Seized Pistons

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sparkplug

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Jan 10, 2006
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Hello:
As I've pointed in other threads, Madrid is just too hot in the summer. This added to new piston rings (and not too much patience...) has made my engine melt literally!
I'm going to rectify the cylinders and place new pistons, but now I have to get into all this trouble, what else do you think I should do to get everything well done? I've thought of pistons bearings and valves...

How can I take the cylinders off? (they're welded to the pistons!)

Thanks for helping! :wink:
 
Is it as bad as it sounds? If it's done anything more than "nip" up then I think that a full engine strip is needed. I've seen seized Comandos on the track and they will usually free-off when they cool down.

I've never had to do it but if the motor is not at TDC, you may be able to raise the cylinder and pistons high enough to part the crankcases and unbolt the big ends, leaving you with pistons and rods still in the cylinder.

You can then try a bit more force or alternatively drill the piston crowns to remove as much material as possible and then attack the skirts with a Dremel or some such until the pressure releases.

In the meantime, some sort of penetrating oil in the combustion chambers would not do any harm.
 
Thanks, 79x100.
It's really bad: I've waited an hour and a half for the pickup truck to come and I've tried to move the engine several times meanwhile without results. I'll pour oil into the chambers while new pistons come...

What is a TDC motor?

Thanks again!
 
Sorry, TDC just means "Top Dead Centre" - simply that the pistons are at the top of their stroke. If that's the case, I don't think that you will be able to lift the cylinder enough to part the crankcases.

Are you sure it's a piston seizure ? How suddenly did it stop? Did you run out of oil ? How hard were you thrashing it ? :evil:

Most piston seizures are just restricted to a highspot and can be freed off.

You don't have an oil tap that you forgot to turn on do you ?

I would certainly take the rocker covers off to make sure that it hasn't dropped a valve in or something, Check the spark plugs for alluminium deposits and perhaps the timing cover off to make sure you haven't got a loose pinion or something there.

If it makes you feel any better, my 850 rattled to a stop with a stellite cam follower foot taking chunks out of the components within a month of buying it ! Even worse, all four followers were intact. Some twat had replaced them without looking for the broken bits :?
 
Sorry I just didn't read it right... :oops:

I'm not sure it's a piston seizure, but I guess so... It stopped after a second of making a mecanical strange noise. Just like that!
Oil was 20W50 fresh (changed 15 days ago). It all happened after about a 15 mile ride, I was passing a car at no more than 4500 revs... Back wheel got stucked and quickly grabbed the clutch to reach the road side.
New piston rings weren't still ridden enough and today temperature has risen up to 44ºC (that means that every part of my body that was uncovered is now barbecued)

I don't have a tap, but I do have a non-return valve I really believe to work fine (at least it has for the last few months...) Actually I was riding and thinking: what a good torqueing work I've done to this cylinder head, not an oil drop in the whole trip! (maybe there was no oil getting there at all? :? )

I'll dismantle everything next week as I'll be on vacation and check in there. I'll keep you informed with the results.

You're really helful!
 
do people ever apply piston crown / piston skirt treatments to their commandos? it's not the most expensive thing (~$100 for both pistons) but i have no idea if it is worthwhile on these engines... (i am new to commandos, my newly acquired 1971 is getting its tank sealed as we speak!)

an article on ceramic coating (to the piston crown):
http://www.strappe.com/plasma.html

a place with some pricing info so you have a basis for comparison:
http://www.swaintech.com

in short, ceramic goes on top to help keep the piston from absorbing heat from combustion, and the piston skirt treatment helps the piston retain oil...
 
Sparkplug,
Did you check the ring gaps before you put the new rings in the motor? They often need a little bit filed off to get the correct gap which is an absolute must before you put them in an engine.
 
No, I didn't... Never heard I should do so... anyway rings had made good 600 miles before yesterday and seemed to be fine.

How is it done?

Thanks.
 
You check the piston ring gap by putting the ring in the cylinder (I normally push them down the bore a bit using an old piston to make sure they are straight) then simply check the gap between the two ends with a feeler blade. Manufacturing tollerances on our old clunkers are much greater than new bikes so the rings often are slightly larger than they need to be. To trim them down simply hold one side carefully in a vice near the gap and carefully file it with a small fine file, then recheck the gap.
 
My guess is that you have a lubrication problem rather than a ring gap problem or, perhaps a rod bearing failure. You mentioned that a noise preceded the failure, was it a knocking noise?
 
I'm inclined to agree with Jason. I've never had piston troubles on an 850 Commando. Components always seem good and oily on dismantling.

I have, in the dim and distant past had lots of experience of seizing two strokes and rather than hearing mechanical noise, there is suddenly a complete absence of noise.

Could it have dropped a valve in ? or let a broken piece of throttle slide catch under a valve head ? There are all sorts of mechanical things which can lock a motor solid.

