Saturday 1 August 2020

Piston and Rings

Back in the days before I retired, I was travelling to Chicago frequently on business – at one stage I was spending one week per month there. Chicago is a lovely city, at least if you avoid the South Side, and I would often arrange to fly home on a Sunday morning so that I could meet up with a good friend that I had got to know well over a period of years.

I first got to know Dave Smith, who lives out in Plainfield in the Western suburbs, as someone who came recommended to me for machining a pair of Velocette cam followers for a Thruxton. In due course we met up and Dave, at that time, was running a small Velocette spares and rebuilding service. Dave also turned out to be one of the best engineers I have ever met in my life – and I’ve spent 45 years of my life working as an engineer in the engine industry.

Anyway, we would meet up every Saturday morning and over a lunchtime beer one day we were discussing Mk 1 cammy Velocette pistons. I had (and still have) a number of these bikes to rebuild, including 2 very early flat-tankers and 2 KTTs, but I was in need of pistons. Dave was, at that time, having pistons made by JE Pistons in California for the later pushrod Velos and suggested that if I could get him a good example of a Mk 1 cammy Velo piston that he could get a small batch made as he had a good relationship with JE. I duly delivered an original 74mm bore cammy Velo piston from KTT 55 to him and a batch of 4 new pistons was made.

Before making the pistons it was important to decide on the ring pack and, not unreasonably, JE needed to know the ring details before making the pistons. Dave had rebuilt a number of Velos with good success using a Nikasil plated bore and we decided to go this route. However, Nikasil is exceptionally hard compared to the original cast iron and needs different ring technology. We therefore approached Total Seal and procured from them 4 sets of rings suitable for running on a Nikasil plated bore.

The excessively large gudgeon pin diameter of the original was reduced slightly in the new pistons and an option to coat the surface below the rings to aid running in was taken up. The picture below shows the original piston, one of the new JE pistons and the rings (unfortunately in plastic bags – I don’t want to break the seal until I use this set). The weight of the new piston + rings + pin at 217.4 gm is a few grams less than the original.

And installed in the engine with a Nikasil plated liner, although that can’t be seen with the piston at TDC!

I was warned that when starting an engine for the first time that uses this technology the engine would be extremely tight but the high friction would soon disappear and it would “bed in” very quickly. This proved to be the case and, as at the time of writing, the bike has covered a few hundred miles, including my 2019 annual pilgrimage to the Manx GP, and has excellent compression but is not “tight” in any way.

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