The DOHC 250 Velo Engine: The Final Build – Part 1

Before starting the final engine build there were a couple of jobs that I had postponed, simply because up to now there was always something more important to do.

The first of these was to replace the small end. The small end bush that had come with the MOV flywheels/connecting rod assembly had just a bit too much wear and so a new phosphor-bronze bush was made on the lathe and milling machine,

an oilway along the length of the bush put in with a long 1/8” end mill

and then pressed in and reamed to fit the gudgeon pin that had come with the engine.

The second outstanding task was to check the crankshaft balance. I would assume that my Ebay-acquired MOV crankshaft would have been balanced to an MOV piston and the only difference is that I am now using the steeply domed race piston that came with the DOHC engine. Nevertheless, the balance needed to be checked. The crankshaft must balance all of the rotational mass + a fraction (the Balance Factor) of the reciprocating mass.

The reciprocating mass is the mass of the complete piston assembly, m_p, plus the small end, m_se.

By mounting the crankshaft assembly so that it can rotate freely and adding weights to the small end the balance mass, m_bal, can be determined.

First, the small end was weighed

and then the piston + rings + pin + circlips, m_p, (which came to 347g) before the flywheel assembly was mounted in a pair of used main bearings on vee blocks on the milling machine table.

I have used this arrangement rather than the crankshaft balancing jig as the assembly can be rotated without the end of the conrod hitting the deck!

The first step was to ensure that the existing balancing puts the big end at the top, rather than one side, when rotated.

It does. And so, the next step was to add weights in a small polythene bag to the small end so that the crankshaft would stop in any random position, without any preference, when rotated.

The mass added to do this, m_bal, was 214g. The balance factor, BF, is then defined by:

BF x (m_se + m_p) - m_se = m_bal

Which gives a balance factor of 73%

Is the acceptable or not? It is not easy to find documented “hands-on” experience of balance factors for MOVs but I did come across one article titled “Making a MOV Go” on page 6 of Fishtail 18 (October 1959) which stated:

 “ ….to enable a balance factor of 70% to be achieved. No experiments were made with other balance factors, the motor proved smooth between 4000 RPM and peak revs of 8500-8700 RPM.”

and so the value of 73% in the DOHC engine would seem to be just about right.

I have seen extremely high values of 85% quoted for MOV balance factors but I can find no evidence to back this up. If I substitute the raw piston weight of 8 ¼ oz (235g) for a 6.5:1 compression ratio piston that is given in the Velo technical data here then this would give a balance factor of 80% and this value would be reduced if the slightly heavier 10:1 race piston data were used. For the record, the raw piston weight of the DOHC piston is 307g.

I had already sorted out the lower bevel gear meshing (see here) and now the solid steel dummy “bearings” that I had been using were removed, the oil pump was inserted into its hole, the new (modified) main bearings were fitted, the crankshaft was put in and the crankcases sealed – exactly the same procedure as described earlier for KTT 305. Prior to assembly, the modified Woodruff key that engages with the lower bevel gear was heat treated.

The K-45/2 timing gear cover plus all the internal bits and pieces were assembled. 2x 1.25mm thick steel discs were placed behind the oil pump driving piece to get good engagement of the K-34 and K-72 gears as described in a previous blog. There are 2 different gaskets (K-68 and K-68/2) depending upon the timing crankcase/timing gear cover combination – needless to say I had the wrong gaskets in stock and so had to make the other one.

I have also fitted a new spring and ball to the oil pressure adjuster.

With the changes that I have made to the lubrication system these will no longer be used to adjust the oil pressure but they do need to be there to block the hole that connects the chamber to the feed side of the oil pump.

The inner timing case then needed modifying

to enable it to fit over the boss that I had attached for the crankshaft quill oil feed.

If the magneto chain ran in an oil filled chamber then there would be a problem ….but it doesn’t.

I had checked the piston, piston ring and bore dimensions some time ago and found clearance of 0.008” on the thrust/antithrust faces at the bottom and the 2 compression rings had a 0.032” gap. Recommended values for these quantities are generally quoted “per inch of bore” and to be able to make comparisons my measurements equate to 0.003” and 0.012” per inch of bore respectively.

So, what values are recommended? That depends where you look…. For engines in standard trim (ie, non-racing) Hepolite/Heplex recommended a comparable piston/cylinder clearance of 0.0015” per inch of bore (see eg The Vintage Motorcyclists’ Workshop, Radco, page 53). On the other hand, Phil Irving (page 68 of Tuning for Speed) gives a value of 0.0027” – nearly double. The piston/bore clearance dimension on this engine is very close to Phil Irvings value (0.003" versus 0.0027”)

For the compression ring gap, Racdo quotes a value of 0.012” (which, for comparison, equates to 0.0045” per inch of bore) for a Velocette KTT whereas Phil Irving suggests 0.010” per inch of bore – again, double.

I checked the ring thickness against published Velocette MOV data and they were the same and so I bought a set of new rings. These fit perfectly and gave an “off-the-shelf” value of 0.025” for the compression ring gap. This equates to 0.0093” per inch of bore which is pretty well exactly Phil Irvings recommendation.

So, it would seem that the clearances in this engine conform pretty closely to those recommended by Phil Irving.

Incidentally, I was amazed that I could simply phone Cox and Turner

and buy a set of NOS (New Old Stock) piston rings that arrived the next day for a fairly obscure British motorcycle that ceased production in 1948!

The next step was to put the piston + rings on the connecting rod and put on the barrel and cylinder head. The circlip grooves in this piston are flat-bottomed and so new Seeger circlips were used to hold in the gudgeon pin. There are not that many occasions when I need circlips but I bought a box of assorted imperial-sized circlips many years ago

and these do come in useful when needed.

The barrel and head had been given a coat of cylinder black before assembly. It was now time to set up the vertical drive.

The drive shaft (which would usually be part number K-49 on an early “K” engine) that came with engine is substantially longer, 7/8” longer in fact,

and although I don’t have a Mk 2 shaft in my workshop (except in the engine of the bike I recently purchased) my guess is that this is a shaft for the later engine. It also looks to be new with only minor witness marks of having been used.

This required a pair of Oldham couplings - the question is how thick should the flange on the couplings be? The shaft was put in place and a bunch of washers + shims + a feeler gauge placed between the shaft and the drive at the lower end

to find out. 2 new Oldham couplings were then made with a slightly thicker flange (0.218” here vs 0.188” for the thickest commercially available part (see here)) from O1 tool steel,

finished to size and heat treated

before cleaning up

and fitting. In the above picture, the 2 new ones are on the left and, for comparison, a standard “K” coupling on the right where the difference in flange thickness is apparent.

I have used a slightly higher tempering temperature than usual (260⁰C) for the heat treatment to trade off some hardness for additional toughness. The thicker flange should also help in that respect.

A check after assembly showed 0.008” vertical clearance

which should be fine.

Much of the engine build is now complete. The main outstanding tasks are: check the cambox internals, complete the lubrication system and fit the magneto.

And then it will be Christmas and I will be banned from the workshop for a few days.