Tuning an old Velocette for Sprinting
This article was written by Dai Gibbison, about his 1937 Velocette MSS 500cc race bike. Dai is a Velo Fellow of the highest order, with an encyclopedic knowledge of Veloce design and engineering details. Click the link here to see a portion of it his digital library and notes.
In the UK they call it Sprinting, but American readers will know it as Drag Racing. The bikes must be light to get maximum acceleration (F=MA and all that). But they also must not blow up during the short runs at maximum RPMs. Dai’s bike has been updated, modified and tuned to become a very quick, and very reliable bike. The best 1/4 mile runs Dai has to date have been at 13.87seconds at 101mph.
Building and Tuning a Pre War MSS Engine for Competition
Text and photos copyright 2010 Dai Gibbison.
In 2002 I campaigned what was basically a standard MSS running on methanol with 1 3/16 10TT9 fitted in both NSA and VMCC sprints. The only other modification was to remove the head gasket and fit a one inch spacer between the head and carb.
The pre war MSS is red lined at 6000 RPM, and based on good advice I was informed that over revving the long stoke engine was impossible. The combination of tuning the exhaust and inlet lengths, lack of a rev counter and over enthusiastic use of the right hand caused the exhaust valve to hit the piston. A contributory factor to this was a gradual stretching and tuliping of the exhaust valve. I should have suspected something was awry when I completely lost compression and exhaust valve clearance. However as the bike started and performance was not affected, I just carried on. The outcome of which was the valve and piston had a very brief sparing match. Fortunately the valve bent and subsequently hit the spark plug, closing the gap and so little damage was done to the head or barrel. I have now fitted a rev counter!
A (later) Venom motor shown in cutaway. Note the changes from the earlier cast iron motors in the hairpin valve springs and the oil returns from the rocker chambers to the pushrod tunnel.
To avoid a similar situation occurring again, up grading of the valve gear was the order of the day. The first task was to lighten the valve gear, but not so radically as to affect reliability. The cam followers were replaced with Venom followers to increase the valve opening and lightened by removing redundant metal. First stage was to grind the bulk off on a bench grinder, followed by further lightening using a Black and Decker Powerfile. Finally, the follower was buffed on a polishing mop to remove any potential stress concentrations.
Similar treatment was applied to the rockers and you can if you want to get the last once of performance you can waist the centre portion of the rockers to reduce friction / drag.
The top and the bottoms of the pushrods were lightened, again a similar three stage process. New pushrods were made up to the exact length using 3/8” HE15 tube, the ends being a pres fit into the ¼” reamed tube.
Finally the standard springs and cups were replaced by uprated components from Nick Payton. The springs needing to be shimmed to achieve the correct length for spring pressure.
Next for attention was the valves themselves. Ideally I first thought I would have liked to use Nimonic 80 for the exhaust, but the cost of manufacturing a small number was prohibitive. Further investigation indicated that 21/4N,a high chromium steel that retained its strength even up to 800 degrees C was more than adequate for race use with methanol. It also had the benefit of being slightly lighter than Nimonic 80. So exhaust valves of G & S manufacture were obtained from the VOC spares scheme and sent back to G & S for nitriding to stress relieve the material and provide a hardened coating that prevents and pick up in the guides. As an extra precaution the exhaust guide was reamed out one thousandths over size. An inlet valve also of 21/4N was fitted. As the inlet and exhaust valves on an Iron MSS are the same dimensions except for the stem diameter, you can safely use a 21/4N inlet valve as a lighter exhaust valve to reduce weight. I hope to cover more technical information on valves in a later article.
Cast iron guides were retained, as while they do not have such good heat conductivity as aluminium bronze they have better self-lubricating properties.
One problem with iron M series engines is that the valve spring cups fill with oil, in time finding its way past the valve and guide into the engine (which can oil the plug). A common racing modification to the cups is to update them to Venom specification by fitting oil drains to the cups.
To increase the compression ratio I used a 80 mm JAP piston, with an extra oil scraper ring fitted. This involves sleeving and shortening the barrel to get the correct compression height. The compression was set at 11.33 :1, as experience of others indicate that going above this level with an Iron MSS results in split crankcases. Shortening the barrel involved extending the head steady, new fuel pipes, shorter pushrods and a lot of fiddling around to get the exhaust right.
The original 5/16” MSS studs were replaced by suitably modified Venom items. This involves changing the stud inserts in the crankcases and opening out the stud holes in both the head and barrel.
There’s a fine line to be drawn between using the existing heavy fly wheels to give good acceleration off the line, and lightening the flywheel to get better top end acceleration. Anyway, the flywheels have been lightened a la Geoff Dodkin. A lucky find at Stanford Hall was a lighter con rod of the same length as a standard MSS rod. The rod was polished and shoot peened to hopefully reduce any chance of failure. A MOV big end was fitted on the MSS pin to reduce inertia and friction.
Finally a M17/8 cam was fitted in the place of the M17/2, which involved all sorts of shenanigans to avoid everything touching where it shouldn’t.
The original oil pump was retained running at single speed as the bike is only used for brief spurts, so the engine and therefore oil does not get a chance to get really hot. If you decide to fit a twin start oil pump worm to an iron engine, remember to change the oil pump spindle drive too – or you will end up with bronze coloured oil. The timing cover was modified by Bob Prior to accept the later cam steady plate and oil jet and rev counter drive gearbox.
Finally, I eventually fitted a John Watson clutch and primary belt drive, which to date has proved to be faultless.
Veloce advertisement for the 1937 MSS
Tuning for Speed, Phil Irving
Getting Down To It (Part 2), Nick Van, Fishtail 71, page 14
Preparing the MSS For Racing, Laurie Nunn Fishtail 101, page 53
Race Tuning the Thruxton, Marin Violette, Fishtail 105, page 34
Venom With A Sting, Motor Cycle Mechanics December 1971