Thursday, 10 December 2020

Camshafts, Bearing Housings and Vernier Adjusters

As with the AJcette project, I planned to use Velocette K17/2 cams. Later cams have a smaller diameter base circle and are superior in the respect that the rubbing speed at the cam/skid interface is lower but I already had a number of K17/2 cams “in stock” that were in reasonable condition. These benefitted from a clean-up of the surface with a visit to Newman Cams and are now effectively “as new” cams.

There are a number of components to be made and careful measurement of all dimensions is necessary to ensure that parts end up in the right place and with the correct clearances. The picture below shows both bearing housings before any holes were drilled and were made in high strength 7075 T6 Aluminium.

And now completed and ready to fit

Here, I decided to re-thread the 3/16” BSW screws that Velocette used to hold the bevel gear housing onto the cambox. A number of these threads were worn after nearly 90 years of use and, luckily, the thread #12-24 UNC has the same pitch (ie 24 TPI) and very slightly larger OD at 0.19”. Each cambox was carefully set up on the milling machine and a #12-24 tap run down each thread carefully with no drilling required – the tap will pick up the existing thread.

Camshafts and Vernier adjusters were made

and together with new tappets and collets from a small batch that I had made previously

and also new cylinder head bolts (En24T)

The camboxes could be assembled and, together with the cylinder heads, put on the engine.

A few points to note here:

The camshafts now have flats milled on the threaded end to drive cambox scavenge oil pumps.

The Vernier adjusters have 16 holes on the sprockets and 17 holes on the hub, giving a resolution of 360 / (16 x 17) = 1.30 of cam rotation or 2.60 of crankshaft rotation. This is quite sufficient for setting up the valve timing.

The camshaft and the Vernier hubs are made of EN24T. There is a keyway and 0.001” interference fit between the shaft and the cam.

The thick washer with the pin to secure each sprocket to its hub is made of EN8 and the 3/16” pin of silver steel which has been threaded into the washer and then silver soldered on the rear. The washer/pin is then tempered at 2400C to eliminate any brittleness of the silver steel potentially introduced by silver soldering. It is silver soldered because I have had one of these pins shake lose on a previous engine with a bad outcome!

Spacers of nominal 2mm and 4mm thickness have been machined and one of each has been used under the cylinder barrels to ensure correct tension of the cam chains. Cam chain tension is extremely sensitive to the distance been centres of the chain drive and camshaft and the correct tension must be achieved at this stage as there is no good way to correct it later. In fact, even a change of distance of 0.010” between centres can make the difference between a correct chain tension and either a sloppy tension or a chain that can’t even be put on the sprockets.

The next step is to complete the machining of the timing cases and to make and fit the scavenge oil pumps.



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