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Replacing alloy idler with steel

youngjohn

Well Known and Active Forum Website User
Non-VOC Member
I'm going to replace my twins alloy idler with a nice new steel one. Any advice or tips on the easiest and most painless way to do this would be most appreciated. Do I need to retime the valves and order half a dozen crank gears of different sizes or are there good tricks to simplify things?
Thanks.
 

timetraveller

Well Known and Active Forum Website User
VOC Member
Remove spark plugs so that there is no compression and then remove all tappet adjusters so that there is no tendency for the engine to be turned by valve spring pressure. You might have to rotate the engine to remove some of these. Then mark both the cam wheels and the breather and magneto/distributor drive gears with a marker pen and continue these lines onto the crankcase face so that you know exactly where these gears lined up. You can then remove the ali idler and replace with the steel one. If there is now too much or too little clearance between the large idler and the half time pinion you will have to replace that. If so ensure that the new one goes in so that the teeth line up and one of the key ways lines up with the slot in the mainshaft. Nothing should move. If sod's law applies then you will find that there is no position whereby the half time pinion can simulaneously line up with the teeth and the slot in the mainshaft. If so then make the absolutely minimum rotation of the engine. It should be no more than plus or minus one and a half degrees'
 

b'knighted

Well Known and Active Forum Website User
VOC Member
stopping accidental rotation

You may need to rotate the engine, as Norman says, to take the load off the pushrods. If you do that by selecting top gear and rotating the rear wheel you can then lock the crankshaft in a stationary position by winding one of the brake adjusters up tight.

Cheers,
 

A-BCD

Well Known and Active Forum Website User
VOC Member
So you have no slack at all in primary and rear chains, and no backlash at all in your gearbox ?? !!!
 

vibrac

Well Known and Active Forum Website User
VOC Member
and you may need an extractor for the small idler

One lump of steel flat 3 2ba clearance holes one 1/2 bsf tapped hole a few screws and a bolt jobs done (and dont put a point on the 1/2 bolt)-
 

Bazlerker

Well Known and Active Forum Website User
VOC Member
It took me 5 or 6 different pinions to get the right size. Go slow- its a very worthwhile task. Measure the backlash at various places while you rotate the engine- not all idlers are dead round.
 

bmetcalf

Well Known and Active Forum Website User
VOC Member
I'm sure you have read in the books that the large idler spindle is adjustable, so you can make your new gear mesh properly with the cam wheels. It is unlikely to be identical to the old one.
 

Howard

Well Known and Active Forum Website User
VOC Member
I'm sure you have read in the books that the large idler spindle is adjustable, so you can make your new gear mesh properly with the cam wheels. It is unlikely to be identical to the old one.

Good point Bruce. It's easy to forget when you don't do these things often. If the old idler is worn, the new one might not fit without slackening the spindle nuts first.

H
 

wmg73141

Well Known and Active Forum Website User
Non-VOC Member
One reason why the involute form is almost universally chosen for gear teeth is that it is to a degree tolerant of small inaccuracies in pitch centres. That is not saying that it is entirely acceptable, just that mechanically it’ll still work OK. In a perfect world with perfect centres and tooth profiles the contact faces of the teeth will have a true rolling contact thus promoting minimum wear, constant velocity transmission and less noise.

In post #6 it is stated: “not all idlers are dead round”. Well if it ‘aint send the b****r back ‘cos it is N.B.G.! A properly cut gear will be absolutely concentric with the centre hole and the correct tooth profile is is built into the calculations when the gear is cut.

The old Toolmaker’s adage that; “The time spent making a jig is never wasted” is true here, it would be well worth the Club considering making such a jig and hiring it out so that members can the idler shaft perfectly positioned.

The correct centre distance between crankshaft and idler is 3.781” (96,04mm) and between idler and camshaft 3.031” (76,99mm).

However the expansion of the crankcase should not be ignored, if the crankcase gets to around 60°C then the crankshaft/idler centres will increase by 0.005” and the idler/camshaft distance by 0.004” when using an alloy idler that is mitigated somewhat as it too will “grow” but a steel idler will expand by considerably less than that. Also regardless of material for several reasons the idler is unlikely to attain the temperature of the crankcase.

A council of perfection? Well maybe, but it is worth considering the customary engine operating temperature and modify centre distances to suit.

For those who don’t want all that faffing about it is worth noting that the correct backlash for 16DP gear teeth is 0.0098” (0,025mm). Make that 0.009” (0,023mm) to allow for crankcase expansion and if feelers of that size will fit snugly between the meshing teeth the centres won’t be far wrong.

Finally, worn or not there is only one correct centre distance . . .
 

bmetcalf

Well Known and Active Forum Website User
VOC Member
I'm not questioning the distances, but what is your source, out of curiousity? The Works drawings I assume would specify it.
 
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