Recommissioning- those blasted tappets and lengthening a pushrod!

The bike is back together now, but before I try to fire it up I wanted to check the tappets. These use the eccentric cam system and I've never done these before. I have two Jubilees queued in the wings and these use the same system so I'd better get used to it! The principle here is that turning an eccentric cam within the rocker shaft moves the shaft up and down and thus moves the tappet closer or farther from the valve stem head. In order to keep tabs on where the shaft is there is a flat on the adjuster



Rockers inside the box. The rockers consist of a hollow rocker shaft visible here and through which is passed an eccentrically mounted cam on a solid internal shaft. There are washers on each end of the shaft to allow movement without wearing the ally rocker box case.



This is the internal shaft removed from the hollow rockers which could then be lifted out. Note the cams which move the rocker


The eccentric shaft screws into the back of the rocker box and is usually prevented from rotating by a lock nut but this has been removed here.



The other end of the eccentric shaft carries a slot so it can be turned and a flat to monitor how far it's been rotated with reference to the cam position. If everything works out perfectly then when correct rocker clearance is achieved the flats should face each other across the rocker box as shown here at app. 3 and 9 O'clock

There is a nice video here.

The trouble is all the sources I have found just reproduce the same stuff from the manual and no-one really tries to expand or explain it: For instance, the books all say that the flats should be in the vertical position facing each other before starting... Well they won't be will they? They will be in whatever position they were last set? So should they be turned vertical first and if so in which direction? Does it matter?

Secondly the sources stress that the rockers should be adjusted by turning them inwards -towards each other- they don't say this but it would be clearer to say "turn the inlet adjuster clockwise and the exhaust adjuster anticlockwise" ... if indeed that's what they mean! When adjusted the flats should be still in the half sectors facing towards each other across the rocker box- an ideal adjustment being 3 O'clock on the inlet and 9 O'clock on the exhaust for instance.... unless you have an early motor ( before C25 2050) in which case this is all reversed... turn the adjusters the other way and ideal adjustment is then 9 O'clock on the inlet and 3 O'clock on the exhaust... that's not confusing at all is it??? (Luckily, the early rocker boxes carry a spring washer under the eccentric shaft lock nut and further, the early rocker boxes have an internal strengthening rib in the centre of the "horse-shoe shape which is missing or machined out of the later type). Mine is a B25 motor and the number is higher than this, it has no such spring washer and no internal rib so all signs point to it being of the later type,

Obviously as you turn the shaft you will cause the shaft to advance into the threads on the far side of the rocker box. Eventually you will run out of thread and the shaft will be screwed tight into the box. At this point you need to turn the shaft back 360 degrees to restore movement. Although this will restore rotation, does it affect the amount of adjustment possible?

OK here is what I have gleaned from taking the rockers apart: The eccentric does indeed move the rocker shaft (and thus the tappets) up and down... BUT it also moves it to and fro; towards and away from the cylinder centre. When the flats are facing one another and within 180 degrees then both rocker shafts have been moved inwards toward the cylinder centre- and I am guessing this is when the tappet pads make best contact with the valve stem heads. The movement isn't large and I'm not convinced that it would make much of a difference, but I suppose "best is best". The requirement to have the flats vertically at the start is to make sure that the shafts are in this stage of their rotation, I presume just in case there is so little clearance that its not possible to turn the shafts back through the vertical position to make the adjustment if you are on the wrong side: Well this is getting to my experience, and I can say that so far its my opinion that there is fat chance of that! In my experience there is so much clearance that you can always turn the shaft a full 360 degrees to get the flats on whatever side you want so this initial setting doesn't seem important.



I'm trying to find out how much movement is provided by the cam. Here the flat on the adjuster is fully outwards and I marked a line before...



Turning the flat through 180 degrees to be fully inwards. The shaft has moved over by about 1mm

Testing with a scribed line (and also with calipers) suggested that the cam provides about 1mm (25 thou) of movement. This then is my problem: I was able to successfully set the exhaust at 10 thou- but the inlet was far too wide- best I could do was 18 thou and this was with the flat fully downwards!



Best settings I could get: Exhaust (right) is just about OK but angled more down that I'd like but the inlet (left) is horizontal at the bottom- this corresponds to max adjustment and it was still nearly double the clearance needed.

