It’s always good to have a plan. In practice, most plans usually don’t work out exactly as anticipated and need modifying along the way because of something that you hadn’t thought of but without a plan a project has no structure.
So, what is the plan? The main elements so far are:
1) Check if the engine will actually fit in the Mk1 frame – because that’s what I have available
2) Check where the piston will be at TDC with a 68mm stroke and determine what modifications might be needed
3) Build the crankshaft with a 68mm stroke (to give 250cc) and with mainshafts the same as the 350cc engine to accept the bevel drive on the timing side and a taper on the drive side
4) Make offset/eccentric studs/adapters to move the centres of the holes for the cylinder barrel/head/cambox inwards
5) Modify the lubrication system to provide a positive oil feed to the cambox (the cambox has 2 entries for supplying oil and one drain pipe to scavenge the oil by gravity)
6) Check the cambox internals and set up the bevel drive shaft/couplings etc
7) Complete the engine build – piston/rings/valve timings/carburettor/magneto etc.
Although the project milestones can be summarized easily, most of these tasks are a substantial amount of work.
The first step was to check if the engine would fit in the frame. This was fairly simple as the chassis for the “cammy special” has already been built so it was just a matter of trying the engine for size.
It didn’t fit! The cambox would not fit under the lower (curved) frame rail; after removing extraneous bits and pieces (gearbox etc.), chopping off the extended bit of the frame that would support a hand gearchange lever
and removing the bolts connecting the engine plates to the frame the engine could be lowered into a position such that the top of the cambox cleared the lower frame rail and could be built or removed with the engine in-situ.
In the above picture, the steel rod that I have inserted through the cambox and cylinder head bolt holes is to ensure that they are aligned and the cambox valve pushers are resting on their respective valves with the cams set to “valves closed”. In practice, the engine height would be increased slightly with a valve clearance (say 0.020” plus a cylinder bases gasket (0.040”)) plus (as will be seen shortly) a spacer of around 0.2” under the barrel but this exercise is sufficient to determine that the engine will need to be lowered by around 0.75” at the front and 0.5” at the rear. The carburettor that I found among my collection is an AMAL RN and has the correct choke size. It fits OK although a remote float chamber will be needed – luckily, I have one.
One thing that became immediately apparent is that this engine will need a considerably longer K-49 shaft connecting the top and bottom bevels. The picture below shows a standard-length shaft held in position.
I have made a first estimate of where the piston would be relative to the top of the cylinder at TDC. I could calculate this but, as I have acquired an MOV crankshaft for this project
which has a 68mm stroke it is just as easy (and probably more reliable) to make up some dummy steel “bearings”
to be able to insert the crankshaft into the “K” crankcases.
It turned out that the MOV crankshaft is about 0.160” longer than the distance between the internal faces of the MC22 Bearings (22mm x 50mm x 17mm) that would normally be fitted and so I reduced the width of the “bearings” although on the timing side this didn’t help because the big-end nut fouled the “bearing” and this meant that I couldn’t completely close the crankcase halves.
Nevertheless, the assembly was sufficient to be able to put the piston in place on the con-rod (luckily the MOV small end is the same diameter as that of the DOHC piston) and slip on the barrel to see where the piston would be at TDC.
So far so good.
However, when I put the cylinder head in place the exhaust valve fouled the piston and so the barrel will need to be raised by putting a spacer beneath. I don’t know exactly how much yet – my guess is around 0.2” and I will need to make an accurate estimate based on piston piston/valve lift curve profile/compression ratio etc. in due course. As I mentioned, this will have a knock-on effect on overall engine height, length of the “pear-shape” section on the cylinder retaining studs etc. that I’ll need to take into account later in the project.
The lubrication system will need completely revising. In the picture below of the underside of the cambox, I have circled round the oil feeds that direct oil to the extremities – they squirt oil directly onto the cams - and also the drain pipe which would have scavenged the oil back into the engine (or, more likely, directly into the oil tank).
This suggests to me that this engine was designed to use a lubrication system akin to the Mk V KTT in which oil, under pressure and directly from the pump, is fed to a quill into the end of the timing-side mainshaft via internal passages and to jets in the cambox that direct oil onto the cams; this became standard practice on later engines.
The picture below of the (as received) KTT 581 engine shows the feed that comes from the pump and is connected to the cambox.
To do this, the lubrication system had been completely revised. The oil feed at the exit to the oil pump that on earlier engines pressurized the lower bevel chamber and then, by virtue of the pressure in the chamber, fed the oil up the drive to the upper bevel and via the slots in the shaft and large phosphor bronze bearing into the cambox had been blocked off. The internal volumes were not pressurized and oil was fed strategically to the crankshaft and camshaft as described above.
For the DOHC engine to operate on the same principle a number of modifications are required:
1) The small passage that connects the feed side of the pump to the lower chamber (indicated with an arrow on the picture below)
needs blocking off. (I have made this modification previously for the Velo oil pumps on both the AJcette and the AJS V-Twin – see here) by building up with braze and machining.
2) The drive-side crankcase needs modifying with a union to take the oil directly from the feed side of the pump (as in the picture of KTT 581 above)
3) The timing gear cover, K45/2, will need a quill and an oil feed to provide oil into the end of the crankshaft for the big-end
4) The crankshaft will need to be modified at the end of the timing-side mainshaft to accept the inserted quill
5) Oil feeds to the cambox (2) and to the upper bevel will be needed. A drain back to the oil tank will also need to be added.
Needless to say, the bevel drive shaft will need to be considerably longer but that will be one of the last parts of this project and when I see where the cambox is eventually positioned.
So, I have the outline of a plan and some of the implications that go with it.
In preparation, the MOV crankshaft has been stripped and cleaned, the valves removed from the cylinder head, the valve gear cleaned up in the tumbler and the main aluminium components (except the cambox) have been vapour blasted.
The first sub-project will be to rebuild the 68mm stroke MOV crankshaft with mainshafts appropriate for the “K” engine.
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