hi huys,
Old Wheezer (WM20) needs a top-end job. I have sourced a standard NOS BSA piston and rings and ordered a replacement liner. I am 3,000 miles away from a brit bike mechanic, but have a great machinist to replace and size the sleeve for me.

I'm wondering if anyone could give me some advice I can pass on to the machinist? Grit sizes, best clearances etc? This is not something I have done before so I need all the help I can get!

email (option): cas.vanderwoude@gmail.com

After the old sleeve has been removed and the barrel cleaned up the interference fit between the outside diameter of the liner and the barrel should be .002" -.0035"...
A press with a minimum capacity of 5-7 tons will be required to fit the new liner..

Note that at the lower end of the liner there are two 'scoops' ground into it (and the barrel)...These provide clearance for the conrod and must be reground into it after the liner is fitted...(the dimensions are shown in the manual/factory worksheets)..

The liner is of a 'top hat' design with a larger diameter shoulder around its upper end so it must be pressed out from the bottom upwards and in the reverse direction when fitting..

With a solid skirt piston, rebore and hone to a piston clearance of .004"-.006" measured on the thrust faces 'fore and aft'...Ian

email (option): ian@wright52.plus.com

Thanks Ian - as always, good detailed advice. DO you have a recommended grit grade for final honing? I have a feeling I read somewhere it should be a bit coarser than current standards - Am I right to say 150-180?

email (option): cas.vanderwoude@gmail.com

Hi Cas.
Just been re sleeveing a C15 trials with a friend and he used a but welde Hss tool on a slow water cut to get a really good finish down to 2-3 thou of finished size, a fine grinding wheel attachment for within 1/2 thou and final hone with quite a fine hone to gently finish off. Wanting as good a finish as possible it turned out damned near perfect in the end.

It's a no. 3 spare engine for him and so not a rush job, so he had plenty of time to get it right.
Regards
Darren

120 is what I use for all old Pommie iron.
150 would be the upper limit.
A lot of people go with 180 because it is the coarsest bottlebrush hone you can get down here but I think it is too fine.

email (option): bsansw1@tpg.com.au

thanks Trev,

I'm pretty sure that's why a lot of rebuilt Trumpies here in the US smoke like blazes - too fine grit and the rings won't bed-in.

email (option): cas.vanderwoude@gmail.com

I have never really got to grips with bedding in engines.

I have rebuilt many, many chainsaw engines and just fired them up and gone to work with them with no bother at all (used genuine Stihl pots and pistons, but of late used Chinese ones, again with no problems.).

I have rebuilt bike engines, but run them in first. Usually with no problems. Though the M20 seems to mark up at one side (valve side) at the very top of the piston above the rings.

Tractors (i.e. diesels) I put them to hard work straight off. However, one of them when I put it back to haulage, oil spraying out of the exhaust. I have rebuilt it twice and same thing - all exhaust ports weeping oil (injectors rebuilt, head refurbished, new liners, pistons and rings).

Bore glazing is a real pain in the arse. Do you cane engines on rebuild or bed them in gently? Depends on the engine I suppose!

email (option): sacombsashtrees@hotmail.com

Chainsaws run seel rings in chromed or nikasil plated bores.
I now fit them dry and ride the bike like I stole it.
The 2 big problems are too fine honing and mechanics leaving the bike idleing for 20 minutes while they stuff around with the carb.
No high compression loads = no high pressure behind the rings to force them into the bores.
Thus they slip up and down , barely touching the bore and glaze it before you leave the workshop.

When you rering a chainsaw, you start it, work the throttle up, down up down for a few minutes till it is warm then WFO while you adjust the main - high speed jet and then worry about the idle.
Bike owners start the bike then spend 20 minutes adjusting the idle so it will stay running without your hand on the throttle, exactly the beast way to glaze the bore.

Put your hat & gloves on, start the bike, if it actually starts runs, jump right on and give it some full open throttle to get the gas pressures up behind the rings.
Most of us can hold the throttle open just enough to stop the bike stalling out at traffic lights.

email (option): bsansw1@tpg.com.au