Hitchcocks Forum

I've just had my 500 Barrel measured and the engineer says is oval so worn-out. Having rang up for a new piston I'm told the barrels are meant to be oval? So if I have a new piston and a reborn to 87mm how does the machine shop get the oval shape correct? Just a bit confused and wondering if my barrel is still OK? H

"the barrels are meant to be oval" methinks someone is pulling your wire.

Perhaps it's time to go to another machine shop, preferably one that's not used to dealing NR500 barrels all day long

Hi H, The pistons are very slightly oval - maybe someone is getting their wires crossed? The bore in the barrel should be perfectly round, straight and give the correct skirt clearance for whichever piston you intend to fit. Regards, Paul.

Your barrel will be perfectly OK if you bore your 500 barrel to 87mm and fit 535cc piston that will solve any irregularity if done by a good workshop.

As B W says the bore is round, the piston is slightly oval as part of it has more metal and this allows for the different expansion when hot, any rebore and oversize piston will sort the problem.

A bore will wear oval as the most thrust from the piston is fore and aft instead of sideways. As others have said, a new bore must be round.

Hi if you bore out to 535 hitchcocks do a nice accralite piston 8.5 cr bit pricey but way ahead of any other crap ones, I would have my crank rebalanced to 63% much smoother.carby 120 main jet 27.5 pilot. You will be pleasantly surprised how much more grunt you have, I moved up to a 19 tooth gearbox spkt.all the best.

H, the best people locally for your rebore is Alex Carr, they understand the correct clearances for these engine types and have reasonable charges, pop in with you barrel and they will tell you what oversize of piston you need.

Rebore and piston should be supplied together, ideally re-bored to the piston, and then honed to fit. After honing there will be a definite front and back to the piston, so ensure you hone the right way round (this may have already been done). Piston and bore will eventually wear oval, eventually.