Measuring Cam Wear

baz · May 12, 2023

As a rule I'd always replace the the rings and I agree with the above
I don't like disturbing the rings
But I have also read that the rings rotate in use?
It's something I have never thought to check
I always set the rings so the gaps are 120°
So the next time I strip one of my engines I can easily check

Stephen Hill · May 17, 2023

I discovered the timing gears were not assembled correctly. The dot on the timing pinion (the small one) does not line up with the dot on the larger intermediate gear at tdc. The intermediate gear is rotated one tooth anti-clockwise, which best I can figure will advance the camshaft.

The number of links between the two sprockets is standard at 10. I am pretty sure the cam is a 2S.

I can't find any situation where the misaligned dots would be done intentionally.

I am chasing a high load mid range and above misfiring issue and am running down a few theories, such as low compression, worn cam, and cam timing.

Any thoughts on how the misaligned gears will affect how the bike runs?

mean gene · May 17, 2023

You may be looking to far. I have an Atlas that had a low speed misfire as it turned out , after I rebuilt engine, it was the timing/point lobe worn. I set the gap a little wider and the miss went away. Just a thought or another thing to check

Fast Eddie · May 17, 2023

I don’t think cam wear, or timing (within reason), would cause the kind of misfire you describe.

Sparks or fuel is my bet.

Stephen Hill · May 17, 2023

I have already ruled out fuel and ignition.
And I just found that the timing gears were not installed properly. Might be part of the problem.

I also know I have a low compression issue. Next I will dismantle the head and barrels.

Fast Eddie · May 17, 2023

Advancing the cam is a thing that folk do, especially with the sporty cams. IIRC it helps improve the mid range.

ntst8 · May 18, 2023

I'm guessing you have done this but it is worth a reality check on the dots. A different beast but when playing with my now sold 16H i had to mis-align the dots to get the timing right.

Stephen Hill · May 18, 2023

The NOC has some details on advancing the 2S cam to improve the midrange, pasted below. Quite different from just moving the intermediate gear one tooth counterclockwise.

I am not sure what effect the one tooth move produces, as in how much advance. It could explain why with the 2S cam at tdc I should get .157 intake lift and instead I measure .178. And I should get .144 exhaust lift and instead I measure .200. Those are big differences. I will retime to stock settings and measure how it changes these numbers.

The 2S and 3S cams may be advanced up to 3o to restore some lead and presumably result in a less peaky power curve. Advance by 5o as above, then file a tiny step on the camshaft key to turn the sprocket 1o anticlockwise. The net advance is then 3o.

worntorn · May 18, 2023

Fast Eddie said:
Advancing the cam is a thing that folk do, especially with the sporty cams. IIRC it helps improve the mid range.

Ludwig posted an interesting chart that showed the effects of advancing and retarding a cam.
From that, you are correct, advancing the cam adds some midrange at the expense of top end.
You wonder if the end result of an advanced performance cam is just about the same as back to stock power, except with some added valve train wear?

Glen

Stephen Hill · May 19, 2023

According to the Mick Hemmings engine build video, cam lift at TDC is the critical dimension when it comes to valve timing.

At TDC for a 2S cam inlet valve lift should measure .157"
Dots misaligned as found (timing to intermediate) .178"
Dots aligned .135"
Timing advanced (see below).141"

At TDC for a 2S cam exhaust valve lift should measure .144"
Dots misaligned as found (timing to intermediate) .200"
Dots aligned .165"
Timing advanced (see below) .142

Timing advanced by rotating the cam sprocket one tooth clockwise and the intermediate gear three teeth anticlockwise.

I am not sure why the exhaust is so close (.002') and the inlet is off by .016".
I am guessing this is about as good as it gets.
Comments?

Fast Eddie · May 19, 2023

With .200” lift @ TDC I am thinking you may have had valve to piston interference. I’d check for bent valves and pushrods. A bent valve not seating correctly could be your loss of compression issue.

Looking at your figures you need to do some more juggling IMO so you have .010-.015” more on the inlet vs the exhaust.

If you have to compromise, compromise the exhaust, the inlet figure is more critical than exhaust, at least that’s what I was always told.

If the head is off I’d be pulling the barrels to check things, in particular the condition of the cam and followers, if they’re worn, you’ll never get the figures right.

