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hondatl125

Honda Tl150Cc Conversion Help

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Hi there,

Im in the middle of doing a top end rebuild on my little TL I want to bore it out to a 150cc iv'e got the 124cc barrel just wondered if anyone knows what piston I can use and where I can get one from. Im based in the UK.

Many Thanks

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think the old CB750 piston fits.....but makes the cylinder liner VERY thin. So it's best to fit thicker liner to the barrel also.

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Tons of info here, especially the FAQs.

CB550 piston at 58.5mm will give you 135cc with decent liner thickness, the 150 kits use a 61mm piston which will leave you with a liner 0.7mm thick. Plenty of folks have fitted the Sammy Miller kit over the years without problems so you pays your money and takes your choice...

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I got my piston off Sammy Miller many many years ago when he was selling them as 150 conversion kit. Took my barrel to local boring shop and he refused to do it because he said liner would be too thin.

I then took it to Ray Cunliff at Ilkley. He bored it spot on and although liner was very thin it never gave any problems

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blimy roy cunliffes at ilkley is going back a looooooong way.. closed when i was a kid at school must be 35 years ago

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Thanks everybody for your replies. I've purchased a Honda CB750 K2 standard piston but It appears when measuring against my 125cc piston to be approx 1.7 mm higher from the gudgeon pin hole to the crown of the piston. Will this be okay or will it touch the valves ?

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You might need to cut the valve pockets deeper, you'd need to box it up and check the clearance to the new piston as it goes over tdc on the valve overlap to be certain. I do it with a clock gauge on the spring cap and a tool to push down the rocker arm but you need to lock the engine as the crank will move when you take the weight off the camshaft. The valves are closest to the piston at about 5 to 10 degrees either side of TDC. You might also run into problems with the compression ratio as the original 1 PC TL head has a very small combustion chamber and dished piston. It's not an easy problem to overcome as you can only raise the barrel about 0.5mm without the cam chain becoming too tight. Maybe someone else can tell us if the SM 61mm pistons were dished.

Doing your own big bore is never straight forward, that's why a lot of folks prefer to buy a kit, hopefully then someone's worked it all out for you...

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Mine is flat, not dished, with pockets for the valves,

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I’ve got the receipt when mine had it’s 150 conversion done by the dealer in 1975 before my dad bought it new. Still going strong today! 
 

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