Vincent Firefly

by Mick Newman

I am nearing completion of a Vincent Firefly that I purchased in bits at Newark autojumble recently. I have been lucky enough, through club contacts and adverts in this journal, to acquire most of the key missing parts but one difficult problem was finding a replacement piston. The original standard sized piston was damaged around the ring lands and the barrel was quite badly worn. I have been completely unsuccessful in finding an original Vincent piston either in standard or any oversize and it became apparent that they are just unobtainable. As I have now solved the piston problem and my Vincent is now running well, I thought it may be useful to other Firefly owners if I passed on the solution.

While there are no Firefly pistons readily available, there are plenty of modern machines with similar sized pistons that could possibly be adapted. With a two-stroke, the piston serves the additional function of timing the inlet, transfer and exhaust and well as the normal duty of compressing and containing the burning charge. Selection of a candidate replacement piston was, therefore, undertaken carefully, otherwise the port timing would have been disturbed and the engine would not have run properly. The piston that I have used is from a Honda MT/MR 50 moped; it closely matches the Vincent dimensions. However, a number of minor modifications are necessary.

The standard Vincent bore is 38mm while that of the Honda is 39mm. Two options are open to you: either carefully turn down the Honda piston or, as I did, overbore the Vincent. There is plenty of thickness in the over-bored barrel, even the vulnerable spigot area.
The Vincent gudgeon pin is 15/32" (11.91mm) whereas the Honda piston is bored for a 12mm pin. I reamed out the standard Vincent little-end bush to take the Honda pin.
The height of the Honda piston above the gudgeon pin is identical with the Vincent and I can detect little difference in the dome. Exhaust and transfer timing will, therefore, be unchanged, as will the compression ratio.
The length of the Honda piston is about 2mm too great so, in order to maintain the same inlet timing, this amount was trimmed from the Honda piston skirt. Furthermore, it was necessary to cut a V notch in the inlet side skirt to clear the con-rod, as with the original Vincent piston.
The Honda rings are pegged in the wrong place to suit the Vincent ports and it was necessary to remove the pegs and install new pegs at locations to match the Vincent. The pattern piston I used had plenty of thickness to allow this.

The cost of this was not great as I undertook most of the work myself. The Chinese piston was £12 complete with rings and pin; the rebore cost £20. Add a further £8 to buy a 12mm reamer and the total comes to slightly more than I paid for the whole cyclemotor. The alternative of having a special one-off piston made would have cost much more and would not be economic.

I hope that my experiences may be useful other Firefly owners and it would be interesting to know if there are other modern pistons that can similarly be adapted to keep other types of cyclemotor running now that original pistons are getting thin on the ground.

First published - April 1997

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