If it was me, I'd have had the head and cylinder off as soon as they were cool enough to touch ! :)
 
After dismantling the engine, what I've found is that a con rod was broken into peaces, whipping the whole thing inside the crankcases and breaking part of the barrels, valve heads, etc, etc... before the rear wheel stopped turning. So I'm thinking of rebuilding the whole engine myself or asking someone to do so. As this is something I've had in my mind before, I always thought I'd ask Mick Hemmings to do it (as I told L.A.B. a few months ago). Do you think this would be a correct choice?
What could happen if I safe my money and do it myself? (Im not an expert, but I believe I could manage to do it... :? )
 
Mick Hemmings is a very sound choice to rebuild motor, particularly if you have had a blow up like that.
 
:( Bad luck, Sparkplug.

Still, at least it didn't let go at high revs. Sounds as if you still have your crankcases and you haven't gained a modified frame cross tube.

Have you worked out what happened ? Did the rod loosen it's bolts or seize ? Old damage on the leading edge is often a starting point for a break.

If you don't have previous experience of the engines, with the ability to check everything, I think that you would be well advised to call in specialist help on this one.
 
Could have been some sort of lubrication failure that caused the conrod to break, or maybe just bad luck but I am sure Mick Hemmings would do a first class job of rebuilding your engine, and could be worth the extra money that it will cost to have the rebuild done professionally if you do not want to try it yourself.
 
79x100 said:
:( Bad luck, Sparkplug.

Still, at least it didn't let go at high revs. Sounds as if you still have your crankcases and you haven't gained a modified frame cross tube.

Have you worked out what happened ? Did the rod loosen it's bolts or seize ? Old damage on the leading edge is often a starting point for a break.

If you don't have previous experience of the engines, with the ability to check everything, I think that you would be well advised to call in specialist help on this one.
Click to expand...

I don't know what could have caused such a damage... My father, who was a Bultaco semi-professional racer in the sixties, got shocked when he saw how everything looked after all this happened! :shock:
 
Bad luck on the engine blow-up, whichever option you decide to pursue make sure that you thoroughly clean the oil tank and all the oil lines prior to putting the engine back in, there is almost always contamination of these parts after a major blow-up. Also make sure that you have a good supply of fresh oil going through all the hoses before you start the new engine. You would be well advised to fill the crank shaft with oil and fill the rocker feed pipe with oil before starting. Rolling the bike around your driveway with the plugs out, fuel off and the bike in gear is a fairly lazy way of getting the oil flowing to all the right places.
 
You have my sympathy. My engine blew nearly 4 yrs ago when a NEW (less than 8000km) rod bolt broke. I had it examined and where it broke there were signs of an old crack on the narrowed diameter with oil staining. I could not be completely sure that was the cause but the supplier did say there had been a batch of bolts he would not be happy with! When asked who supplied them I told him "You did". That ended the discussion.
It may or not be necessary but it is a good idea to polish bolts on the rough machined diameter to remove any machining marks where cracks could start.
If you saved the cases you are lucky. I lost the front of the LH case, rod, piston, exh valve and expensive Steve Maney aluminum barrels. The explosion (at 110k in 3rd gear at 5000 rpm) also bent the front ISO bolt and pushed the LH frame rail forward by 7mm. The crank was marked but survived without getting bent or cracked. Even so I probably should not be using it but could not afford new or find a good used crank.
Yes, Mick Hemmings would be a good bet.......buena suerte!
 
Is the big end of the rod (or what is left) black? Is the crank journal scarred? Is it the timing side rod? If the answer to these is yes, then be certain to carefully examine the oiling system when you rebuild. Take the crank apart and thoroughly clean the sludge trap. Look for little balls of RTV silicone. This has a nasty habit of clogging crankshaft oil passages. I strongly suggest using anaerobic sealants for case mating and avoid RTV anywhere there is circulating oil.

You mentioned you have a non-return valve in the oil line. I would carefully surface the oil pump, make sure the spigot seal fits properly and put the valve on a shelf.
 
Ron L said:
Is the big end of the rod (or what is left) black? Is the crank journal scarred? Is it the timing side rod? If the answer to these is yes, then be certain to carefully examine the oiling system when you rebuild. Take the crank apart and thoroughly clean the sludge trap. Look for little balls of RTV silicone. This has a nasty habit of clogging crankshaft oil passages. I strongly suggest using anaerobic sealants for case mating and avoid RTV anywhere there is circulating oil.

You mentioned you have a non-return valve in the oil line. I would carefully surface the oil pump, make sure the spigot seal fits properly and put the valve on a shelf.
Click to expand...

Ron:
Have you got into my garage during the night and seen the bike or what? :wink:

It's exactly as you describe: previous owner had put silicone EVERYWHERE. Fortunately it was red, so I could see every spot and remove it everytime I worked on the bike (or at least that's what I thought) The broken conrod is the one you point at and the answer is yes to all... :shock:

I'll keep away non return valve, hoping the engine rebuild will decrease oilleaks and turn the old engine into a more cohabitable one.

Thanks a lot to all of you for the help.
 
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