Nasty Surprise

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Nasty Surprise

Postby SoloFrenos » 13 Dec 2014 15:24

Hi, i have had to strip my new rebuilt motor to cure a gearbox issue and upon removing the rear cylinder came across what looks like signs of a seizure (see photos). There are three marks on the piston one on the front directly behind the pushrods and the other two directly behind the gudgeon pin. The barrel has only been marked in two places.

The engine has only covered 30km (18 miles) since the rebuild, so i am wondering what may have caused it. It looks to me like it the barrel has not been bored correctly and as the piston has expanded it has nipped up. But i'm no expert so would welcome peoples comments. On the final journey, about 10km into the ride the engine felt like it had lost sparks and stopped, so i cruised to a halt checked a few things but then it started again and rode happily back the couple of Km's to my house.

Any thoughts anyone? Tony
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Re: Nasty Surprise

Postby mad muller » 13 Dec 2014 19:30

I take it this the engine you had difficulty with when kicking it over after rebuild , it certainly looks like seizure marks on the piston skirt, you need to check piston to bore clearance it may well be to tight , you will have to remove the piston from the conrod and with piston rings removed measure the clearance between piston and cylinder , for a rough guide use feeler gauges or take it to whoever did the rebore, check pistons were the right way round , its easy to get wrong . muller
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Re: Nasty Surprise

Postby buell1203 » 13 Dec 2014 19:58

Mad muller is correct..seizure!! Oops!!! Bugger!!!!

You might get away with cleaning up the bore and piston but experience warns that a replacement piston would be wise.
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Re: Nasty Surprise

Postby SoloFrenos » 14 Dec 2014 11:04

Hi Muller, Yes it is the same engine as i was having the kickstart problem with but that cured itself when i rebuilt the clutch with the dished plate in the 'old' position of last plate rather then first. I do hope its salvageable as they are brand new 375cc pistons so cannot bore the barrels anymore. Going to check the piston clearances this afternoon but ultimately it looks like its back to the engineers to see what they have to say :o( Looking back i guess the 'loss of spark' was in fact the engine nipping up, lucky my clutch hand has not forgotten my two stroke riding days :D
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Re: Nasty Surprise

Postby SoloFrenos » 18 Jan 2015 16:52

Does anyone know if the piston /bore clearances quoted in the blue manual are correct (0.05 - 0.055). I have now had mine measured and they are 0.0508 on the front and 0.071 on the rear, which would suggest that they are ok, but in which case i don't understand what caused the rear to seize in the first place, as it is slightly bigger than recommended.
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Re: Nasty Surprise

Postby smotorboy » 19 Jan 2015 15:19

Those clearances sound like they are more in line with a "water-cooled" engine than an air-cooled one.

Are your Cylinders "all alloy" with a coating or do they have the steel liners ??? It makes a big difference as steel doesn't expand as much as alloy for any given temperature.

The type of alloy the pistons are made with also is a big deal when it comes to expansion rates. are these Morini pistons?

I can tell you for a fact that the rear cylinder runs 100 degrees F (or more) hotter than the front cylinder. I have CHT gauges on each cylinder head of my LSR Morini and I have sheet metal air ducting and H2O misting on the rear cylinder to keep it from melting down. It should have more clearance to start with.

Last thought, cylinders are honed round then put in place and tighten down. Guess what ? They go "out of round" from the clamping pressure so extra clearance has to be added to allow for this. The wear in process makes them round again after a lot of running. High level race motors are honed with a "Torque Plate" bolted to the cylinders to simulate the clamping distortion so the bores will start out round when the engine is all bolted together .

As far as saving the pistons/cylinder, on old racing two-strokes in the 60's we used to raise up the cylinder, without the rings coming out, and lightly sand away any scuffing marks on the piston skirts. We'd then run it some more, check the skirts, and repeat the process till the scuffing ceased. We used #600 grit paper if we had it and #400 grit if we didn't. A little light oil helped the process along and controlled the dust. If the cylinder and/or the piston is deeply scored, then replacement is in order.

