(Above photo is me winning best of show at triumph day 1992)
This page contains information about factory and aftermarket pistons for the A65 and A70 BSA motors
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This is a selection of various BSA factory pistons. Starting from the left are 1) #??? 9:1, 2) 68-906 9:1, 3
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4) 68-742 10.5:1, its interesting one has the small valve cutouts and one has the whole side relieved but both pistons are stamped with the identical #'s. The piston with the treated skirt in the middle originally came out of a low mileage wrecked 66 spitfire so it was used with the larger 1.6 in. intake valve. 5) My personal favorite for its lightness, and efficent designed dome is the 67 spitfire piston # 68-891.
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This is a picture of a hepolite replacement A65 piston on the left compared with the BSA OEM part on the right. The hepolite piston, in addition to being slightly heavier, is, in my opinion (and we know about opinion's) weaker because of the slot for the oil drainback and expansion control.
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This is a shot of the underside of the hepolite and BSA pistons. On the left is the hepolite and you can see the extra material near the skirt bottom and adjacent to the wrist pin boss. This was probably required as the sides had to support the pressure on the piston dome. This contributes to the extra weight.
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Here is the hepolite weiging in at 335 grams.
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A BSA 9:1 piston weighing in at 317 grams. both this piston and the hepolite are 1MM (.040) over. A fact of life is that the same mould is used for all the pistons so as you go oversize the weight goes up.
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The lightweight champion!! The 10.5:1 std size 66 hornet-spitfire piston. This weighs in at 287 grams. I didn't have a 10:1 67 spit piston with the rings off so I'll look for a used one to weigh.
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Here I'm comparing a stock A65 9:1 piston with a factory 9:1 A70 piston (made by hepolite, note that it dosent have the oil slot to weaken it) You can see that the wrist pin location has been raised to accomodate the extra 10MM of stroke, and the skirt has been shortened to clear the flywheel at BDC.
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Here it gets interesting!! This shows various pistons with different wrist pin locations. The piston on the left came out of the late Warren "Specky" Sherwood's BSA daytona racer. Note that it has the highest wrist pin location of all. We'll get back to why in a minute. The next piston is a A70 piston with its raised pin (not as high as the first). the next is a MC forged replacement BSA piston with the standard A65 pin location. This is the same forging as the sherwood piston. The piston on the right is a used 10:1 BSA spitfire piston. Until I decided to take pictures of this stuff for the webpage I never noticed that the pin location of the Sherwood pistons were non standard. However they were 650 pistons and his racer was supposed to be a 500. The following information is from memory which is in fast decline so don't take it as gospel. I was told years ago that the nutley NJ BSA guys (and Sherwood worked there for a while) used to cheat and use 650's with the top fin milled off the block. I couldn't figure out how they prevented the piston from protruding above the cut down deck. It wasn't until I just noticed the wrist pin location that it all became clear.
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This is a shot of a stock MC BSA piston and the sherwood MC piston. The wrist pin hole is 1/4 inch higher (approx) than stock. This is about one fin's worth off the barrel. He also cut away the skirt to clear the flywheel and milled the pin bosses to lighten up the pistons. Sherwood raced at daytona from 1950 to 1972 never missing a year and racked up the most miles raced by one person at Daytona. He started with goldstars then raced the twins. Interesting history. If anyone knows anything about this or the factory daytona bikes I'd love to hear about it. You could email me at
dukie@prodigy.net
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