If he can't post pics, I will when I receive it.

[HeadTrauma]

I'd like even a spent HRC piston so I could measure the actual dome volume. Knowing the valve relief/dish volume of all the other pistons would be a good addition to the chart too.

[oldxr1]

Piston Porn

[valvesrule]

About the hrc 628 piston-its got a 30.25mm compression height.There is a 4.5 mm wide squish band then a 1.5 mm dome.the valve reliefs are cut very deep

Thanx Bud! Nice to have the information even though the HRC piston is likely no longer available. These dimensions could help a builder figure out his motor combinations much better. HT, I've started to make up a cc plate a few times and got diverted every time. I'm more concerned with an accurate combustion chamber volume than anything else. With the somewhat vague specs we have so far I decided to use the XL600 head on my 641 build. It hauls ass, starts easy and doesn't have any indication of detonation. I've got it jetted rich for now cause I always start there and go down. Now that it's running though I regret not using the XR500 head with the small chamber. Oh well, another teardown. Edited April 14, 2012 by valvesrule

[valvesrule]

Piston Porn

Yea, that's got to be getting a bunch closer to 10.5/11-1. Edited April 14, 2012 by valvesrule

[brianhare]

Brian, I'm buying this piston from oldxr. If he can't post pics, I will when I receive it.

Cool.thanks Bruce,i`d like to see some pics of one.hey bud,,i`ve got a rebuild thread on advrider if you want some reading,,good for a laugh anyway,,,,,,i went a little nuts this year ?

B

[brianhare]

Piston Porn

Interesting.........looks like a forged slug.........or am i wrong..usually a cast piston is easily ditinguished...

This has been highly machined unlike the stock cast piston for sure,,nice piece of alloy right there ?

?

B

[HeadTrauma]

The HRC piston is indeed forged. It's not so easy to tell from the outside. The underside, however, has telltale forging marks.

It also appears to be a thinner forging than other aftermarket pistons.

[HeadTrauma]

That was nice of them to put it right on the spec sheet. What I did is just change the labeling for the JE to include the Arias since they effectively are the same height.

Right Click > Save As.

I'm just moving this most recent chart to the last page to make finding it more convenient....and to bump the thread.

[brianhare]

I should have taken some measurements of the 11.25:1 J.E. XR650L 102mm piston i just installed in my XRL huh.......never thought of it till now ?

B

[Page Mohler]

I should have taken some measurements of the 11.25:1 J.E. XR650L 102mm piston i just installed in my XRL huh.......never thought of it till now ?

B

Running something above 91 octane correct?

[brianhare]

No..........i`m running 91.......but it`s not 11.25:1 as advertised...

B

[heart_of_darkness]

Piston Porn

Some aftermarket pistons for the smaller XR engines have that distinct domed shape. I've wondered why it wan't used on the 600s.

The smaller XR aftermarket pistons seem to also run high compression ratios (or at least claimed ratios). Maybe the smaller combustion chambers have fewer detonation problems.

[heart_of_darkness]

Have the measurements on the chart been double checked? The 600 pistons go from 9:1 to 10.5:1 with a change of 0.08mm height. The 650 pistons go from 8.6:1 to 10.5:1 in a change of 2.123mm height. I realize there are other variables and that comression ratio change isn't perfectly linear, but those numbers seem widely different.