Inliners International Forums General Discussion Hi Performance Cam gear to crank gear tolerance

Hello all, first time poster here. I have a GM 250 getting machine work done at a very reputable shop here in central Iowa. We are in the assembly stage of putting the long block together, and have run into an issue with the cam gear lash. As it sits right now, we have between 1 and 2 thou backlash using an aluminum gear for the cam. Desired clearance is 5 to 8 thou. We've had two different gears on now with no difference between the two. My machinist and I are at a standstill on what to do. We both feel that most, if not all of these aluminum cam gears come from the same plant in China, so ordering more and changing them out likely won't solve the issue. Just wonder if any of you have run into the same situation and what you did to solve it. I've even thought about using the old gear because it likely has enough "wear" the it might fall into acceptable tolerances, but have spent way too much money to this point to take a chance on a parts failure.

AFAYK has the block been line bored? Any cap replacement?

This is a likely culprit!!

Yes,it has been align bored, but my machinist never goes beyond 2 or 3 thou on the caps for this very reason. Still the original caps.

AFAIK cam gear failure is pretty rare even with high valve spring tension. Most of the drive fails I'm familiar with occurred as a result of violent RPM change (seizure, backfire), key rejection, something caught in the teeth, no oil, etc.
I think the small lash you have will be smaller at operating temperature (250° F? oil), which will tighten it farther.
Is Cloyes China?
Back in the day, a machinist would make a fixture (or use a dead engine) to run either gear with an electric motor or lathe, and lap the gears with very fine compound. This has been done many times (60 years) to reduce parasitic load on Harley-Davidson cam gears. Will the lower load per tooth but softer material on the cam gear make it wear faster? IDK
Just mentioning it, not advice!

I have a crazy thought, and I’m not a metallurgist by any means, but my machinist used the gear heating method for sliding it onto the cam. Any chance that once cooled, the gear didn’t come back to exact tolerance? Said he heated it to 375 before it would go on. We’re only talking about 3 thou difference of where we need to be.

The plan right now is to slide the old cam and gear back in and check the lash, and if it’s within tolerance, I’m going to use the old gear.

If it is within spec how do you plan to divorce said gear from the old cam? Typically gear removal techniques are for replacement not gear reuse.

Since you are entertaining crazy thoughts why not try tweaking the harder crank gear to open up the tolerances?

Also, what is the source of the .005 minimum target?

This timing gear design dates back to the stovebolt era. With fiber gears in a late forties 216 the spec is a narrow and tight .003 to .004. By '55 the spec had loosened and widened to .004 to .006 - but that is still a fiber gear: Old Car Manual Project
which may not expand much at operating temperatures.

I also agree your clearance is too tight. Set the cam up in a lathe , make a jig to hold the crank gear on the tool post, add some lapping compound and let it run. It will develop some of the needed clearance.

Have you had to do this before Tom? Is this a common issue?

It is known to happen when you line bore any block, But it shows up much more in a Gear to Gear set-up. If you can not set it up on a lathe. I have Heard (key word Heard ) Nor have I seen it done. But you can do it while the cam and the crank is in the Block. With flywheel,Starter. NOTHING else NO rods no pistons.
Main Problem I see is keeping it OILED Good. Yes it would set the wear and clearances your after. JUST not a 100% how good of a bright of a idea that is.????

This is only acceptable for very small corrections, since it only removes material from the contact points of the teeth (not the entire gear) which does not "miniaturize" the tooth profile.

The suspect 54 tooth cam gear could be compared to another by inserting a small drill shank (IDK the correct size, Harley-Davidson has used .105") in between a pair of teeth on opposite sides of the gear, and measuring the distance across the pins with a micrometer.

Did you replace the crank and cam gear at the same time? Should be a matched set.

Lapping the two gears to each other will mess up the involute tooth geometry. This is because gear teeth slide on each other everywhere but at the pitch circle. More wear will occur inside and outside while less wear at the pitch circle. That said, a little lapping is unlikely to cause a problem.

Gear teeth "roll against each other" is NOT TRUE!

Gear teeth "roll against each other" is NOT TRUE!
Thank you!

I like where this post is going. Hoping to learn something about fitment of gears cut from involute segments of a circle.

With straight cut involute gears the so called pitch circle is an imaginary circle upon which meshing gears roll against each other without slipage as depicted here:

Prior to (and after) this point of contact there is slipage - but the whole point of involute cut gears is to minimize this slipping effect - instead trying to maximize the the transmission of force from the driving gear to the driven gear along the tangent depicted by the blue arrow along the blue dotted line (with minimal wear and wasteful heat generation).

In the theoretical gear-set depicted above there is zero lash.

As I understand the engineering (and stand ready to be corrected) adding lash to the above diagram would create a gap between the valley of one gear and the tooth tip of the other gear. This leaves the pitch circle unchanged but moves the tangent away from the base of the driving gear teeth and closer to the tips of the driven gear teeth. The lash is measured cold with idea that when heated both driving and driven gears expand to roughly zero lash. And moreover both the driving and driven gears have teeth of identical specifications.

In this theoretical world it would seem that lapping cannot create lash.

So now let's move to the real world animal:

where the gears are diagonally cut - which takes part of the tangent driving force and uses it to hold the gear against a thrust face - and there are huge tolerances between gear tip and valley bottom.

