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Re-assembly Order Big 4

Submitted by ian_macdougall on Wed, 13/12/2023
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I have been helping another member to overhaul the engine and gearbox of his military Big 4. We are getting close to putting these lumps back into the frame (open type). I am contemplating adding the rear engine plates and magneto, then timing the mag while the engine is still on the bench. How feasible is it to then install the engine/plate/mag assembly into the frame?

My thoughts are that it may be possible to lower the frame, without forks and maybe with the gearbox already hanging from the top mount, onto the engine. My previous experience is with the full cradle versions of the frame where this approach would not work. Anyone tried this method?

Cheers, Ian McD

Submitted by ian_macdougall on Fri, 15/12/2023

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We have that manual. The method described may well be the only practicable way of re-fitting the engine & magdyno. However, the manual starts with the premise that you have a fully assembled motorcycle which will constrain the options available for each procedure described. I still wonder if there might be other ways of offering engine to frame (or vice-versa) if the starting point is a bare frame. I like the idea of setting the ignition timing on the bench, but this offers no advantage if the engine & magdyno then have to be separated to put them back into the frame.

Submitted by richard_payne on Fri, 15/12/2023

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I think that it could be done, but the engine plates would have to be loosened off to pass around the frame lug, so the Magdyno, although timed, will be loose on its pivot. With a new chain, it probably couldn't jump a tooth, but I wouldn't be certain with a worn example.

My general practice is to reduce the weight of the lump to be lifted as much as possible.

Submitted by david_cooper on Fri, 15/12/2023

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Timing is pretty simple anyway, with nice long stroke engines.  No messing about with TDC finders and degree discs.  Piston drop is fine.

Submitted by ian_macdougall on Fri, 15/12/2023

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Before re-uniting engine with frame the engine has to make it out of my garage and into the car, and from there into my friend's garage. Probably best not to add unnecessary weight. If the whole job was happening here I think I would try lowering the frame onto the engine then lift the assembly on a chain hoist to fit the forks & wheels.

Agreed, piston drop is fine although the degree disc is already fitted to sort out the valve timing. Proper long stroke on this engine - 120mm.

 



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