Here we go again....

[Paul Slootheer]

A friend of my, who a owns a classic motorcycle repair shop and does reboring and hone as well, first heaths up the piston to a specific temperature (forgot how high…) and measures them. This enables him to see howmuch the piston expands and to what size he must do the rebore.

[nevhunter]

The Pistons you have NOW are "W" slot A car type piston where every motor is water cooled so the BLOCK doesn't get that hot. When the Block gets really hot that"system" does not work.. What Paul has described is one way of "getting an idea. But the TYPE as well as the Metal makes a lot of difference. Slots stop the heat travelling to the Skirt so the crown runs hotter. The WAY your pistons are grabbing indicates INSUFFICIENT Clearance whatever the clearance IS How many reconditioners are FAMILIAR with AIR cooled Motors these days that are NOT High oil flow motors? These things are seizing up commonly as people use fits that will never survive The fits prescribed by the MAKERS of the TIME are still valid today and remember WE tend to run them Harder because the Modern traffic goes faster.

[Simon.Gardiner]

In this case, all the pistons are manufactured specifically for this application (air-cooled motorcycle, BSA/Ariel 650). The manufacturers (JP and Wassell/Hepolite) specify the necessary clearance. Both engineering firms are very familiar with these exact engines, they have done this work before without any issues.
However in this case one lot seem to have cocked-up and bored to only half the specified clearance!
(Presumably the later seizure indicates lack of clearance because there was lack of clearance! However the JP was running in the same bore with the same lack of clearance - JP and Wassells actually specify exactly the same clearance figure.)

SG

[nevhunter]

We are NOW a very small % of the total market For Piston suppliers and a lot of the Old blokes who Knew about early stuff are Long gone from the ranks Pre WW2, everyone rode Motorcycles and sidecars and they weren't locking up all the time for good reasons which I've outlined, briefly. Most modern stuff is alloy with Nikasil bore and High silicon Pistons and internally showered in OIL. Designed to sit at High speed all day long if required. ALL 50's bikes are is 30's technology with enclosed valves which makes them cool worse.. Recurrent seizures are dangerous and remove metal from cylinders unnecessarily when it has to be rectified. Overbored and sleeved barrels can also Break. It's no fun riding a bike that MIGHT NIP UP on a freeway. Don't risk it. Nev

[nadab_sardar]

Worth keeping in mind that the clearance figure only means something if it's measured properly, whoever does the work. Mic the piston at the skirt, 90 degrees to the gudgeon pin and at the height the maker specifies, then measure the bore with a proper bore gauge at a few heights and directions. Don't rely on a quoted "bored to size" figure from the shop. It takes ten minutes and would have caught a half-clearance job before the engine ever ran.

Paul's heat-and-measure method is fine for fitting up oddball pistons, but with a known maker's piston like a JP or Hepolite for this engine, the published clearance is the number to trust. If the bore comes back tighter than that, send it back rather than hoping it'll free up with running in.

Also worth checking the barrel for pickup marks from the earlier seizures and getting it honed to remove any high spots. A bore that's been scuffed will grab again even with correct clearance.

[nevhunter]

For the fits we are talking of, a feeler gauge is satisfactory. If the fit is a Bit Marginally tight it will take ages to wear much metal off and be ready to pull the clutch in if it does or you might chuck it down the road as well as damage the bore and piston skirt. We ride bikes faster than they did when they were new because the roads are better and WE JUST ride them faster. Really, IF there's any risk of seizing, it's unsafe and it's likely to happen when you are overtaking someone. nev

[chris.vredenbregt]

I don't know if you ever heard of it but I use TSL in all my vehicles car ,motorbike (race)diesel camper its an oilupgrade you can find it on YouTube better starting smoother ride and prevents lock ups not cheap but worthwhile anyway cheaper than pistons