I had hoped that the rocker shaft/rocker cam might be tapered so that as you screwed the rocker cam shaft into the rocker box it would increase the amount of adjustment possible by one revolution. Sadly this seems not to be the case and the rocker shaft seems to be straight bored, you get the same (inadequate) amount of adjustment in whatever position the shaft happens to be across the box.

A little research showed that several others have reported difficulty in getting the right clearance on other forums. Although others have posted such a problem, I cant find anyone posting the solution! In my case its not a large discrepancy, the motor will run, although its difficult to start, runs poorly and only at rich mixtures (possibly as the inlet valve isn't open for long enough) and the tappets sound awful- but it does run. The bike was running before although it did have noisy tappets even then- I haven't touched carburation or timing and I still have very decent compression at 145 psi.

So suggested causes for this problem are :

1. Too thick a gasket at cylinder base or below rocker box- I have fitted only the paper ones so presumably this isn't the explanation. I have fitted a new head gasket but again I believe this is the correct thickness.

2. Too short a valve stem- I have fitted GS valves and although I didn't measure them, they should be OK. Of course they have been in storage for years and so could conceivably have got mixed up- I hope not as this would mean stripping everything down. Note head seat erosion or re-cutting should decrease tappet clearances not increase them.

3. Wear in rocker valve operating pads. Certainly there was some but I didn't think it would be this bad and it wasn't obviously worse on the inlet vs the exhaust.

4. Incorrect push rods. The push rods are not of equal length in this motor and it seems that BSA changed the push rod length a few times. They could be simply wrong- or possibly switched as the inlet and exhaust push rods aren't the same length. I'm certain I put them back the way they were, but I didn't measure them while they were out (now I wish I had). The exhaust rod was still marked red, however it does have the look of a home-made red mark. I guess I can't be sure a PO didn't switch them and mark the wrong one?

However, if the push rods were switched then this would only make sense if the inlet rod is longer than the exhaust; so that a shorter exhaust rod would then give excess clearance if incorrectly fitted to the inlet valve, but the longer inlet rod would require less adjustment to seat correctly on the exhaust. After that its a question of finding what the lengths should be- and here I hit a problem- the only reference I can find is in the Rupert Ratio book which incidentally gives quite a lot of detail about the eccentric - in more ways than one- adjusting system. This gives three entries that might be relevant; the C25 motor (both up to 2050 and after engine no 2050)... and for this the exhaust is actually longer so swapping wouldn't account for the problem. However he also lists the C25/B25 (meaning what as distinct from the C25 above??) which he says, has had several different push rod length during its production. To find the right one (from scratch?) he suggests starting (?) with an inlet rod of 6 15/16" and an exhaust rod of 6 7/8" - i.e. an inlet rod 1/16" (62 thou) longer than the exhaust so the theory lives on! There are other bikes listed but the C25/B25 the only case of exhaust being shorter than the inlet.... is this an error I wonder? i have also never come across the concept of choosing your own push rods but I guess it depends on how much bespoke engineering you require.

Plan-

1. Try to swap push rods- it may be possible to do this without removing the head if I can find a way of levering the valves down to provide enough clearance to detach the rods from the balls on the rockers.

2. Fit a longer rod for the inlet. This can be done by lengthening the existing rod by inserting a metal shim below the steel cap or cutting down a longer rod- e.g. from a C15. I'm not sure how you work out just how much longer you need to make the rod though. Rather than ruin mine, and since I am on a learning curve, I have ordered a used B25 inlet rod to experiment on.

3. Fit a lash cap (valve stem cap) this goes over the top of the valve stem and reduces clearance. I have found these made for 5/16 valves by CCM Britain ltd although RGM Norton also do them. They are obviously not std for the B25 so I will test them for goodness of fit and effectiveness in clearance reduction.

OK so much for the worry- time to do something. I was unwilling to take everything off but in fact if you remove the nuts the rocker box can be raised sufficiently to allow the push rods to be removed.



raise the rocker cover and...



... the push rods can be finagled out.

I measured the rods as best I could (steel ruler-) Obviously this isn't as accurate as the daft values given by BSA but it was clear that the exhaust was actually shorter than the inlet rod by about 1mm at 174mm in length. Swapping the rods cannot therefore be the explanation.



Measuring the push rods.



