A Question on Tappet Clearances for the Gurus

[rustybronco]

Isn't the base circle of the cam lobe going to be ground the same radius all the way round until the contour changes when it comes to the cam lobe or ramp?

It's very possible the base circle it isn't completely round. that may the reason, or part of the reason, for your noticing the .1mm difference in clearance.

[7981GS]

You have a learning opportunity here Daniel; your checking method is wrong for the reasons explained. Beyond this point, the choice is yours.

Whatever you guys choose to believe is fine with me.
Have any of you actually talked with a camshaft manufacturer and asked them how camshafts are made?
Didn't think so.

Daniel

[hampshirehog]

Whatever you guys choose to believe is fine with me.
Have any of you actually talked with a camshaft manufacturer and asked them how camshafts are made?

Yes.

But that's irrelevant. It's got sod all to do with how the cams are made, it's to do with clearances on the cam journals and how adjacent valve spring pressures can distort the readings. Try it when you next set your valve clearances - you'll see that Mr Suzuki was right.

I think max clearance on the cam caps is .15mm and Don's photo appears to show .1mm of this (not really as you have geometry that comes in to play, I can't be bothered to do the maths but it does show it in action). Bugger - I think I can't remember how to do the maths anyway.

[TheCafeKid]

Whatever you guys choose to believe is fine with me.
Have any of you actually talked with a camshaft manufacturer and asked them how camshafts are made?
Didn't think so.

Daniel

Actually there are more than a couple ways different manufacturers make cam shafts. Hell, Pops Yosh used to make his own or re-shape them himself with a file. I don't understand how that's relevant to this conversation, however. It's simple physics. If there is spring pressure on the adjacent lobe, its going to push up on the shaft. This may be less noticeable on the 16v motors because they have more cam caps holding them down, but unless the clearance between the cam and the cap is zero, which would be bad, it's got at least, however minimal, some room to move. I dont understand why this is an argument?

[Nessism]

Whatever you guys choose to believe is fine with me.
Have any of you actually talked with a camshaft manufacturer and asked them how camshafts are made?
Didn't think so.

Daniel

Daniel,
I think you are the one that doesn't know very much about camshafts. No worry though, I'll be glad to teach you.

In a fantasy world, with a one lobe camshaft, you could position the cam lobe over the bucket in a wide range of positions and as long as the base circle is positioned over the bucket that would be fine for measuring the lash. The "base circle" is truly a circle thus the lash won't change regardless if the lobe was pointing up, forward, or back; as long as the base circle is positioned over the bucket you will get the same lash measurement. In the case of a 4 cylinder GS Suzuki though, we don't have the luxury of multiple acceptable camshaft lobe positions.

The camshaft has clearance all around the journal bearings, from .04-.07mm, so the camshaft can float around a fair bit in this clearance. Suzuki's specified inspection method places adjacent valves (such as E1 & E2) on their base circles at the same time, so neither valve spring is pushing up on the camshaft and skewing it in the journal clearance. When the cams are properly positioned this way, the the most accurate lash reading can be made and the valves should be adjusted together in pairs.

If some unknowing person miss positions the camshafts, with one of the adjacent valves pushing upward on the camshafts skewing it in the journal clearance, a false reading will be taken. For example, suppose a valve has .05mm lash, but someone mispositions the adjacent lobe thus skewing the cam in the journal clearance.05mm, the valve will measure as .10mm instead of the correct .05mm.

Is the light starting to come on now? Hopefully yes.

[tkent02]

For example, suppose a valve has .05mm lash, but someone mispositions the adjacent lobe thus skewing the cam in the journal clearance.05mm, the valve will measure as .10mm instead of the correct .05mm.

And if you then change shims to get that .10mm clearance down to the middle of the correct range at say .05mm, you have moved the true clearance to approximately zero.
Which would be bad.

[pete]

Daniel,
I think you are the one that doesn't know very much about camshafts. No worry though, I'll be glad to teach you.

In a fantasy world, with a one lobe camshaft, you could position the cam lobe over the bucket in a wide range of positions and as long as the base circle is positioned over the bucket that would be fine for measuring the lash. The "base circle" is truly a circle thus the lash won't change regardless if the lobe was pointing up, forward, or back; as long as the base circle is positioned over the bucket you will get the same lash measurement. In the case of a 4 cylinder GS Suzuki though, we don't have the luxury of multiple acceptable camshaft lobe positions.

The camshaft has clearance all around the journal bearings, from .04-.07mm, so the camshaft can float around a fair bit in this clearance. Suzuki's specified inspection method places adjacent valves (such as E1 & E2) on their base circles at the same time, so neither valve spring is pushing up on the camshaft and skewing it in the journal clearance. When the cams are properly positioned this way, the the most accurate lash reading can be made and the valves should be adjusted together in pairs.

If some unknowing person miss positions the camshafts, with one of the adjacent valves pushing upward on the camshafts skewing it in the journal clearance, a false reading will be taken. For example, suppose a valve has .05mm lash, but someone mispositions the adjacent lobe thus skewing the cam in the journal clearance.05mm, the valve will measure as .10mm instead of the correct .05mm.

Is the light starting to come on now? Hopefully yes.

Thanks Ed, I thought what you and Tom said earlier was nice and clear to me, but seeing that highlighted with the diagram makes my little brain take it in a lot clearer now.