Stephen Hill · May 19, 2023

Barrels come off today, which will give me a chance to look at cam and lifter wear.

Previously I took johnm's advice and clocked the cam in 5 degree increments.
It showed max intake lift at .388 (not TDC) and max exhaust lift at .338 (not TDC).
These numbers look very similar to Dyndo Daves cam profile for a 2S. This was done on the left hand cylinder only.

I will do the right hand cylinder before I pull the barrels.

johnm · May 19, 2023

I don't know the data for the 2 S cam. I tried to look for the instructions Mike Hemmings sent me for the PW3 cam but no success as yet.

Hence my question. Does your hemmings cam sheet give the inlet lift at TDC measured from the pushrod on the cam or at the inlet valve itself. There is an approximate 13% increase in lift at the valve due to the rocker ratio. 15 years plus down the road I can't remember if he specifies at the valve or off the cam.

You said above

"At TDC for a 2S cam inlet valve lift should measure .157"
Dots misaligned as found (timing to intermediate) .178""

How many degrees is this estimating from the lift diagramme you made. It does look pretty advanced.

For a street bike I would install the cam exactly as Hemmings says for inlet lift at TDC. With the lift curve you have measured you can estimate what a 5 degree advance or retard of the cam will give you in terms of lift.

For a race bike I might try 5 degree crankshaft advance for the cam. You can definitely try remeshing the gears and chain to do this. Or else I understand RGM sell cam sprockets with offset keyways which seems a way better way of adjusting it than filing keys. Don't use a vernier for other than short term testing. They are not reliable longterm.

I have more info in my files but pretty busy at present and it will take time to dig it all out sorry.

Atlas Commando · May 19, 2023

Coming back to assessing the wear of a cam, I have seen a Norton cam that had lobes worn slightly on an angle. It appeared the followers would reach the expected max, but maybe 10-15% of the lobe on one side was worn down. How such a wear pattern could happen is beyond me, but I recommend lifting the barrels for a proper look at the wear surfaces.

Stephen Hill · May 19, 2023

Barrels are off. No appreciable wear visible on the lobes. However, since the lifters are not ground square, I think it's fair to say the two surfaces have worn together. Which of course means when I clean up the spotty lifter faces and make them square, the non square cam will need to wear into a square surface. Not ideal, but what can you do. Not to mention that the lifters were installed backwards (chamfer facing rear) so they can't go back on the same lobe. Not that it matters if I am dressing them.

johnm · May 20, 2023

Stephen Hill said:
Barrels are off. No appreciable wear visible on the lobes. However, since the lifters are not ground square, I think it's fair to say the two surfaces have worn together. Which of course means when I clean up the spotty lifter faces and make them square, the non square cam will need to wear into a square surface. Not ideal, but what can you do. Not to mention that the lifters were installed backwards (chamfer facing rear) so they can't go back on the same lobe. Not that it matters if I am dressing them.

Well. I talked to my friendly local engineer who has spent a lot of time in Norton motors and he said. " mmmm. Well it's all a matter of degree. " so not a very definitive answer but as he said you would need to measure carefully to see just how much a problem there really was.

Firstly he said it's not too difficult to get the facing of the followers wrong and get an angle on them. So that probably means someone has been here before.

If it's not too bad and it's only a short run weekend bike you could just put it back together again.

On the other hand refacing the followers, installing them the right way round probably should have a new cam.

He said the 2S cam on a combat is a reasonably high wear area. He recommended careful inspection of rocker geometry and how the rocker works on the valve. It can be poor even on original bikes.

SteveA · May 21, 2023

baz said:
As a rule I'd always replace the the rings and I agree with the above
I don't like disturbing the rings
But I have also read that the rings rotate in use?
It's something I have never thought to check
I always set the rings so the gaps are 120°
So the next time I strip one of my engines I can easily check

Rings rotate....think about it....2 stroke engines have pegs in the piston ring grooves....why? To stop the rings rotating, which is important in a 2 stroke so that the ring ends don't get caught in the ports, but, it just confirms that they do rotate.

Rings in a 4 stroke however may stop rotating when the ring grooves are gummed up, that isn't good either.

