Piston clearance with Yoshi cams?

[Guest]

G'day all,

Currently doing a resurrection of an old GS1000; you can read about the project thus far here, if you're interested.

Anyway, the good news is that the cams I got for the thing have turned out to be Yoshimura cams. Not sure what 'Stage' they are. They don't have a lumpy look, but they do provide extra lift:

Standard intake cam, overall lobe height..... 36.34mm (approx.)
My Yoshi intake cam................................ 37.16mm
Standard exhaust cam, overall lobe height... 35.79mm (approx.)
My Yoshi exhaust cam.............................. 36.56mm

So as you can see from these figures, the intake cam provides approx. an extra 0.82mm of lift, and the exhaust cam provides an extra 0.77mm of lift.

Question: Will this extra lift mean that my valves now risk clipping the crowns of my standard pistons? Or have I got enough room to spare?

The reason I ask is because a recently acquired Yoshi advert from Oct. '81 edition of Cycle World states:

For proper valve/piston clearance, Yoshimura camshafts should always be used with Yoshimura pistons.

Is this really the case, or was that the Yoshi sales dept. just trying to make sure that people bought more stuff than they really needed?

So if anyone has Yoshi cams with standard pistons, let me know what you think.

Cheers,
Mike.

[GregT]

The motor currently in our race GS1000 is using bigger cams than that - a lot bigger - with uncut GS1100G pistons. No clearance problems on 107 - 109 lobe centers. The 1100G pistons have pretty much the same cutaways as the std 1000 pistons.
You should be OK but check it anyway.

[gearhead13]

Whether or not your valves will hit the pistons has much more to do with where you degree the cams than the lift.

[bbjumper]

Got this from Eric....

He has one and dealt with this issue specifically, It runs fantastic and is scary fast.


"Check clearance with clay or silly putty to be positive.With stock low-compression pistons/reliefs, they should clear.
I opened up my valve reliefs .040" to be safe. Giving them .025" clearance.
Factory minimum is/was .010" IIRC.
But, I used custom-made Manley 2mm larger intake valves and had to get the reliefs cut wider anyway. "

[tfb]

Well thanks for those useful replies there folks! It seems I'm going to have to do a 'dry build' and put a bit of putty across the pistons to see what the clearance is in practice. And yes, how the cams are degreed will make a big difference... but I will just try setting things up as stock as possible, and take things from there.

One final question: what might be the valve-shim clearances required for these GS1000 Yoshi cams? I know that most aftermarket camshaft manufacturers call for wider clearances with their cams... but does anyone have the recommended clearances on hand?

Cheers again,
Mike.

[rapidray]

You DON'T have to clay the pistons or disassemble anything to do it the way a lot of us check clearances. Put a dial indicator on the top of the bucket & pry it down to measure the clearances. Much faster & MORE ACCURATE than measuring the thickness of clay. Ray.

[GregT]

Ray's right of course - and we've discussed this here previously.

I'd run .005in inlet and .007in exhaust. That's what I've just shimmed a street bike head to which is running stage 2 Yosh cams - and a mint set of 73mm Yosh pistons too.....A lucky find.

[Big Jay]

If it has stock valve springs, shoot for .080" exhaust and .065" intake.