Installing Mk3 Swinging Arm

I have just been through a similar situation, I am rebuilding and repainting my Mk3. As the pin and bushes were in good condition  I intended to reuse them, during the lockdown here [New Zealand] I placed an order with one of the major suppliers in the UK for a replacement pair of seals, with the new paint  and the UK order here a friend who manages an Engineering shop refitted the bushes and new seals with his press, no way could I get the swingarm back in the cradle, it appeared  there was a difference .035" between the cradle and swingarm, as I had damaged the seals I ordered a replacement set this time from Andover, on arrival my friend set out to identify the problem, the outcome was that the original  bushes that we had reused had an o.d. of 41.26mm where as the first set of replacement seals were only 41mm between the lips of the seal therefor stopping the bush from seating properly, the second set  of replacement seals [from Andover] measured 42.4mm as did the old set that were fitted when the bike was pulled apart, thickness for both [excluding the lip] was 1.143mm.

Neil, the seals I brought from Andover were fine, it was the first set I brought from another supplier that caused me grief. 

Cradle width 3.767" or 95.68 mm as you call it.

The swinging arm should fall under it`s own weight when assembled on the cradle but even with ten thou clearance, the seals are so tight that it`s stiff to move. The problem with skimming the bushes is that they get recessed further into the seals which themselves remain unchanged. I think if I were doing it again I would remove material from the swinging arm inside faces that the bushes and seals are pressed up to, which would keep the bush to seal relationship the same, as they would both move outwards to create the larger gap needed. Best done on a milling machine of course but careful use of a large flat file would also do as it wouldn`t have to be perfect because once the bush is fitted, the rim would be square to the axis anyway.

Regards, Al.

P.S. Could just be that all these NOS swinging arms and cradles we`ve been buying have been in the back rooms all these years for good reasons!

It would be very useful to have a summary of good solutions to this probably common problem as a post on this thread, including where to buy correctly dimensioned seals and bushes.

Hi, had the same issue whilst rebuilding mine a couple of months ago, i too cleaned all traces of paint from mating faces even removed the paint right to the edge of the cradle to allow a lead in, all to no avail.

In the end i purchased a pair of new seals from AN and found these to be more supple and slightly thinner (sorry didn't log the dimension), greased them and they were still tight but went in with no damage. The previous seals were from another Norton dealer and were a bit cheaper, but happy in the end with the AN more expensive ones, paying more hurts sometimes but pays dividends.

Really don't want to do that job again.

Cheers, Ian

Only last month I too had this problem when renewing my swinging arm bushes and seals. On closer inspection I found the metal backing of the seals was over 0.030" thicker that the original ones. I explained this to the supplier and eventually he offered me my money back! (£22.00 +VAT!)

I did eventually manage to skim the seals where the flange of the bushes sit, in the lathe, to give the appropriate clearance but would have prefered to have the correct seals in the first place!

Andrew:

Are you sure that the swingarm that you got back from the coaters was the original swingarm that you supplied to them?

I would think that the coaters have a number of swingarms going thru their shop at the same time and possibly yours got mixed up. Was there anyway for you to confirm the return of the one that you supplied to them?

Mike

I bought a new swing arm kit for my Mk1 850, the spindle had seized in the bushes and was badly corroded, having hammered the old one out, the bushes seemed fine.

I was told that the bushes are extremely hard wearing and would probably not require changing. I checked the new spindle in the old bushes and it was a good sliding fit.

It's all together and is fine.

I purchased a new Mk3 interstate in 1975 when they went on sale in 76.,I loved the bike but since I owned  a74 850 and a 70 750 with RH shift I kept having trouble lifting up the brake lever and and pressing down on the shifter to brake. Neither of which worked well. In 1977 I took the rear swing arm off of my Mk3 and adapted it to my 74 850 complete with rear disc and added an AP Rear Master Cylinder to make my race bike. It worked very well. Many years later (now) I was thinking about adding a MK3 swingarm to my MK2 for the support and strength however how do I mount my rear drum brake? Any one have ideas or have tried it.
mike