The DOHC 250 Velo Project: Engine Plates

I was already aware that the engine would sit lower in the frame – see here – and having completed the engine build it was now time to install it properly. I’ve been making engine plates on-and-off for nearly 60 years – see here – and I would say it is one of those not-very-inspiring-but-necessary jobs without which a project such as this can’t progress.

The first job was to position the completed engine (minus carburettor, to avoid damaging it during installation) into the frame.

It has been positioned so that the top of the cambox is ~ ½” below the lower curved frame rail and is in pretty well the same place as when I first  tried it for size some months ago before the engine rebuild. Looking at the position of the holes in the original front engine plates that are fitted and those in the front frame lug the crankcase is about 11/16” lower than usual. The cable ties around the cambox and frame tube are to help position the top of the engine near on-centre as possible.

I have evolved a well-established routine for making engine plates, the first stage of which is to cut out a cardboard facsimile of the front engine plates,

mark the outline on a piece of 6mm thick steel plate (the steel plate used for these were left over pieces of scrap steel from making the plates for the AJS V-Twin and the 33/7 projects)

and cut it out with a plasma cutter.

The picture below shows the 2 plates before any cleanup.

After cleaning up the rough edge and finishing approximately to size using a flap disc on the angle grinder the 2 holes for the engine blots were drilled using an original engine plate to locate the centres accurately and then clamped to the frame

to locate and drill the other 2 holes. A centre drill inserted into the end of a 3/8” diameter steel rod

is used to start the holes.

The plates were then finished to size: there is one long straight edge on these plates and this was machined in the milling machine

before final profiling with a file.

The rear engine plates have been made in exactly the same way

and the engine is now firmly secured in the frame.

There will be another hole to put into the rear engine plates to secure the strengthening bar fitted to KTTs that ties together the engine plates with the rear lower frame section (part Nos. FK-167 and FK-167/2) to help avoid breakage of the lower frame lug and to mount the footrests and brake pedal but I have yet to make these parts.

One thing that I have noticed is that engines get heavier over the passage of time; I’m now 75 years old and although I don’t have any quantitative data to back this up, I'll swear that these engines weighed less 20 years ago.