Camshaft assembly procedure for ohv Douglas

[cardan]

Like most Douglas OHV motors, the RA and TW crank cases are split vertically, and the camshaft is supported by a ball race at each end, one in each crank case half.

Trial assembly of a motor went fine, using old races and not paying too much attention to fit. But for the final build, I find an interesting problem.

The cam shaft for the TW (stamped T.T. as I think are all or most RA cam shafts) is in pretty nice order. The supported ends measure very uniformly 0.473" and there is no visible damage. Now 12mm equates to 0.4724", half a thou smaller than the shaft, so there is an interference fit of 12mm ID ball races on the shaft. No problem as such: if I heat the races in front of the heater they will go on the shaft quite readily, but the fit is certainly interference rather than sliding.

I haven't tried, but it seems that the OD of the races will also go into warm cases. They don't slide in cold.

Can anyone advise of the approved way to assemble the cam into the crank cases?

(I think I now understand why the alloy was broken on the drive side end of the cam: belting the pinion end of the cam with a copper hammer would be one way to get the assembly apart!)

Thanks,

Leon

[Doug]

Leon,

Most of the OHV Dougie engines that I have split (with or without copper mallet!) tends to leave the ball bearing behind in the flywheel side of the case. the odds are about 50-50 on the timing chest side. This is suggestive that the outer race was intended to be a interference fit. If so, the inner race should be a slip or a light tap fit. This would make sense as the aluminum crankcase would be expected to expand more than the steel camshaft, and so would be fitted with the tighter fit to compensate. Both should not be an interference fit, or assembly would be difficult, as you surmise.

0.0005 inch interference on a 12mm shaft actually seems like a lot, and I would expect to be the maximum interference permitted. I would expect something like 0.0002 inch, or better yet 0.0002 inch clearance and make it a interference fit in the case (by about 0.001). But you are stuck with what you got. So you will have to install the ball bearings onto the camshaft, and then heat the crankcase to assemble over the cam bearings.

The later engines (i.e. OC, TT, DT) use a 0.625 i.d. x 1.375 o.d. x 0.2813 wide inch dimension bearing (size R10, still available) for the camshaft. 

-Doug

[cardan]

Thanks Doug. Yes interference fit in the case and sliding fit on the shaft sounds good. I have found another cam shaft, also stamped "T.T.", which is precisely that. Unfortunately it has a chip on the peak of one of the cam lobes, so I will use it for trial assembly but not for the final build. Other TT shafts are also interference fit in the shafts. It may well be something that was used for the racing cams, as it makes things very difficult for assembly and disassembly.

I've now found that the gear I have for the end of the camshaft has a different taper from the cams! Nothing is easy.

Leon

[Doug]

Leon,

I forgot to mention that "T.T." has been seen stamped on some S1 camshafts too. Or I think they were S1. Perhaps they were in fact RA? At the time I assumed the RA had the 5/8" journals, so figured the cams to be earlier. I will have to hunt down the photos and examine them again.

-Doug

[chris mac]

In the past I have honed out the inner races of ball and roller bearings so that I could use them for trial assembly and centralizing crankshafts..    0005 interference on 0.5 is still a press fit, I would hone out the bearing to size or -0001
My experience is not Douglas related so please feel free to disregard
Chris Mac