EM002 MAC ENGINE

After a very busy Spring I have at last found time to investigate the growing clatter from the top end of the MAC. As anticipated the rocker spindles are very worn. Whilst I know that I can obtain and replace the rockers I have no knowledge of the availability of the bearing block and the caps which sadly are also showing signs of wear. I am also unsure about what seems to be a very hit and miss alignment when assembling the bearing. Anyone help? - Peter Swift 28/5/2001

Try Criterion, they produce a complete replacement assembly with improved lubrication, £90, in their last catalogue. Contact them on 01793 790219. If you need rockers themselves, try Grove Classics on 01582 873066, M9/2AS, M9/4AS £24.50 +VAT. - Dai Gibbison 16/10/2001

I had the rockers refurbished by a local engineer and they are now very good. However the clatter/audible knocking, did not stop. just moved down a bit. The focus, with the bike running, seemed to be the mag. With the auto advance removed and turning the engine over by hand it was obvious that there was a very audible clonk from the camshaft as the cam followers went over the cams. With the push rods out and no load it was silent. Looking further and checking against what little information I had available I found that: 
The cam is an M17/7 not the standard M17/6
The bottom rockers are bushed (non-standard?) and had lateral play on the spindle
The bush on the inlet is eccentric and had scoops worn on the crankcase/ thrust washer face
So here's the question.
What effect might an M17/7 cam have on the performance of the engine and would a standard cam create less of a loading on what should be a well lubricated part of the engine.
Have Alloy Macs ever been fitted with bushed rockers or is this a repair?
And now having foolishly removed the timing gears to check for wear are all cam wheels marked with the same timing marks? - Peter Swift 16/3/2002.

The later MACs from engine 23193 had the M17/7 cam as standard, along with the early Viper and the springer MSS. But it does require a different profile follower from the earlier MACs - I guess the same radius as the Viper and MSS, 1.0" 
The first alloy MACs had an M17/4 cam, then the M17/5 (I don't think there was an M17/6) but both used a very different radius bottom rocker - 0.375" radius?
So you might have the later cams with the early bottom rockers, which would certainly affect performance and could cause clatter.
Lateral play also causes clatter.
I don't think the bottom rockers were bushed when new, though I have only been able to check a later set.
If their bush is eccentric enough that would add to the clatter as well as affecting performance.
All springer cam wheels are marked originally by whoever at Velocette built the engine, using either a punch or an electric scribing pencil
But of course the cams could have been pressed out and replaced in a different position.
It seems like you'd better check your valve timing anyway - the ones for M17/7 are on Dai Gibbison's tech info site (see our tech info page) under MSS. - TW 16/3/2002

I have just returned from the Newark jumble where I managed to find a complete M17/5 cam and timing gear train with cam followers in more than acceptable condition. It seems that I did have the later and larger cam followers which I now know matched the M17/7 cam. The only minor drawback now is that the intermediate gear does not mesh with the fitted crankshaft pinion and will have to be changed to the one which came with the set, sadly it does not have any visible timing marks. however the M17/5 cam wheel does mesh with the original idler gear. Is this an acceptable proposition, just to change the cam shaft and smaller cam followers? I was hoping not to have to remove the oil pump and pinion as I do not have an extractor for the latter. It's a bit like working in the dark with all the minor quirks so any help is really appreciated. - Peter Swift 17/3/2002

I've just realised there was something puzzling about your first note, when you said:
"The bottom rockers are bushed (non-standard?) and had lateral play on the spindle The bush on the inlet is eccentric and had scoops worn on the crankcase/ thrust washer face."
There should be a "belleville" thrust washer between the rockers and the crankcase, which should take up any normal amount of lateral play. I guess it might not get along with a bush and so could have worn the scoops instead?
Anyway - when you say the new intermediate pinion won't mesh with the crankshaft pinion, is it just too tight or is it the wrong helix angle?
If just tight, then that's why the idler gearwheel spindle is adjustable....
It might be a different helix - Velocette used three different angles but if you mesh them tightly on a plate you can see if they stay flat. You can test against the previous idler wheel or the camwheel or mag wheel, no need to remove the crankshaft pinion.
If the new wheels check out as the same angle helix as the old, I would mix and match. Not ideal, but not likely to be a problem on today's mileage. And you have to get the idler wheel mesh adjusted right, as per the manual - negligible backlash but not tight or it will whine. - TW

I think that you are right and that the thrust washer is the culprit for the wear, as there are corresponding marks on the washer. Checking against the drawings it was fitted the wrong way round ie convex face out. the helix angles are the same throughout the train so I think that I will take your advice and go ahead with the original crankshaft pinion and idler wheel ( the replacement idler wheel central bush is 5mm shorter on the crankcase side)and an M17/5 cam and appropriate followers. I was a little thrown when I checked the technical site and read that the Alloy MAC was fitted with a M17/6 cam. All very confusing for a 'beginner' with singles. I now have to check that the push rods are ok with this cam,check the valve timing and we should be ok, famous and optimistic last words! A busy time at work coming up so it could be a while but I will let you know how I get on. I'm still unsure how this cam will affect the performance and why veloce chose to change it ( was it just a matter of production to match in with the newer engines?) or were there problems with the older and smaller cam follower design? Was it an attempt to obtain a little more power? I'm hoping that a lower cam profile might reduce stresses on old metal and...owner! - PS 18/3/2002

I think the M17/6 cam might have been just for alcohol and/or for the US dirt-track racers. I didn't think it was fitted to a road MAC but I may well be wrong. - TW

M17/6 is a racing cam for a MAC - as difficult to find as hen's teeth! The larger radius followers keeps the valve timing the same as the smaller diameter followers, but open the valves quicker and improves the breathing. If you read Phil Irving's Tuning for Speed, this also reduces the stresses on the valve gear (which I don't fully understand).  
Shimming the cam is not recommended, as when they wear they tend to disintegrate and spread steel slithers through the engine. 
The steady plate is flexible enough to deflect a few thou when tightened, so just make sure that the spindle protrudes though proud of the cam spindle by 0.0015. 
One way to do this is to put a 0.0015 shim between the crank case and cam, heat the crank case before gently taping the spindle in flush with the cam, this should give the required clearance after the shim is removed. It worked for me! - Dai Gibbison 16/4/2002