Moreover, in fitting the gears the shop manual seems completely unconcerned with the distance between the tooth tip of the driving gear the valley bottom of the driven gear - instead focusing on the space along the diagonal face between gear teeth:
The Old Car Manual Project Section 6 Page 42

This suggests (IMHO) that the teeth of the driving (harder)gear and driven (softer) gear are similar but not identical. And that lapping can therefore create lash.

But in my mind the question remains - is there a better way to reduce the size of just the harder gear to obtain desired lash?

regards,
stock49

That is a nice writeup Stock49. Same thoughts I had. It is not the tip, but the lateral sides of tooth/ teeth we are concerned with. I may make a jig to perform this procedure in.
Actually line honed my first block yesterday. A junker just for practice. May start performing this on more blocks.

So bottom line is It does not or will not change the Depth? As I see this the side loads Or clearance Would change and not in a good way. Because now as you get on the gas or off you may now have back lash of the contact points. And as normal wear takes places you now have even a wider gap on the contact area,Would it not?

Something that I think MAY be being overlooked is that the camshaft and valve train assembly has little if any kinetic energy of its own. Between the limited friction of the cam bearings, the drag of a distributor, (if used) the resistance of the oil pump drive, (again if driven by the cam) and the resistance of the lifters and related components all combining to try to stop the camshaft from turning, inertial drive against the drive gear on deceleration would likely be minimal if there actually is any at all.

Except at extreme engine speeds, since there is no flywheel or drive train to drive the camshaft during deceleration, the rest of the valve train is acting as a brake and thus the crankshaft gear is STILL driving the cam gear. Bear in mind here that the camshaft NEVER actually freewheels.

I'm certain that there is some thrust due to anything that the cam might be driving, but that is MOSTLY inward, often by design. We put a cam "button" that rides against the timing cover on some high performance chain driven valve trains and there is a retainer on many gear driven cams, but the emphasis is on inward thrust and its prevention as a general rule. Many stock chain driven cams don't have ANY kind of provision to keep the shaft from moving out of the block!! The angled gear drives mostly are designed to push the camshaft inward. That "cam button" on some high performance valve trains is to keep the cam from moving at all so as to prevent damage to roller lifters on deceleration, but that's because the chain driven camshaft can slide forward because of its weight and the weight of the driven sprocket, not because of any thrust applied to the cam from the valve train or any driven pump or distributor.

Some of this was observed in Smokey Yunick's electric driven engine assembly, years ago. My little brother and I got to see that demonstration at one of the PRI shows. Little brother got to spend a lot more time with Smokey and see more of those internal workings. Wish I'd had the time to spend.

I think I would try forcing the gears together (less than zero lash) with extreme pressure lube first before using an abrasive. Seems to me this would slightly crush the machining marks in the aluminum gear and give a bit more clearance when installed. Tom's suggestion to do this in a lathe setup would work well. Switch the mesh repeatedly (simulate a "hunting tooth") to equalize and also move the gears back and forth axially a bit for the same reason. Finish with timing marks aligned.

Have not done this. If you do, please post result.

That's my thinking exactly. These timing gears are involute but they are not precisely involute. Meaning that the tangent doesn't necessarily need to traverse the entire face of the gears (as it does in the theoretical model above). And therefore the (tip-valley) depth can vary greatly based on machining variances.

It appears to me that the whole point of the lash spec in the service manual is to make sure that the 'trailing' shoulders of the gears are not being jammed together as the engine approaches operating temperature.

These gears only run in one direction so there is no reason for the trailing faces to be in contact (as they are in the theoretical model above).

Gonna sleep on this one . . .

Generally agree that the gearing will remain in contact on the drive side of the gear mesh due to constant loads on the camshaft from valvetrain friction and oil pump.

However, I would not be at all surprised if torsional vibrations of the crank allow the gearing to rattle between drive and coast side of the mesh. Likely will be witness marks on a used set of gears to confirm. More wear on drive side, less but still some wear on coast side as evidence.

IIRC Spintron showed slight but constant cycling between drive and coast especially at harmonic orders.

Given the close tolerances of the gear sets and the expansion due to heat, I would agree that there will be contact, and thus friction on the trailing faces of the gear teeth AND that harmonics will likely play a role!

Since there IS contact, there will obviously wear and evidence of it. I've been rattling these things around in my head, (a majestic cavern, uncrowded by grey matter) in moments of idle thought and my recollection of all of those times where I replaced a timing gear set and the observation of worn and damaged gears seems to come back that the driven side of the gear was always the side that was damaged. This, of course, only stands to reason, but I don't recall EVER seeing the coast side more than simply lapped smooth from contact!! It'd be harder to see or say on a gear that's been wiped!

DAMMIT!! I just threw away a used timing gear set that might have shed more light on this! It was only worn, not really damaged and likely COULD have been reused.

My next thought runs to straight cut gears!! I'm planning on using a straight cut set in my 153" 4Banger hop up. That's going to be a hydraulic roller engine!! Bigger valves, heavier springs, roller rockers! Makes me curious about similar characteristics AND different ones as well.

I still have some used sets laying around, For that matter they are STOCK GM alum an steel gear set. I know I turned one set into a Staging/Drag Tree.