Good luck and let us know what it turns out to be…

Robert in California
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Re: Nasty Surprise

Postby smotorboy » 21 Jan 2015 14:15

A thought about this came to me….. The clearances you quoted seem way too tight if we are talking about "Diameter" clearances…..However, Radius clearance is another matter as that would be a doubling of the Diameter clearance and seems like it would have a chance of working.

Robert in California
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Re: Nasty Surprise

Postby norbert » 21 Jan 2015 18:50

The clearance of the nikasil cilinder/piston is much less. Your pistons are not for the nikasil cilinder. Maybe there could be a reason?

norbert
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Re: Nasty Surprise

Postby SoloFrenos » 22 Jan 2015 16:56

Hmm this is all getting confusing now, having looked in the Harglo manual, it states that the piston/bore clearance is 0.1 - 0.12mm. Completely different to what the blue book says, but would back up the suggestion that they are radius clearances in the blue book. Thanks Robert for all the info, interesting a well known dealer told me that the rear cylinder runs cooler than the front, but could not tell me what the clearance should be, which seemed very odd to me! Also interesting, exactly as you described the engineering shop that did the rebore did questioned if I had over tightened the bolts causing the barrels to go oval!

Just to clear things up these are 64mm big-bore pistons going into lined alloy barrels not nikosil bores.

I have emailed Vertex who made the pistons (both in Italian and English) asking for the clearances but as yet no reply....however they are Italian so maybe tomorrow :o)
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Re: Nasty Surprise

Postby smotorboy » 26 Jan 2015 15:17

Everything you said makes sense, except the dealer saying the Rear Cylinder runs cooler than the front…..

Harley Davidson and a lot of other Vee-twin air-cooled makes used different carb jetting (richer) and retarded timing on the rear cylinder to keep them healthy….

As the heat flowing off the the front cylinder goes right over the rear cylinder, then how can they possibly run cooler unless measures are taken to even thing up a bit.
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Re: Nasty Surprise

Postby SupermotoDave » 27 Jan 2015 18:23

Conventional wisdom is that the riders legs channels some air onto the rear cylinder but not the front, and that more oil is flung up under the rear piston than the front due to the direction that the crank turns.
David
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Re: Nasty Surprise

Postby norbert » 27 Jan 2015 22:17

SupermotoDave wrote:Conventional wisdom is that the riders legs channels some air onto the rear cylinder but not the front, and that more oil is flung up under the rear piston than the front due to the direction that the crank turns.
David


I also think that this is the reason why the morini twins normaly don´t have problems with overheating the rear cylinder.

norbert
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Re: Nasty Surprise

Postby EVguru » 28 Jan 2015 11:23

Don't forget the offset cylinders with both exhaust ports out in the breeze.
Paul Compton
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Re: Nasty Surprise

Postby smotorboy » 28 Jan 2015 14:26

Good points about the riders legs and the oil sling….

I realize that my temperature readings are not valid compared to a stock bike. It's because my legs are way to the rear and all the extra hardware is making the airflow go in different paths...
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Re: Nasty Surprise

Postby mbmm350s » 12 Jul 2018 07:52

Hi SoloFrenos,
Sorry for resurrecting an old thread..
Did you find out the cause of the seizure. I have exactly the same seizure pattern on 62.4 mm Vertex piston in standard steel liner
after 3000 miles on front cylinder only . I think the seizure pattern is almost a four point seizure at 45 degrees.
There is no sign of overheating of the crown nor any scoring above the piston rings exactly as your picture.

The rear piston is perfect but its an ASSO-werke also 62.4mm

Just an observation the front gudgeon pin is very tight, but then the piston is very distorted now,
my understanding is that an oiled pin should be able to be pushed into a warmed piston by hand.
I didn't build the engine so i assume different pistons were used because that was all there was.

Thanks
Mark
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