The ends of both rods have taken a bit of a hammering over the years, and although neither was bent (rolled well on a flat surface) its possible that wear at the end has maybe shortened the rods. According to Rupert Ratio's book (and converting the daft fractions to mm) then the rods should be; Inlet 176.21mm, exhaust 174.61 mm. It thus appears that both my rods are around 0.5-1.2mm too short, (although I can't measure them to that accuracy!). This fits with the max adjustment in the eccentric being about 1mm as well and explains why it has run out in the case of my inlet which seems to need about 1.2mm. Mr Ratio also says that the rods can be lengthened by inserting a shim below the steel end cap.- of course to do this you need to know what thickness shim which presumably should be in the range above... and how to get the end caps off the rods!

The exhaust rod end cap turned out to be only a loose fit! I'm pleased to have discovered that because I hadn't noticed during my initial reassembly and I'm sure its not right. At the least cap movement would make the bike noisier than it needs to be and at worst could come off during use!!



Exhaust push rod end cap just slid off in my hand!

Sadly its the inlet rod I'm more concerned about ans this was a really tight fit. It wouldn't pull or tap off so I reasoned that if it had been loose like the exhaust, then perhaps it had been Loctited on. I therefore tried heating it as Loctite will only let go when hot... no joy, if anything the fit got tighter. Finally, I put the rod in the freezer for 30 mins and then returned it to the soft jawed vice- the cap no knocked off easily so I guess this is the way to go.

Given the reasoning above- that the movement in the adjuster is about 1mm, the rods are 1.2 and 0.5 mm too short and that even max adjustment doesn't generate a small enough clearance, I reasoned that I could extend the inlet rod by about 1mm and the exhaust by say 0.5. This was actually a moot point because I only had mild steel sheet at 0.75mm (22G) so that was going to have to do for both. Mr Ratio doesn't say what sort of metal should be used as a shim but I am hoping this will last a decent time. Perhaps a small (0.5mm) diameter hardened washer would be better?



My metal 0.75 mm thick

I cut a 0.5 cm square of the metal with tin snips and then ground this down to a rough disc shape that fitted easily into the rod end cap



Roughly ground disc, its an easy fit in the end cap.



Obviously I need this to be flat so I tried to remove any burrs with a fine file and then hammered it onto the anvil section of the vice before slipping it back inside the cap and slipping that onto the push rod. The inlet needed to go back into the freezer before the cap would fit back on.



Inlet rod extended. Before I removed the end cap I had scribed a line on the rod. This is the refitted cap with internal shim and the original scribe mark is clearly visible; the rod is lengthened by 0.75 mm or so.

Having modified both rods I test fitted them, I wasn't altogether sure that the exhaust rod needed a shim at all and I didn't want to Loctite it in until I knew it was going to work. Once again make sure you have actually engaged the rods in the cam followers at the base of the pushrod tube- harder to do than it sounds, before lowering the rocker box fully to engage the ball ends of the rockers..



I should have thought of this earlier- a mag pick up tool lets you position the rods easily beneath the ball ends of the rocker shaft and fitting was a doddle compared with the last time.

I tightened down the box using half of the nuts and checked clearances. This time I was able to set both valves at the desired clearance and with the flats in the inner section and facing each other. One point to note though is the desirability of having the end caps firmly fitted to the rods. As I removed the exhaust rod to Loctite it, it simply fell apart and the cap and small shim plunged down the push rod tube.



What no cap! Shock horror! The cap has vanished.



A few moments of panic later, as I recalled that the push rod tube isn't fully open to the crankcase at its base, I was able to fish both the cap and shim out again using the magnetic pick up tool.

 

I reassembled the exhaust rod, coating the base of the shim and the end of the rod in green Loctite 603 retaining compound. This is a pretty good "stick" but I left the rod for 24 hrs to be sure it had gone off before reassembling. So final refit tomorrow. I hope the box gasket will still do its job as I haven't got another. I have been careful not to disturb it and I remain hopeful that I can avoid yet another strip down. I am hoping that the lash caps will arrive tomorrow. Although I don't need these now I still want to assess their suitability as a solution before I put everything back together.

Note added- Put the bike back together again- remember to check pushrods engaged in cam followers- mine weren't (...again!), but eventually sorted, tappets adjusted nicely and I was able to run the bike enough to warm it up and adjust the carburation- seems to be idling nicely now if maybe a little fast, so all good.

The lash caps arrived too late for me to check them but they look like a simpler solution to this problem although they decrease valve clearance by 40 thou (just over 1mm) so might need to be ground a little depending on the amount of excess clearance encountered.

I had to revisit the tappets using a different approach to setting the clearances. In fact this seemed to work better so see my next posting "Jobs I hadn't expected to do again: tappets".