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Can I calculate lash pad size from change in lobe base circle?

Charlie Boy

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So short story is I’m replacing a slightly worn Schneider 300f cam with a Isky L490 so a fair jump down. My question is can I get close to calculating new lash pad size by the difference in base circle?

300f base circle is 27.9mm

L490 base circle is 31mm

cam base circle delta is 3.1mm so 1.55mm closer

current lash pad size is 6mm

is the following the correct calculation? 1.55x1.48 rocker ratio =2.294mm so new lash pad size of 3.7mm?

is it a safe bet to go between 4.45mm to 3.7mm or do I need to go lower? Wipe pattern was even on the 300f cam

I can purchase a few pad sizes just need to know the range I’m working in so if anyone more experienced can offer some advice it would be most appreciated.


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Actually, with today's CAD programs you could probably just draw out the picture above, make the changes and let the computer do the work.  I think that there are even free simple online programs out there.  Gotta sell those 3D printers.

Sometimes I do long division with paper and pen just to reawaken that old shriveled part of my brain.  I wonder if it's even taught in school anymore.

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I have modeled something up but need to measure a rocker arm when I get home. Unfortunately I'm on another continent to the car so cant get any other measurements of the valve train geometry. So other than looking at previous pictures and taking a guess it might be a bit pointless task doing that. Hopefully someone as gone through the same process and can chime in with some suggestions.


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Thanks, seems like the rocker ratio shouldn’t be used for lash pad calculations as you would always move the pivot with base circle changes which makes the 1-1.45 ratio redundant. My rockers look fairly parallel so I’m guessing I should just move the 1.55mm on the pivot and lash pad which would put me around MSA available 0.175” size. Maybe I'm missing something though.


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Funny, looking at an actual rocker arm it seems almost reasonable to just use the base circle change directly.  Because the contact pad is almost over the valve stem.  And there is some room on each end of the wear pattern for error.  Seems like you could take your 1.55 mm number and just fudge it based on the existing wear pattern, adding a little bit for the relatively small distance the lash pad is extended from the rubbing pad.


you could just stick a 1.55 mm shim under the 300f base circle and take a measurement at the lash pad.  Since it's getting bigger.  Replicate the L490.

One question for you would be whether or not the valve tips on your current setup have been ground in the past after a valve job.  Apparently the good machinists do that so that all of the lash pads are the same size.  From what I've read here on the old forum.  So Diseazd's recommendations might not work well since it's probably based on standard valve sizes.  Not an expert, I just like to talk about things.



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Thanks, no chance the valve tips have been ground down, the seats would have be machined to match. They are probably set a bit deeper as the head has 2mm cam tower spacers to compensate for high compression and chain geometry.

Anyone know what is easier on the cam, wipe pattern closer to the valve or to the pivot?

i have new rocker arms to go with the cam.




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So I model up the cams and rocker arm. Not having access to any head means I’ve had to make quite a few guesses to angles and position of the rocker arm relative to the cam center line etc.

It does seem though when I do the 1.55mm parallel move the wipe pattern always moves forward towards the valve no matter where I move things within a sensible range. I guess this makes sense as the valve and pivot move away at an angle. If I go from 0.175”(4.45mm)-0.160”(4.064mm) lash pad it recenters, if I go down to 3.7mm then it moves closer to the pivot. Anyway looks like the available 0.160” pads from MSA should work. Fingers crossed 


Edited by Charlie Boy
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