Valve clearance changes with cam rotation

PTwo

Hello,

1992 851, just finished with some head work and valve lapping. Reassembled heads and while doing the valve clearances, I found that one horizontal exhaust valve cam base circle appears to have excessive runout. When measuring opener gap, it can vary between 0.05mm and 0.13mm, depending on rotational position of the cam (base circle portion).

Thoughts?
Can I set it tight on the closer end, around 0.12 so the other is 0.2?

*or*

is this a real problem and I need to source another cam?

EDIT: as I'm doing this on the bench, I noticed this issue. If I'd had the cam belts attached, then rotation wouldn't have been possible and I would've most likely ended up with it being too tight on a good portion of the base circle, potentially holding the valve open too long.

Has anyone else seen this issue?! Kind of scary that it could go unnoticed using the normal, installed process.

Thanks,
Peter

griff851

What is it in the centre of the base circle? 0.08/9? You don’t want any binding as you rotate the cam through a cycle, springs out. I’d set up with mid tolerances shims at lobe centre, then rotate to bind point and mark, then reverse and get bind point and mark. Grind section with bench grinder. Mark you boundaries with perpendicular line, and roll the face against the grinder wheel, then clean up the surface face with emery cloth wrapped around a square section stick of some description. Check with feeler gauges against the rocker. Repeat as necessary. If that doesn’t sound like fun or you blow it, and it’s a standard cam shaft, they’re cheap. One one eBay in the USA for USD$30 at the moment. Not real confident
Brads website diaries/ camshafts has what cam is what. http://www.bikeboy.org/duccamspec.html
O for orizontal S for saused which is close to exhaust. 😁

PTwo

Thanks Griff.

Just curious, have you heard of this before?

Good point on the grinding, thanks, hopefully the case depth is thick enough if I go that route.

Peter

PTwo

Further investigation...

I put the cam on a lathe and measured with a dial indicator. It just confirms the problem. The other lobe is almost perfect.

These pics show the extremely on-sided loading on the lobe. The other one looks much more even and less intense.

I'm really surprised it didn't chew up the follower, at least on the edge contact point.

Bad lobe on left side:

griff851

That’s not real good, although a quick survey of a few sets of standard cams I have, had one closer running off centre. Not as badly as yours however. The valve clearance previously set doesn’t look real crash hot either. Set a tad tight. The base circle from what I can see on my collection usually has blackening, indicating clearance. Pity it wasn’t an OS, I have an extra 2 for some reason.

As you’ve got a lathe, bore a hole in a piece of round stock the same as the base circle. Part off a bit as wide as the lobe, cut it in half. Wahlah template. Use bearing blue to guide grinding. You could really get into it and mount a grinder in the tool post, and rock the chuck back and forth.

Ducati cams like components of old, appear to be made of stupidly hard steel. I’m pretty sure I had a conversation yonks ago with Ivan Tye the cam bloke here, I don’t think they are surface hardened. I have a VA still tagged with a flank seriously ground into when he went looking for soft steel. 60-70% is virtually non contact in any case.

marvin

I'm sure I hae one around here someplace. I should have a 916 cam or a '92 851. Free if you pay shipping.

marvin

P.S. I just looked and I do have a pair of 851 heads that the rocker arms have been robbed out of. I assume the cams are better than yours although I have not removed the cover to look.