Model 18 Cam Follower Profile

Thanks Jon for highlighting those, I hadn't spotted them. They are not an exact match for mine, not shown on my photo is a socket on the upper face which locates the end of the pushrod. Would have been good if the seller had shown a side view to compare the profiles, but I could request that.

I am fairly sure that no-one on the Forum is going to come up with a Norton factory drawing of the cam profile - rocking horse poo would be easier to find. I wonder though if photos, or better still, a tracing of the profile of a good follower might be sufficient for Newman to refurbish mine with a fair degree of confidence. I would understand if someone was reluctant to let an original good spare out of their sight. Just for curiosity I wired a strip of 0.008" shim steel over the worn area of the exhaust follower. It stayed in place long enough to confirm that the valve timing then came very close to the expected values.

Cheers, Ian McD

I think when everything is new the contact between cam and follower is a line, with the cam always touching the follower at the same point. As wear takes place the line slowly gets wider, with the initial contact point moving towards the follower pivot. This advances the valve open point, while the width of the worn area maintains contact with the cam longer and delays the closing. I think your suggestion of trying to mirror the existing unworn radius will be the most practical approach. My test with the shim gives a fair indication of how much material needs to be added. I had hoped that Newman's past experience would have given them a good feel for how to proceed. Probably just being cautious, a known template being better than an educated guess. With hindsight I could have made up a template with my own approximation of finished profile before sending them away.

Thanks Jon but maybe I should say why I have sent the cam followers for refurbishment.

I am trying to resolve a valve timing anomaly which may or may not be responsible for reduced power from the Model 18. The open durations of both valves are longer than they should be, by approx 30 degrees for the exhaust, 20 degrees for the inlet. Thinking there was something odd about the cams I swapped them for another set. It made little or no difference to the timing figures. In "Tuning for Speed" Chapter 11 Phil Irving describes how wear on radiused cam followers can extend the valve open duration (as described in an earlier post). As an experiment I wired a strip of .008" shim to the exhaust follower, covering the worn area. The shim stayed in place long enough to check the timing figures over 1 cycle. The exhaust values were restored almost to maintenance book values. From that I think it will be worth having the followers restored to original profile, or at least a close approximation.

Cheers, Ian McD

Like Jon (I suspect) I am pleasantly surprised by this suggestion. How difficult would it be to search the archive - has someone compiled an index/database or would it be a physical trawl through a stack of drawings? It would be very useful if a copy of the original drawing was to turn up but I am preparing for disappointment!

Cheers, Ian McD

Much appreciated, certainly worth the try. I will ask Newman to make an educated guess based on the profile of the unworn side of the follower.

Cheers, Ian McD

I have spoken to Newmans this morning and given them the 15/32" figure. Ken must have had my follower somewhere close by as he was able to say that mine is nearer 1/2" but not a constant radius. He will do as we discussed, build up the worn area then regrind to a close estimate of the original profile.

Richard's blueprint highlights another possible area of confusion. The part numbers on the blueprint of the later engine are the same as the earlier engines eg 9046 is the cam follower/rocker for the early 30s sidevalves and for the later engine shown above. There must have been several versions of the original detail drawings, plus information either on the drawing or in a separate document indicating the model year or engine number to which they apply. One less worry for us as we don't have the drawings anyway.

Thanks again for your help, hopefully the reprofiled followers will bring the valve timing of my engine back nearer to spec.

Cheers, Ian McD

After a longer than expected delay the cams and followers came back from Newmans and look good. Valve timing has been restored to book values. In the meantime I discovered that the valve guides also needed replacing and that further delayed the rebuild. Just this week the Model 18 went back on the road and is running so much nicer than before. Performance is crisper and once at cruising speed the throttle can be backed off to maintain that speed. Previously larger throttle openings were required just to maintain progress and it felt like there was little in reserve. A big improvement, definitely worth having the work done.

Thanks Ian, I'm surprised that such an article included dimensioned drawings. I will look more closely at old copies of The Motorcycle when they come my way.