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Cam Sprocket Autopsy 

Thanks to David Deines - updated  9/8/01


I am now in receipt of Don Harold's camshaft. Or the 'collective camshaft' as it were. Remember, I have very little 'hands on' knowledge of these things but I will offer my observations.

1. The bearing on the outside of the failed sprocket has a groove about 1/32" deep. Does anyone know if that is supposed to be there?

2. The sprocket moves about 1/16" side to side between the 'nut' on the inside and the race on the outside. I understand where the clicking comes from now. I can see the splines are still intact on the tube, and cannot turn the sprocket more than about 1/16" by hand.

3. ALL of the bearing surfaces have a raised land in the center of the surface and score marks all over. It takes a fingernail to tell that there is a difference, but it is easily seen.

4. This thing is DENSE! Frankly with all the talk about how low weight it should be I was surprised. I expected this to weigh half what it does. I have held hi-performance cams for 350 chebbies, and they only weigh about twice this.

5. The cam lobes look as clean as a baby's bottom. No marks, no wear. Don's car had almost 100,000 miles on it.

I'm going to have my father-in-law take a look at this, then I'm sending it to Kirk for his examination. The last time I spent any quality time inside a motor was in 1981 with a 352 Thunderbird Y-block.


'98 TR
$1000 piece of pipe in hand

PS: No signs of overheating. All the lobes and bearing surfaces look normal, no blueing.

Groove in bearing may be a an oil groove? Paul Nimz - does yours have one?

Tim Wright

What's interesting is I would have expected strictly radial wear/grooving as the sprocket attempted to turn on the shaft. Once it let go I could then see it threading, corkscrewing or whatever. Seems that there is some linear exertion being placed on the sprocket. WRT to the wear factor. Don't forget that the lobe mat'l and shaft mat'l are and have to be different. Yamaha could make the lobe as hard as req'd to negate wear without anything else to worry about. The shaft had to be designed and heat-treated keeping in mind that a deformation/expansion process had to take place to secure the lobes and sprocket in place. Therefore the mat'l selection and the degree of hardness probably was not optimum but satisfactory in their minds. If the shaft was hardened to the same degree as the lobes, say 65 RC, then it would be impossible to put it through the expansion/assembly process. It would probably just explode like a piece of porcelain. I wonder why they didn't heat treat the entire assembly after installation. Probably warpage, parts moving etc. It would be real interesting to find out the hardness values are for the individual parts. That would go along way in determining the logic that was employed during the process.

When you rock it back and forth, is there more play at one end of the sprocket relative to the other. I know that is not an easy question to answer. But you might notice more of a gap between the sprocket and shaft say at the end with the sprocket located directly over top vs. the end that has the hub only. Just wondering if having the sprocket being offset on the hub is causing a small amount side force/load. Out of curiosity, where are you guys located.


John Hamilton

The cam you have is the "exhaust" cam, on the left or front head. It has not yet totally failed. I caught it in time. The way the cam sprocket is loose, tell you why I could hear a ringing sound coming from the front of the engine. I thought it was a timing chain, but I now believe the noise was the cam sprocket starting to go. As David noted, the cam lobes look like new for an engine with 105,000 miles on it. I was not impressed with the cam journals though. They look a little rough, like an oiling problem, but I don't think there was a "bearing" like a main bearing on a crank. Can any of the rest of your verify that. this surprised me...It was like the cam rested right on the head casting and was restrained by the top clamp ??? Anyway, other problems existed and, timing chain, guides, tensioners on timing chain as well as the tensioners on the slave chains were replaced. All other cams were tight and did not need replacing. Cost....1671.00 on parts....22.5 hrs labor. I now feel good about the top end of this engine. When do the rings go ???? on a 105,000m engine.....can I go another 100,000m now ??

Don Harrold 


Hardness can be determined at any time using a hardness tester. Most basically look like a small drill press. It has an interchangeable "penetrator" instead of a drill chuck. The part or location on a part is cleaned if req'd, then placed under in the machine on an anvil just under the penetrator. Rockwell "C" (RC) is the most common scale used for general purposes. For RC a 120deg. conical diamond penetrator is inserted in the machine. The part is brought up to meet the penetrator by means of adjusting/raising the table. The table is then raised a bit more until the penetrator has just started to penetrate the part and the weight scale on the front of the machine reads that 10kg of preload is now being applied. A handle is flipped and 150kg of weight is applied to the penetrator further pushing it in to the part. The load is removed after the needle stops moving indicating full penetration allowed, downward movement has stopped. Voila, whatever the dial indicates is the hardness of your part (depth that diamond has penetrated). There are even portable hand held units. We have some at work, but their accuracy, repeatability is not that good.

From what very little I know about motors, a regular camshaft is hardened to say 55-60 RC, then ground to final dimension. At this time we don't know anything about our camshaft wrt hardness or mat'l composition. I suspect that the shaft is still relatively soft, which would account for the marks on the shaft bearing surfaces. These marks on a normal "hard" shaft would not be present unless there was a problem. The reason I think the shaft is still soft is the assembly process used. From what I can gather, all parts were made individually and then assembled (not a revelation I know). Lobes were probably hardened to req'd hardness that all cam lobes require 55-60 RC? This hardness is obviously required for wear, but also req'd because in the swaging process one part has to be sacrificial, or soft and the other hard. Same goes for the sprocket. It had to be hard wrt wear, so it might as well be hard for the assembly process as well. That leaves the shaft. The shaft has to be somewhat soft for two reasons. First, to allow the cam and sprocket spline to bite into it in the swaging operation. Second, to remain soft (ductile) enough to allow it to be expanded during the assembly operation and keep it's integrity wrt cracking, flaking, shrinking, profile

This is speculative on how they assembled the parts. A fixture (jig) was made to accurately locate all the components in a very rigid manner. An anvil was hydraulically driven up through the shaft thereby expanding the shaft and causing the interference fit at the lobes and sprocket. If hardness testing revealed that the shaft was say >50 RC, one would assume that the assembly was subjected to heat treatment after assembly. I would think that to be a risky affair, warpage, part movement etc. They would almost surely have to re-grind the assembly after such a procedure. So I bet it wasn't done, Yamaha trading off the premature wear of the shaft bearing surfaces to allow them to do this process.

It would still be nice to know though. If the shaft is indeed hard, then wear marks would not be good. If it is still soft, it would account for the wear marks. And if there is an oiling problem which again I'm not in any way discounting, then the wear would be even more premature. Maybe this could be adding unforeseen friction/load on turning the camshaft. That load would be seen at the sprocket.

So why did they use this process to make the camshaft? Money? Does it allow for higher RPM's? Did Yammy feel like experimenting with an engine they would not be held accountable for?

It would be nice to hear from a gearhead as to why they think or know what the advantages of this style of cam are.
If it's strictly a production/cost related shortcut, I'm going to be really, really pissed!

Sorry for the major ramble, just thinking out loud. I was only going to talk about hardness testing. But in true brainstorming, all concepts are laid out on the table. There could very well be someone sitting back taking all this in and connect all the dots for us.

Hammy, caffeine level now pegged!

John Hamilton
'97 ES
90k km
Multi-ported Airbox

What exactly failed?

Do the little teeth fracture? Or does the bond between the teeth and tube fail?

How tall are the teeth?

If the teeth are very hard and brittle, and can't take any motion (rocking caused by the oscillating loads) then welding them may not be the best fix? If the teeth fracture in shear then weld will have to connect tube to the lobe above the teeth.

Are there little bits of teeth still embedded on the tube?

Anybody catch what I am trying to ask?

Tim Wright


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