ET: Engine (Twin) Large Idler Boss/Shaft Assembly

[MichaelOrgzey]

Hi everybody!
I would be interested if there is an actual difference between ET 51/1 and ET 51/2?
I assume so, otherwise there wouldn't be a difference in number. Is there any technical development or are they both equally fit for the job? I'm switching from an alloy idler to steel, so would be handy to know if it makes sense to swap the boss as well. Since a didn't have the timing cover off at the moment I don't Know which boss I have at the moment but I do know that the idler is alloy. Bike is a B series Rapide.
Cheers and merry Christmas everybody. Michael

 

[timetraveller]

I've just checked and although the ET51/1 is shown in the Series D parts list only the ET51/2 is listed on the Spares Company site. It might be the only one available. If when you get inside the bike you have the two pieces spindle, that is alloy triangular back and steel shaft, then definitely change it for the later one piece steel component.

 

[Sten Jensen]

Actually a 'one piece spindle' I bought many years ago turned out to be 'two piece' but both made of steel and probably with a tighter fit than original. I guess this is strong enough for most uses.

 

[Cyborg]

I bought one quite recently and it is steel, but 2 piece. Bought one for the Comet maybe 3 years ago and it was 2 piece as well.

 

[davidd]

Maughan makes a two-piece steel item that has never had a problem, to my knowledge. In the Comet engine, the large Idler spindle, which is one of the two pieces. is also used without the mounting base as a small idler spindle sitting to the right of the large idler spindle (running the ATD). So, the same spindle is used for the large and small idlers in the Comet, one with the base, and one without the base as the spindle is mounted directly in the case.

I suspect that this design caused the after-market parts suppliers to be very motivated to make the twin two-piece item work properly because it simplified the manufacturing process for the Comet small idler.

Years ago I began saving the old Series D large idler mounts as they were true one-piece items, but the Maughn-supplied items never failed, so I stopped worrying.

The two-piece items that had an aluminum base always separated, which probably prompted the Factory to change to the Series D one-piece style mount.

David

 

[royrobertson]

The original alloy casting that supports the idler shaft can be like "swiss cheese" inside but look ok and if this fails the damage can be terrible. I once built an engine for someone and this happened. In my racer I kept the shaft and pressed it into a titanium backplate. Like all the titanium studs these have stood the test of serious use since 2000.
roy the racer

 

[Cyborg]

That must turn the entire contents of the timing chest into chutney.…. which in turn has the valves trying to do the same with the pistons. Can’t imagine the toll that would take on my colon.
#deathrattle #moribund

 

[fogrider]

I was most interested in a very recent discussion elsewhere in these pages re the setting of the spindle heights. Someone stated the shoulder of the spindles should be ( either side of ) .420". I looked it up, correct. But checking mine I realised that with the ET173 thick thrust washer under the outrigger plate, that set the total height at the large idler spindle on mine to .450". ET173 is .070" which would mean the idler spindle shoulder needs to be at .350". Attention is required to No1 inlet valve follower spindle as well as no1 cam spindle. The question - what height to set them all to is becoming rather vexed. The others are at .420".

Bop the large idler spindle in to .350" ? Or has my decaying brain got something very wrong here ?
(I bear in mind allowances still required for shims and end float).

 

[davidd]

Bop the large idler spindle in to .350" ? Or has my decaying brain got something very wrong here ?

I am from the school that believes you do not have any choice when setting spindle heights. I will stick with the Comet for this discussion, but it applies equally to the twin.

There are two different sets of spindles. The two Idler spindles, large and small, and then the bank of four narrow spindles at the top of the timing chest. If you want to discuss spindle heights to the shoulder of the spindle, each of these two groups will have a different spindle shoulder height. So, the spindle shoulder height is the same within each group, but different from each other. This is because the shims that go under the steady plate are different, ET173 for the idler spindles and ET98/1 for the four narrow spindles.

Unfortunately, one spindle will not move! The large idler spindle cannot be moved down as it is bolted to the crankcase. This means that when you place the ET173 on the shoulder of the large idler spindle and measure down from the mating surface, that number you measure will be the "under the steady plate" number for all spindles because you cannot go any lower. But, once you know the under the steady plate number, that is the number that all the spindles and including their specific spacers will rise to so the steady plate will be flat.

I believe this is a good approach because it should minimize the number of shims needed during assembly. It will also keep the steady plate flat, which is how it should sit. It should also prevent the rubber washers in the timing chest cover from being squeezed too much.

David

 

[chankly bore]

I am from the school that believes you do not have any choice when setting spindle heights. I will stick with the Comet for this discussion, but it applies equally to the twin.

There are two different sets of spindles. The two Idler spindles, large and small, and then the bank of four narrow spindles at the top of the timing chest. If you want to discuss spindle heights to the shoulder of the spindle, each of these two groups will have a different spindle shoulder height. So, the spindle shoulder height is the same within each group, but different from each other. This is because the shims that go under the steady plate are different, FT173 for the idler spindles and ET98/1 for the four narrow spindles.

Unfortunately, one spindle will not move! The large idler spindle cannot be moved down as it is bolted to the crankcase. This means that when you place the FT173 on the shoulder of the large idler spindle and measure down from the mating surface, that number you measure will be the "under the steady plate" number for all spindles because you cannot go any lower. But, once you know the under the steady plate number, that is the number that all the spindles and including their specific spacers will rise to so the steady plate will be flat.

I believe this is a good approach because it should minimize the number of shims needed during assembly. It will also keep the steady plate flat, which is how it should sit. It should also prevent the rubber washers in the timing chest cover from being squeezed too much.

David

Yup, David. You've got two heights which should be fixed, as I've said before; the ET 162 at .447" and therefore the idler boss assembly plus the thickness of ET173 also coming to .447". If you're lucky enough to own a Comet, add in ET51 plus ET173, same total. Then all you have to do, assuming the timing case wall is not worn and all components including the other shims, hopefully all at .025" thickness, are correctly made, is cut out a bit of aluminium (or aluminum) beer can to make a .005" spacer, heat to 200°C locally and tap in the spindles. Problems arise when the ET173 shims are the wrong thickness or because cams and followers are not made right and don't track properly. Never tap the end threads on spindles, they are often overhardened and brittle.