Pre unit Triumph engine - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 2 (permalink) Old 08-14-2009, 11:17 AM Thread Starter
Production 125
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Join Date: Nov 2008
Posts: 6
Pre unit Triumph engine

I'm building a pre unit 500 engine with 650 barrells pistons etc Barrells are fitted but when fastened down, at tdc, piston edge of crown is still 2mm from top of barrell, is this because the throw on the 500 crank is 2mm shorter than the 650 and if this is the case, is that a problem? If it is an issue do I have to change the crank or take 2mm off the bottom of the barrell flange? Ibought the motor as a basket case and am finding that no two bits are from the same motor but as it was cheap Iwant to carry on with it and get it running. Somebody out there tell us what to do!!!!

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post #2 of 2 (permalink) Old 08-14-2009, 08:51 PM
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Main Motorcycle: '55 6T Thunderbird
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Join Date: Aug 2008
Location: Chicago, IL
Posts: 48
hey red,

Just went out to measure mine, it's a '55 6T 650. The piston crown edge comes up to about .050" from the top of the cyl barrel, at TDC. So a little bit less than the .079" you are measuring. Not an exact measurement on my part since my calipers wont fit between the piston dome and the cyl wall, so I am sort of eyeballing it.

However, I have no idea if that is significant. Might affect your compression ratio.

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