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Hi Folks,
My 81 Bonneville could do with a rebore as compression is down on the LH cylinder.
I fitted new std rings when I had the head off to be rebuilt with new valves and guides but it really needs a rebore.
I am looking for recommendations for which new pistons to buy here in the UK.

Regards
 

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I would be happy with L.F.Harris pistons. I am using Federal Mogul pistons at the moment as they came with my 750 kit. These are fitted with Harris rings which do not seem to allow oil burning.
 

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Hi [email protected], I've been involved with 3 rebores in the last few years.

2) T140. Used LF Harris pistons, rings, clips, pins that came in kit. Both were very close in weight & diameter. Measuring pistons in a few boxes they were very true to size. Basically spot on. The casting replicated original pistons pretty much exactly. Supplied rings were different as original had what looks like chrome 3 piece, while LF Harris had 1 piece oil ring. Clearance .0045" Hone 220 grit. Used dedicated break in oil. 40w. So far has worked perfectly no oil consumption or smoke at all. No piston rattles. These came in the Green/white box with LF Harris printed on the white. I think that is the latest production. I don't know who made them. JCC??

Put LF Harris set in 650. Again very true to size. These pistons were cast differently than original AE (Triumph) factory piston. Again .0045 clearance, 220 hone. Used break in oil. No oil consumption or smoke.

All 3 of these did not smoke or use oil from day one. A puff of smoke on first start as expected, then none again ever.

I was not directly involved with this one. Another friend used Emgo pistons. Different casting from AE (Triumph) or LF Harris. Measuring, weighing several sets they also were spot on. Used rings etc. supplied with pistons. Break in oil used. Has turned out perfectly also. Clearance .0045" 220 hone. I don't know who makes these pistons.

Riken ring makes a set that replicates original T140 3 piece oil ring. I have no idea which lasts longer in real life. I have not had '80 motor apart, so I don't know exactly what oil ring they used.

In my area I've seen many failures to break in not using break in oil. Smart money is use break in oil. Morgo & others sell it. I've only personally used Torco TBO 40w break in oil in all these motors. However others ave used other brands dedicated break in oil, to same perfect results. Use weight appropriate for your temperatures.

I assemble pistons, rings with lots of break in oil. Do not spare oil on assembly. No need to think about dry assembly or break in. I use assembly lube on cam lobes. You can use assembly lube or just oil on rod inserts. I don't find it matters. Before first start I always put teaspoon of oil down spark plug holes. Kick over a few times. This assures good lube of pistons, rings & a good oil seal on rings. Install spark plugs. Get jacket, helmet on & start bike. Look quickly for oil light to go out. Lift seat verify oil return. Briefly look for obvious oil leaks etc. Go for road test. I don't like to let bike set & idle any longer than needed. Go for several blocks. Pull over & look for leaks. If good keep riding. I like to do a good 30 miles for the first ride unless problems arise. Trim up carb adjustment. Check ignition timing.
Let cool overnight. Torque head, check valve clearance. Go for a really nice fun ride.

Read owners handbook for how to break in new motorcycle. Nothing special to do. Basically ride like the book says. For the first 30 miles or so I keep it under 60 mph. Then progressively go faster. I keep in below 5000 for first hundred miles or so. Then progressively faster.

The beauty of dedicated break in oil is you can do a full wet assembly with out worry about ring seating. So you don't have to worry about piston or ring scuffing at start up do to lack of initial lube.

I run break in oil 3-500 miles. Then use oil of your choice.

Do you have real oil filter? I most strongly recommend a real oil filter. For '81 Motao in frame filter is a good choice. Or the Charlies type, which is same but different base plate. Change element with every oil change.

I will use LF Harris pistons when I do my overhaul on my personal bike. In my experience the quality is there.
Don
 

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Hi, Forgot to mention, on all 4 motors the pistons came in a 2 piston set box. So maybe factory matches pistons in the set box?? They were within .2g gram of each other. Way closer than factory pistons were.
Don
 
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