RD350 Piston clearance

[bigmuff]

I am about to get my SR175 barrel bored to 64mm to fit a ProX RD350 piston. What piston to bore clearance should I be looking at? 4 thou?

[bristolmod]

is that an iron sleeved alloy job?

If so I started mine at 4 thou which was rattly- I'm now on another barrel at 2 1/2 thou which I've had no problems with over the last 2000 miles.

Many may consider this tight, but I've found with the sleeved alloys, that they can go "tighter" than iron barrels

Chris

[lifes a drag]

i thought it was 3 thou but if bristol says his is ok with 2 and half you could go with that, just remember to warm the engine up slowley. i hate it when people show me there scooter that hasnt been started for a while then start reving the tits of it from cold, i come from a motocross background and always remember my brother inlaw always warm the bikes up slowly, he was a wiz with engines and his engines won championships so i always took is advice on engine building and running, different metals warm up and expand at different rates so the closer the tollerence the more important it is to let the engine warm up slowly

[bigmuff]

Thanks fellas, i've gone for 3 thou, to be on the safe side. if it rattles like f*#k I can always try 2 1/2 thou on the next rebore. Pistons are only £40

[soullad]

are you running a reedvalve?

[bigmuff]

Yes, I will be. Home made reed block, malossi cage and rubber from MBD, 60/107 crank. Just need to get boost port cut and barrel shortened once I have checked timings. Timings before rebore were 180 ex, 130 trans, 145 inlet(when piston ported). This was with a 1mm packing plate.

[bigmuff]

Picked up barrel after rebore today and fitted piston to check how much needs to be machined from base of barrel. With no gaskets top of piston skirt is 3mm below gasket face at TDC. At BDC piston is below exhaust port 2mm. I reckon i can get away with 2mm off bottom of barrel, none off top, leaving piston 1mm below gasket face at TDC and flush with bottom of exhaust port at BDC.

Looks like it will need a bit of work widening inlet port to clear the r/h piston cut out and boost port adding. May raise the exhaust port to get 185 degrees. transfers should stay same at 130. Or am I talking through my arse?

[soullad]

Sounds like you've a plan....
One of the niggles of the long rodded SR barrel conversions is that often at BDC the exhaust port isn't fully open. Not a showstopper by any means but not ideal either, but your top n tailing will help with this. I'd try and avoid a head gasket too, and if this means recessing the head slightly so be it, theres usually plenty of meat on the 150 heads to do this. A smear of proper sealant both top and bottom of the cylinder is increasingly becoming my preferred choice of gasket. Actually it might be worth speaking to Rich T on here, cos if he hasn't a head for you, he'll certainly be able to get one done / or give you the info to get head machined/profile matched, as he'll have experience matching this piston thru his GT kits.
You could even use this barrel as a test piece. Start your ex timing at say 175 deg and see how this runs with your set-up ..then open to say 180 and so on, its easy to grind / practice and have fun with.

[Knowledge]

Ian's advice above is sound.

I have a piston ported 185 barrel, but I found myself back tracking from my original 185 Ex and 130 transfers in a bid to widen the useful rev range. You may find that your reed valve already compensates for this and you can go 180/130, but it is harder to go backwards once you have opened the ports. One tuner I know says it is hard to demonstrate any advantage in transfer port timing in excess of 124 degrees.

I do not use gaskets at the top or bottom.

[bigmuff]

Ian's advice above is sound.

I have a piston ported 185 barrel, but I found myself back tracking from my original 185 Ex and 130 transfers in a bid to widen the useful rev range. You may find that your reed valve already compensates for this and you can go 180/130, but it is harder to go backwards once you have opened the ports. One tuner I know says it is hard to demonstrate any advantage in transfer port timing in excess of 124 degrees.

I do not use gaskets at the top or bottom.

Got the barrel shortened today, took 2.5mm off base with the aim of getting transfers below 130 degrees. I was using a 1mm packer before when running piston ported with 60mm crank. Will check timing with disk tomorrow when have time to see if i have grasped basic maths. (not feeling too confident). Will post results of my stupidity before i get pissed tomorrow