Douglasmotorcycles.net
General => General Douglas Discussion => Topic started by: saluki42 on 02 Feb 2021 at 10:50
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Hi there the deeper I get into working on my Model Z engine the more I realise it is all but identical, the crank case components any way, with slightly earlier models so I think this query might apply to them. Crankshaft rebuilt, original new old stock, pistons fitted and bores sleeved so now moving onto the crank case. The spindles on which the timing gears run are secured to the supporting plate and the other end located, in my case, into steel bushes which are a close fit into the crank case. At present both of these bushes, which I thing act as load spreaders, are loose and one of the holes into which they fit is slightly oval. Now I can get the holes increased slightly in size to remove the ovality and have new steel bushes machines to fit the new increased diameter but my question is this:
Does anybody know if these bushes should be a semi permanent press fit into the crank case holes or should they be a close sliding fit to ease disassembly at a later date?
I am trying to be as close as possible to the original build details but I don't think and increase in the hole diameter of about 15 thou will be that noticeable.
Stay safe out there
Chris
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Hi Chris,
Bushes like this would almost always be a light interference fit in the case - maybe 1 thou per inch of outside diameter. Less if there are thin parts of the casting around the hole. If the hole is oval you'll need to have it rebored oversize by someone with proper equipment and skills, and a new bush to suit. I'd used phosphor bronze, with a hole for oil entry and a slot for oil distribution.
If things have been banging around in there for long enough to elongate the crankcase hole, check that the shaft on the cam is still round. If not, it can be ground true, then you'll have a bush oversize on the outside and underside on the bore, and all will be well.
Leon
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Hi Leon thanks for your response and yes I agree with the light interference fit though phos bronze with oil holes is unnecessary for this application. The bushes and the shaft are designed to be stationary with no lateral or rotary movement between them the bushes act as locators for the cam gear spindles and are retained by a location plate and nuts via and oil channel providing lubrication. If the retaining nuts are tightened properly as I said there should be no movement of the spindles.
Chris