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Total Likes: 7
Original Post (Thread Starter)
by Bob Fletcher
Bob Fletcher
I have 3 sets of Triumph Twin 750 pistons -- all different top shapes and heights-- is there a diagram somewhere that will detail what compression ratio they are by measuring the height from the wrist pin centers to the top of the pistons? I have a Glenn's Triumph Guide that shows this very well for 650 pistons on page 75---but nothing for 750's------ and-- is the 7.9 compression ratio what I should be building my engine with for normal street use? What other options of low compression pistons are there to choose from? Some of the piston sets are Routts TRW 403-99 pistons. The Routts look to be heavy duty pistons for racing?
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by L.A.B.
Originally Posted by John Harvey
don't think there was anything available lower than the 7.9:1 option.

Also 7.4:1 (71-7085)
2 members like this
by Bob Fletcher
Bob Fletcher
I have enough pistons now to put out a list of what to expect the measurement should be to determine piston compression ratios. The dimension given is measured from the center of the wrist pin hole to the top of the piston. This does not take into account bore size. Accuracy is about + or - .025". Keep in mind this is not a exact science but should get you in the ball park when deciding what you've got in those piston boxes.

650 pistons -----
1.800" 11.1 cr
1.625" 9.0 cr
1.425" 7.5 cr
1.300" 7.1cr

750 twin pistons ---------
1.800" 9.5. cr
1.700" 8.5 cr
1.650" 7.9 cr
1.550" 7.4 cr
2 members like this
by kevin
Originally Posted by 1971tr6
Is there a way to mill down the Triumph heads so they can get 9;1 with a flatter piston like the Norton and eliminate the need for the twin plugs ??? I have two 650s and the one 750, and if I have to go through them in the future I would like to make them the best they can be with a practical amount of time and money.

if you have the head off, rob hall will twin plug it for $100.

use any EI with dual lead coils that add up to the correct resistance.

retard the spark to 30 BTDC.


vastly easier than milling the head and then dealing with custom pistons, pushrod tubes, pushrods, and valve to piston clearance.

ive got two T120s with twin plugs. my older one has been trouble free for 36 years. the next one i do will be 10 to 1 so i can see just how high i can go. you cant see the modification with the tank off, and if you dont like it you can just screw a plug into the holes.

no rocket science here

[Linked Image from]

[Linked Image from]
2 members like this
by Bob Fletcher
Bob Fletcher
I have ordered some new pistons -- after I receive them I will have enough data to put out here everything I know about comparing piston crown heights to compression ratio's---inquiring minds want to know!-- stay tuned
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