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Jamesbon
06-14-2000, 02:56 PM
So........what happens if the clearance between the cam button on a roller cam is too tight or non existant with the timing cover/rear cam "freeze plug" type cap? Does the cam wear a hole through the rear cap or timing cover?

My motor's still in the garage and hasn't been installed yet, but I noticed a hump or lump in the timing cover where the cam button is supposed to have just about a few thousanths clearance. I guess you're supposed to pop that back cam plug and push the cam back and forth to measure the end play. I'd hate to do so, but after all the cleanup work I've had to do after my machinist, I probably should......what do you think?

Nathan
jamesbon@ij.net
or
nschlaeppi@templeton.com

AVickers
06-14-2000, 03:19 PM
The rear cam cap is NOT a cam locating/clearance item. Just be sure that it clears the end of the installed cam and that it's sealed up real good, or you have an oil leak that will require engine removal to fix.

The cam is forward-located by the button and backside-located by the rear face of the sproket riding on the boss on front face of the block. In fact, in many cases, the engines don't have a cam button -- resistance from turning the oil pump against the angle-cut gears on the cam draw the cam back against the boss on the front of the block...

GeneD
06-15-2000, 07:24 AM
I've always wondered why the button was used.
I mean, like, you can't go any further back than the chain sprocket, and the distributer keeps it from going forward.
It can't have anything to do with being a roller cam. The lifters don't keep the cam in place. Why doesn't a regular cam walk forward?
The new GM roller cams have a keeper bolted into the front of the motor, on either side of the cam boss.
Interesting. Are we missing something?

------------------
GeneD
007
Melbourne, Florida

[This message has been edited by GeneD (edited 06-15-2000).]

AVickers
06-15-2000, 08:23 AM
Two theories:

1) Engines with a distributer and non-roller lifters: Without a keeper, the forward motion of the cam is contained by the tension of the cam chain. As the chain stretches over time, the cam can move farther and farther forward when not being held back by the resistance of the angle-cut gears that drive the distributer and oil pump. (Another factor that might tend to hold the cam back against the front boss is that the engine is designed so that the centerline of the cam lobe doesn't line up with the axial center of the lifter. This tends to rotate the lifters as part of distributing wear across valve-train parts. The vector forces that go with this rotation may also work to pull the cam back where it "belongs.")

Anyway, the times the cam could move forward are when the throttle is snapped shut and the cam walks forward as the crank & timing chain slow down more quickly than the cam & oil pump & distributer assembly, thus the cam would walk forward slightly (to the extent that slack in the timing chain allows) as it walks up on its angle-cut gears. This would cause a momentary out-of-time situation with the cam -- as the chain slacks up as well as with the distributer as the gear position changes relative to the position of the crankshaft. From a performance point-of-view, this is no big deal since it occurs under decelleration, but decelleration is a time when engines spit out bad stuff, so this may be an issue in our more environmentally sensitive times... Plus, a cam button was one more thing that the hotrod shop could sell you. {I do have a boat with an engine that rotates bassackwards and the cam button on that engine is there to hold the cam in place just like the the boss holds it in place on engines that turn the other way...the distributer gear on this engine tends to push the cam forward.)

2) The other theory is simpler... Roller lifters require more precise cam lobe location and any forward-backward motion is to be avoided 'cause it causes problems with the rollers...

rayjay
06-15-2000, 08:59 AM
Roller lifters do not offset or bevel the cam lobes as you are trying to keep the lifters from twisting. Therefore the cam is free to walk back and forth to its limits. The button helps to maintain this walk to tolerable limits for the cam, cam drive, and distributor gear. Hot rodders, just like boaters and truckers, stress engine pieces more than the ordinary person. The button, or some other cam locating device, is a good idea for us to put in with non-roller lifters. This will help prevent wear and tear on the cam and distributer drives from sudden accelerations and decelerations, like having the prop coming clear of the water and then re-entering. rj

[This message has been edited by rayjay (edited 06-15-2000).]

[This message has been edited by rayjay (edited 06-15-2000).]

Jamesbon
06-15-2000, 11:17 AM
Fellas,
Your advice is priceless!!! I guess no clearance between the cam button and rear face of the cam sprocket might cause minor friction between the block face and rear face of the cam sprocket....hmmmmm. It's probably worth removing the timing cover and adding a few thousandths clearance, or possibly using a thicker timing cover gasket to achieve the add'l. clearance. So, if I'm in jail Monday, it's because I was so pissed off about doing clean up work after the candy ass machinist who assembled my engine, that I punched the F**K out of him. (sorry, had to vent, I really have better things to worry about...like getting my DONZI back in the water.)

I've always been under the impression that "regular cam lobes" are machined at a slight angle when viewed from the side, hence the pressure of the lifter keeps the cam from walking out or riding against the timing cover and is also the cause of "rotating lifters."

Nathan (Jamesbon)
Still motorless in St. Pete

AVickers
06-15-2000, 11:19 AM
Cool...

GEOO
06-15-2000, 02:12 PM
Nathan, Don't feel too bad your not alone, I'm still motorless!! http://206.150.187.82/ubb/frown.gif Goodluck this weekend. GEOO

BillG
06-15-2000, 05:20 PM
The angle on the cam along with the convex lifter face is to make the lifter rotate to cause an even wear pattern. The roller cams do not have an angle, because they don't have to rotate.