engine seize mystery riddle

rx7171 · Dec 11, 2015

rx7171 said:
>>And after pulling the head, the cylinder bores look nearly perfect. No evidence of scoring or wear at all. I measured them with a bore gauge and they're dead even top to bottom, and both sides are the same.<<

If the bores are stock diameter then the pistons if new will not be oversized and should not create a possible piston seize due to too tight a clearance.

Is it possible to read any stamped oversize or STD mark on the piston tops, which would help to inform if a possibility of a too tight fit.

Triton Thrasher · Dec 12, 2015

rx7171 said:
If the bores are stock diameter then the pistons if new will not be oversized and should not create a possible piston seize due to too tight a clearance.

Measure pistons and bores, don't just read what's written on cardboard boxes.

kommando · Dec 12, 2015

And after pulling the head, the cylinder bores look nearly perfect. No evidence of scoring or wear at all. I measured them with a bore gauge and they're dead even top to bottom, and both sides are the same.

Together with the running on the centre stand being revved at 3000 rpm and then the engine stalling I doubt this is piston seizure, its a possibility but there is something else in play as engines do not seize when there is no load but under load on an incline.

chrismay · Dec 12, 2015

you might check the heat range of your spark plugs. I had a similar problem on my 850 when rejetting the carb. Changed plugs and rings everything was fine after that.

eskasteve · Dec 12, 2015

I too had this same type of mystery seizure back in 1980. It was with a shop done fresh top end on my Combat. Before I ripped into it I called the shop. They had me look into the exhaust ports at the pistons for any signs of scuffing. None. Then the question and answer period. How many miles on the bike? Around 55,000. How is the bike stored? Unheated garage, carport or girlfriends driveway. How is it used? It was my only means of transportation in the always wet Seattle area.
The quick check that they had me do was to run a jumper to the coil pack and bypass the ignition switch. Just like a previous post it worked fine. When the ignition cut out the higher compression ratio of the Combat would lock things up.
A thorough cleaning of the switch and the problem never resurfaced again. Can't say if that's what's wrong with yours but it's a fairly low effort thing to check.

seattle##gs · Dec 12, 2015

One time I had a Triumph TR5T 1972 dual purpose bike. I did a top end and it seized about a mile from the house. We double checked the bore and the pistons, the carb setting, and anything else we could think of. Seize, seize, seize. After several attempts we finally honed the crap out of it to give it more clearance. ( I know, now, about fresh hone and old rings...) but there was no change in it's willingness to seize. So we gave up and rode it untill it seized and let it cool off, not caring 2 cents anymore about the top end. Never did solve the problem. I wonder if Hepolite had some quality control problems?

Also, another bike, I had some Japanese pistons show up with no taper....first and only time.

concours · Dec 12, 2015

mkmachine said:
I finally had a chance to take it apart this past weekend. The oil pump is practically new. And after pulling the head, the cylinder bores look nearly perfect. No evidence of scoring or wear at all. I measured them with a bore gauge and they're dead even top to bottom, and both sides are the same.
I haven't had a chance to pull the rest of it apart, but now I'm stumped.
If the pistons were seizing shouldn't there be wear on the bores?
The problem has something to do with heat and rpms. What else could make an engine seize?
I'm ruling out drivetrain and primary, because I know all of that to be working.
So, before pull the barrels and split the cases, any suggestions for things to look for?

thanks,
mk

Seeing as how your already here, another 30 mins with spanner & feeler gages would rule out someone having left the ring gap too tight.

kerinorton · Dec 14, 2015

I haven't seen any evidence here of removing the barrel to look a the pistons. Stop guessing. Remove the barrel and look. If the pistons are seizing there will be signs on the skirts. You may not see any marks on the bores.

Dereck

mkmachine · Dec 14, 2015

Thanks for all the suggestions. I unfortunately don't have time to work on the bike every day, so it takes me a while to make progress.
I pulled the cylinders the other day and measured everything as best I could. The pistons are .040 over. The numbers look ok. The barrel is about .004 larger than the piston, with a caliper, I need to get a proper mic on it. I checked one ring, and the gap was about 12 thou, wich is ok. But the pistons look a little suspicious to me. I'm going to try to upload a couple pics here:

Rohan · Dec 14, 2015

4 thou is bordering on being on the tight side.
Is that at the bottom of the skirt, at 90 degrees to the gudgeon pins.
(if its measured anywhere else, that is too tight).

Those skirts are showing signs of grabbing, and have a strange pattern to them.
One spot there doesn't look good at all...

If that was mine, I'd be thinking of putting a light hone through it.
Remember. Too tight = not good, a bit loose = never seize.

concours · Dec 15, 2015

Are those the "Sure-crack" type pistons?

Those tiny scuffing marks are not the engine stopping seizure you were looking for. :idea:

JMWO

texasSlick · Dec 15, 2015

concours said:
Are those the "Sure-crack" type pistons?

Those tiny scuffing marks are not the engine stopping seizure you were looking for. :idea:

JMWO

IMO +1 ..... on both points

Slick

johnm · Dec 15, 2015

concours said:
Are those the "Sure-crack" type pistons?

Those tiny scuffing marks are not the engine stopping seizure you were looking for. :idea:

JMWO

I cant tell if they are the "Sure-crack" type pistons. (look at Tech talk piston differences at the Les Emery site http://www.norvilmotorcycle.co.uk/ )

But yes to the seize marks.

And what are all the black spots? Are they actually marks in the aluminium or just something on the surface?

To be honest with the scratch marks as well it looks like they were kicked around the workshop floor a bit before installation. :?

Triton Thrasher · Dec 15, 2015

That piston has seized, recently.

Mark · Dec 15, 2015

^ I agree

Paddy_SP · Dec 15, 2015

I might be wrong, but it looks to me as though it's seized before and someone has tried to ease the marks off with a file...

Triton Thrasher · Dec 15, 2015

Paddy_SP said:
I might be wrong, but it looks to me as though it's seized before and someone has tried to ease the marks off with a file...

Yes, the apparent horizontal marks look strange. Couldn't be a camera flash effect or something?

Fast Eddie · Dec 15, 2015

I also think these are seizure marks. An engine can nip up very tight, but free off when cool leaving marks that don't appear very severe.

The horizontal marks could be the result of filing, but my money would be on them being passed over a linishing belt to 'clean them up'.

All in all, with seizure marks, strange black spots, probable earlier linishing, etc, I would invest in a pair of new pistons and correctly bored and honed barrels if it were mine.

mschmitz57 · Dec 15, 2015

concours said:
Are those the "Sure-crack" type pistons?

Those tiny scuffing marks are not the engine stopping seizure you were looking for. :idea:

JMWO

I agree. Old piston skirts have all sorts or odd wear patterns, marks from shop wear, ring compressors, and those pistons could have come from another old motor.
And there's no matching marks on the bores. I would expect to see some aluminum smearing up by the taper near ring grooves and some debris on the scraper rings.
Of course you've come this far. Might as well hone it and move on.

kerinorton · Dec 15, 2015

Paddy_SP said:
I might be wrong, but it looks to me as though it's seized before and someone has tried to ease the marks off with a file...

I would agree with you [ although emery paper would probably be the culprit ].
Mic up the pistons and get someone WHO KNOWS HOW, to mic up the bore,s then you will find out the true clearances.

Dereck

engine seize mystery riddle

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