Daws -

.004" is generally considered the minimum piston clearance.

 

You've already got less than the minimum clearance specifed by Triumph (0.0046").How come it hasn't siezed?Do you just ride slowly?I don't like using less than 0.005" piston clearance on a 650 Triumph.
0.010" minimum ring gap at tightest point is safe.It pays to make the ring gap on the second compression ring about 0.003" bigger than the top ring gap (stops ring-flutter and helps the top ring seal better).

 

I only put about 800 miles on the bike including some extended highway cruising, but nothing much over 65 mph. The bike had a 17 tooth rear sprocket which I am switching to a 20 tooth, so at 70 mph, she was reving pretty high. I am in the middle of a rebuild, and there was some light scoring on one of the piston skirts which is why I am concerned about the clearance. I have been told that you do not need as much clearance with cast pistons, as they don't expand as much. I would be fine having another thou honed out, but not sure if this is the right thing to do.

Thanks

 

I am in the middle of a rebuild, and there was some light scoring on one of the piston skirts which is why I am concerned about the clearance.

If you do not use an oil filter, then some minor vertical scoring of the front-center piston face simply from trash in the oil supply is to be expected. It will look like someone took a scribe and placed vertical lines about .005" wide, in maybe 30 places. That's completely normal. Like:

A seizure would leave a highly discolored area with abrupt furrows about .035" or wider on the front-center piston face. Wide enough to easily catch your finger nail in. Like:

 

Standard 650 Triumph pistons are castings;not forged.New Triumphs would occasionally sieze with factory clearance,back in the day.
Don't use less than Triumph minimum spec,unless it is highly recommended by the piston manufacturer (and they'll guarantee it if it siezes).

A little looser clearance,but still within tolerance,will usually make more horsepower.

 

I run .0045 with my pistons, and they sound loose. I wouldn't be afraid to use .004. When I say "sound loose", I mean you can hear more piston noise, more than the nice whisper of pistons in perfect spec. Nothing major.

 

Thanks for all the info on this topic. It looks more serious than just some scratches from dirty oil, although the oil was filthy when I bought the bike. I think the PO was running detergent oil with no filter. The sludge tube was clear, but lots of filth everywhere. I cleaned tons of sludge from the tank, and I will be fitting a MAP filter on the return side. As for the piston, I rubbed it lightly with a fine scotch brite pad, but it did not have the severe discoloration like the one in GABMA's picture. I have attached some pictures.

 

That looks like a partial seizure, or a seizure that happened in the past and the pistons were reused. Or did you clean them up?

 

Yes, I cleaned them up a bit, but they never had the bad discoloration that you see in GABMA's picture. Not sure if it would be a bad idea to try to use these again ????

 

Yes Dawsman, this old tart has picked up the piston for reasons that GAMBA has explained. A sorry tale, but one we see often enough.

Personally I would use these pistons. but it largely depends on how much money you want to throw around. Reuse the old head gasket too after properly annealing it.

If mine, I'd hone (or have honed) the bores with 150 - 240 grit, and ask for a lower skirt clearance of around .005". New cast iron rings. Buy Emgo or other reputable brand (there is a lot of poor quality stuff out there so be warned). Set the fitted end gap at about .012" Assemble the rings dry on the facings that meet the bore. Some WD40 is OK if the thought of a dry assembly leaves you faint.

GAMBA has rightly pointed to tuning as the probable culprit, too advanced, too lean and in your case, likely too tight.

Make sure you run high octane fuel, at least whilst running in (beg borrow or steal some leaded racing fuel if you can and mix it 50/50 with your highest octane pump fuel). Avoid lugging this motor at low revs. If the motor labours at all, use the gearbox, that's what it's for. These old girls like to spin, so 4000 rpm will not hurt it at all after the initial bedding in. Bed the rings by several episodes of up to 5000 rpm through the gears allowing the engine to return to lower rpm by natural engine braking. Do not muck around with excessive idling or low rpm upon the initial start. If necessary trailer the bike somewhere where you can stuck into it after a gentle warm up of a minute of two. No puddling about the suburbs at 2500 rpm. OK?

Bedding the rings and running the motor in are important aspects of getting the engine right and oft ignored by some re-builders.

There are two schools of thought on this. The aggressive bed-in and ride it like you stole school, the other being to follow the instruction book. The jury is still out on who is right, but you need to satisfy yourself about this. I am in the the aggressive camp.

Set PRT crush at .030" to .040", no more. Inlet tappets .004" and exh .006". the book will say less than this, but I find these settings sensible.

Retorque the cylinder head after the initial run and reset the tappets.

I'd run a formulated running-in oil for 25 miles then change to a cheapo SG rated 20w-50 oil. Buy and install an external spin on filter and you're good to go.

This info assumes you know your way around engines a bit. HTH RR

 

If it were me I'd get new pistons. That is definitely not normal wear.

 

That piston is getting hot at the crown, not at the skirt. In your case this may have more to do with timing or the carb settings than piston-bore clearance. Do you strobe time your bike? What grade fuel do you use?

New pistons would be up to you. It would mainly depend on how the bike is used. The main concern is aluminum transfer to the cyl walls.

 

Thanks again for all the great info. Yes I did strobe time the bike. Which as GABMA pointed out in an earlier post is no easy task on a
'64. Primary cover off, dial indicator, and degree wheel is the only way I know of. I have a boyer ignition, and the previous owner had a mark on the Boyer plate which my strobe timing verified as correct.
I was running 93 octane fuel.
There were quite a few other issues with the bike that could have been significant. The valve guides were in terrible shape. Lots of carbon getting into the oil, and I suspect the head was warped. I was having dark black oil seeping from around the head gasket which I could not get to stop even after re torquing several times. The PO had a '71 head with the early style rocker boxes. Not a good combination of parts. The '71 has the 4 inner head bolts that torque down directly on the head which can create a small indentation in the alloy from the washer. The inner head bolts on the '64 torque down on top of the rocker box, and can allow oil to get down into the head bolts on this later head. This in itself is not an issue unless your head gasket is not sealing properly. I would assume a warped head could have been causing some of the excessive heat.

I have since found a very nice '63 head which I have had redone with new valves, guides and springs. Can't wait to bolt it on, but I have a ways to go. Today I dropped in the new main bearings and gear box bearings. Everything went in beautifully. I made a bunch of tubular drifts and shouldered drifts for pressing in/out. I heated the cases in the oven and put all my bearings in the freezer (after my wife went to bed of course). Now if I could ever get my crank back from the machine shop, I would be in business.

Very helpful information RR. I will use your advise about running in, and let you all know how things are progressing.