Whilst measuring the ring gap doesn't tell you what the original gap was, it simply tells you if the current gap exceeds the defined tolerance, if it does:

1 The bore has worn
2 The ring has worn
3 Both the bore and the ring have worn
4 The ring gap was not set correctly at the time of the ring installation, for reasons of incorrect ring or bore, or other general incompetence

In most cases, the third of these is going to be the case, no matter how much we may be inclined to think it's the fourth. If we didn't do or watch, and record the installation figures, we are guessing.

But if the measurements are still within tolerance, why change things? That decision should be based on a review of the condition of everything, not just the ring gap, but also on your future plans for the engine! If you are planning a round the world trip, or racing, or you are just nit picky, then of course you want to start from the best possible base and have the required funds.

Reference to aviation practice is completely irrelevant, because the risk factor and environment are different. Your bike is unlikely to fall out of the sky, and you are unlikely to be prosecuted for incorrect maintenance practice. In aviation, you will be able to refer to a legally required service record, completed by aviation authority licenced staff, to establish running hours and condition and parts source. You know who did what and when, with what and to what standard.

Not so in the world of motorcycles. Never trust the last guy to touch it! Especially if that was you!

worntorn · May 21, 2023

SteveA said:
Rings rotate....think about it....2 stroke engines have pegs in the piston ring grooves....why? To stop the rings rotating, which is important in a 2 stroke so that the ring ends don't get caught in the ports, but, it just confirms that they do rotate.

Rings in a 4 stroke however may stop rotating when the ring grooves are gummed up, that isn't good either.

Whilst measuring the ring gap doesn't tell you what the original gap was, it simply tells you if the current gap exceeds the defined tolerance, if it does:

1 The bore has worn
2 The ring has worn
3 Both the bore and the ring have worn
4 The ring gap was not set correctly at the time of the ring installation, for reasons of incorrect ring or bore, or other general incompetence

In most cases, the third of these is going to be the case, no matter how much we may be inclined to think it's the fourth. If we didn't do or watch, and record the installation figures, we are guessing.

But if the measurements are still within tolerance, why change things? That decision should be based on a review of the condition of everything, not just the ring gap, but also on your future plans for the engine! If you are planning a round the world trip, or racing, or you are just nit picky, then of course you want to start from the best possible base and have the required funds.

Reference to aviation practice is completely irrelevant, because the risk factor and environment are different. Your bike is unlikely to fall out of the sky, and you are unlikely to be prosecuted for incorrect maintenance practice. In aviation, you will be able to refer to a legally required service record, completed by aviation authority licenced staff, to establish running hours and condition and parts source. You know who did what and when, with what and to what standard.

Not so in the world of motorcycles. Never trust the last guy to touch it! Especially if that was you!

I believe the aircraft info is relevant as what's bad practice for one engine is bad for all engines.
Interesting side note, on the Cessna forum many of the Cessna engines are showing the same type of premature cam and follower wear problem that Jim Comstock and others have had with their Norton engines. The Cessna owners are muddling around, looking at oils, talking Zddp. Some have been using the Aeroshell oil that failed so miserably in Jim's test.

As mentioned, my own experience with removing and then reassembling used pistons and rings in bores without hone and re-ring has shown that there is something to the belief that well seated rings are best left undisturbed , if possible. I did quite a lot of small motorcycle engine rebuilding at one time.
I just mention this in passing. Of course everyone is free to do things any way they wish!

Glen

Stephen Hill · Jun 9, 2023

Thought I would close the loop on the cam wear/valve timing adjustment thread.
After confirming the cam is a 2S, concluding there is no significant lobe wear, dressing the lifters, relacing a worn timing chain, and adjusting the cam timing the best I can (the timing gears were installed incorrectly before), here is what I ended up with:

Intake valve lift at TDC is .167". Factory spec is .157"
Exhaust valve lift at TDC is .125". Factory spec is .144"

I am advanced over stock specs, but that should return a better midrange. And my intake is .020 over exhaust, which apparently is a good thing. I could jack around with a half key, but don't see the point.

The bike started as a combat, and the head was swapped to a RH6S, which is 9.5 to 1 vs 10 to 1 for a Combat. Intended use is street only.

Any comments would be appreciated.

Measuring Cam Wear

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