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Sherpa 199A Brake Hub Steel Liner Fitting Clearance

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Hi Guys,

My local engineering company who are to ream out and then press in the steel liner into my back and front hubs has been asking what the interference fit should be.  Too small and the hub might crack.  Too large and the liner may come loose.  My man thought it might be either 4, 6 or 8 thousands of inch.  He is reluctant to take it on unless he has the correct spec that works.

Any guidance appreciated.

Thanks 

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Your hubs may split or crack even at the correct interference fit due to the aluminium becoming age degraded...more brittle. Presumably the hub will be heated to expand the housing and the liner shoved in a freezer to shrink slightly. Using this method the liner should just slide into the housing and be allowed to cool naturally. Some people have used a resin bonding agent whilst others have employed locking screws or locking pins, although these methods may increase the fragility of the hub(s). There is info on this web site if you do a search you should find it. More importantly is that you know the inside diameter of what the brake drum should be, I would have thought. Your engineer is obviously being wary and quite rightly so. 

As to interference clearances and taking into consideration heat build up when braking...you would need to brake for a very long time to build up significant heat in a Bultaco brake shoe assembly front or back. Here are some details for you.

The housing is bored out to accept the sleeve, read that as cleaned up. When the sleeve is installed into the aluminium housing, the interference fit is .004. When the sleeve is being installed into a cast iron housing, the interference fit is .002 to .003.
The interference fit is determined by measuring the outside diameter of the sleeve, then boring the cylinder diameter smaller than the sleeve. In this case you do not want to remove much if any material from the hub. The lining needs to be in effect .004 oversize to the hub. Any areas of corrosion in the hub could be treated with a resin adhesive suitable for metal.
The interference fit is a critical aspect of sleeve installation because this step ensures that the sleeve does not move when in the hub. 

Now you have this info I do not accept any liability for the process if you decide to employ any of the suggested methods and it all ends in damage. 

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Hi, Many thanks for your comprehensive and detailed guidance. I am aware of the risks involved and accept that the liability rests with myself. My man is aware of the risks as well, and hence he was looking for  a specific specification which I appreciate is not available.  I explained to him that the bike is for trials and not high speed or even road work and the likelihood of the drum heating up and coming loose is minimal.  Considering what you have said, I may suggest to my engineering shop man (a one-man enterprise, and with highly regarded expertise with engines etc) to machine the hubs to accept the liner with an 2 thou clearance.  ie the liner outside diameter to be 0.002 oversize to the hub for safety.  I am not too sure my man would heat the hub in the oven and freeze the liner, but I may do that myself, if I ask him just to ream out the hub to the 0.002 thou below the liner OD. Then I could tap the liner into place myself.  I may also consider using a resin as well to take up any irregularities within the reamed out hub,

I appreciate your assistance and will let you know how I get on.

Regards

Ian 

 

 

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Unless your man is dirt cheep, or free, I'd stick with recognised experts. Have you considered the suitability of the material being supplied as a replacement liner ?

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When fitting a steel or cast iron liner to a previously chromed drum Bultaco hub, the new liner is sometimes made with an ID smaller than standard/nominal ID to minimise the amount of hub material that has to be removed ie machine the drum just enough to clean up the previously chrome plated surface. The shoe linings are machined down below standard/nominal OD to suit the new drum diameter.

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Thanks for the comments everybody.  The engineer who is doing my hubs is long experienced and highly regarded and knows what he is doing.  The liner being pressed in with 2 thou interference fit is of suitable material. My liner has an OD of 129.96mm and an ID of 125mm.  The standard chrome lined hub is 125mm ID.  The hub assembly with brake shoes fits perfectly within the liner. 

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