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Cub Chain Problem

old bones

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Has anyone had the same problem as me when it comes to fitting a 13T g/box sprocket and a 420 chain, The chain will not sit down on the sprocket as it contacts with the seal and bearing housing.The crankcases are twin roller bearing and the only difference I can see from the old ones that got damaged is that they have two seals on the g/box sprocket while the old ones had only one, 

This makes the sprocket too close to the bearing housing which would cause the chain to wear away the seal lip.

Anyone have any thoughts ?

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Many thanks for all your replies, I have what I think is a Talon stepped sprocket with apart number of TG402   13T (420). If anyone has one that is not fitted to their bike could they measure it and see if it compares to the one I have?

Mine is as follows:

The distance between the sprocket face and the boss that goes through the seals towards the bearing is 16.5mm with a 1.2mm recess where the sprocket nut sits with the part number stamped there.

Does this tally with the off-set sprocket measurements?

It beats me how someone would make different sprockets and give them the same part number but non the less that could be the problem, I even emailed Talon for the measurements but they did not reply.

Anyway, thank you again gents your advice is much appreciated.

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Alan, thanks for the reply, I have no problem getting the socket on the nut to take it on or off, its just that if the sprocket I have is indeed the off-set one then I will not be able to use it as the chain will wear away the lip that keeps the seal in. The original cases were damaged by the chain which was 428 because the bearing moved and the bearing housing was worn away to the extent that the seal failed and was so badly damaged that replacing the seal was not possible ,so, I want to avoid the same thing happening so thought that I would go down a chain size on the new cases to avoid this problem not knowing I would have a problem of the same kind.

This is so frustrating, I bet if I go back to the 428 size or bigger say 520 the diameter of the sprocket could well be a lot bigger and not be any where near the bearing housing.

Perhaps you or someone could advise as to weather going up to a larger chain but still keeping the 13 G/box sprocket is viable?

Your thoughts appreciated or anyone else who has an opinion also appreciated.

Edited by old bones
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420 and 428 are the same pitch so the sprockets will be the same diameter. i think you will struggle to find a 520 front sprocket. i have not come across a twin seal casing, is there enough room to space the sprocket out further without the chain rubbing on the back of the primary chain case ?

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I use the 13t 420 gear box sprocket (stepped type) on both my engines no problem Both have the twin oil seals Both engines have the access hole to change the sprockets Had them off last week to fit a low sleeve gear and lay gear in the good engine and ran it at the weekend ok

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Alan, I think you are right on both points ,I had the same thought about an hour after I wrote the post but decided to leave it just in case someone came up with a solution. If you put a small shim behind the sprocket to allow the chain to sit down properly  there will not be much thread to hold the nut, as it is there is only about 6mm of thread for the nut.

If the sprocket was dished I could make a deeper nut and turn down the bit that contacts with the sprocket face and loctite it on instead of center popping it.


Naichuff, I think the bearing/seal housing must be different as a 13T will not work on my cases. I know everyone says they are the same but clearly some are different or the 13T would work. I have received a 14T sprocket today and it looks like it may just be OK but its very close to the alloy I will put the chain on and see if it clears.

trialsrfun, As it is I will try the 14T with the new 64T rear and see how it goes, As a rule of thumb if you go up one tooth on the G/box you would need to go up two on the rear to stay about right and I'm not sure if 15T and 16T sprockets with the corresponding size increase in the rear would be available.


Anyway thank you all for your input and if you have any further advice ,please let me know.


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Both engine I am using One has the bush timing side the other roller timing side both are distributor type engines so side points crankcase may differ again Seen a cub on Sunday that was fitted with 520 chain but it had sprockets specially made for it

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