How To: Broken Piston, Velocette MSS
While on the Girder Fork Ride two weeks ago, the MSS was down on power during one stretch of road. It hesitated and died, and the roadside prognosis was a loss of compression, with a prod on the footstarter giving very little resistance. The kicker will typically support my weight unless I lift the exhaust valve. (early motors with no valve overlap can be a bit tough to kick through the compression stroke).
Back at home, I knew that there were just a few ways to lose compression: faulty valve lifter, spark plug is loose in the head, failed head gasket, bent valves or broken piston rings. After a quick look at the plug and the valve lifter, I moved on to the time-honored method of diagnosing a loss of compression: pouring oil into the combustion chamber. The spark plug came off, and three quick squirts from the oil can, then plug in and kick over the motor to check for compression. This test is a good one, since it will identify if the problem is in the piston rings or valves. The oil will seal around the rings and give compression. If no compression comes back, then the problem is likely in the valves.
The oil found its way around the rings and I had compression for two or three kicks. Then it went back to no comp. So the problem was in the barrel. Off with the head and barrel, then the piston was on the bench. This particular piston is a Hepolite, original type as used when the bike was new. It was fitted 15 years ago, just before I bought the bike, and has now done something like 25,000 miles, including a dozen of the Velo club 1000 mile rallies at or near full throttle to pull the sidecar.
A crack had developed across the crown of the piston, and down through the ring lands and onto the thrust face. The crack wasn’t the source of the compression loss, but the top rings had been nipped in their grooves and weren’t free to expand into the bore.
One tip: if your piston pin is a tight fit in your piston, warm the piston to expand the hole and give the pin some room. A rag soaked in boiling water will work, or the gentle application of a propane torch.
A call to Ed Gilkison and I had a NOS piston in my workshop. A genuine Hepolite, with the same part number, including rings. And to the same +.030” bore size, so I didn’t have to bore the barrel over to fit a .040” piston. This one was marked Ernie Pico 1973, so it was probably sitting on a shelf at Ernie’s shop in SoCal back when I was still wearing short pants. more Ed stuff here on DQ’s site.
A light hone of the bore with a three-stone expanding hone in a hand held drill, then it was time to check the rings. Typical ring gap is about .004” to .005” per inch of bore, so I used a fine file to cut down one edge of the ring until it gapped at .015” using feeler gages. The photo below shows the general method. Or check in an old book, most of them describe the operation. I should mention that the bore looked pretty good. No ridge or deep grooves, and just a bit oversize for the piston diameter.
The valves were given a quick test by filling the ports with a half pint of solvent. No leaks into the combustion chamber were seen, so the valves were left alone. If yours need some attention, here is a How To article on valves.
Rutland stove paint works well on old cast iron motors. But make sure to paint outside, it smells nasty.
When he laid out the design for the M Series motors at Veloce Ltd, Charles Udall decided to use 19 screws to hold the valve covers together. But Valentine Page used just 8 screws to do the same job when he designed the enclosed rockers on the 1938 Ariel Red Hunter. After fettling each of those over the last dozen years, my feeling is that the Ariel is easier to work on and make oil tight, but the Velo runs sweeter.
your mileage may vary.