E: Engine Damaged quill

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everiman

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54 Comet. So can a damaged oil quill cause a seizure? The oil hole in the main shaft was bodged, closed up so that the quill could not easily be pushed in, not sure what happened there, bike is new to me, (hammered?) The damaged oil hole wore off some of the flat threads where the quill enters the main shaft. Piston seized while about 20 km into a ride, was going a steady 60 mph at the time, bike never went over 65. There was plenty of oil, when I took the cylinder off there was oil on the cylinder below the piston, squirted oil through the intake lifter hole, and it comes out the oil hole in the cylinder. The rubber thingys in the timing case looked OK, no blockage detected when I removed the oil feed screw with the jet. Checked ignition timing, it was close, maybe a few degrees retarded. Plug looked fine, nice tan color, no deposits that shouldn't be there. The professional mechanics I showed the piston all say lack of oil. I can't see how the quill damage will starve the Timing case, given that it is down stream, from the TC, and the end of the quill was not damaged, unless the whole thing was worn down. I guess I will have to wait for the new one to compare the two.
 

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passenger0_0

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I think the oil quill issue needs fixing but is not the cause of the seizure.

Looking at the images provided and in context of your description of the situation I would first check the cylinder liner to muff fit. I have seen a few where the liner is not touching the muff, rather burnt oil varnish is between these two which prevents heat transfer occurring. This leads to dramatic and full piston seizure like yours.
I suggest you stand the cylinder up in the oven to drop the muff off to reveal this issue.

Searching this forum under cylinder muff will provide further discussion on this matter.

Given the degree and severity seizure it might be worth considering a full engine strip to check that the con-rod is straight and that the crank hasn't twisted as a result of stopping suddenly.

Good luck in sorting this problem out. Cheers David
 

passenger0_0

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Further to my earlier post. Have a look at the front of the piston - if it is also showing seizure/scuff marks like the rear piston skirt then it's a heat related seizure rather than lack of lubrication.

When it's a lubrication failure the major thrust face at the rear of the piston goes first as this is taking the reaction to combustion forces - the highest force.
When it's a heat issue the piston grows too big for the bore and seizes on both the front and rear thrust faces. Would you mind posting an image of the front of the piston too to confirm this as you need to know what caused this problem before fixing the symptoms as it could do this again?

Replacing the piston and cylinder is not a big and I would suggest you simply buy a new cylinder with liner and muff assembled as it saves you a lot of additional fiddling. Cheers David
 

everiman

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Sh*t :) I think you called it. The motor never actually locked up, and was free after I stopped, but no compression obviously.
 

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passenger0_0

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I think the images confirm a heat seizure rather than lubrication issue and the gudgeon pin looks rather blue too which is also bit of a give away.

If it was lack of oil it wouldn't have turned over again after the event.

Look on the bright side as it could have been much worse - trust me. :)
 

davidd

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Yes, David seems to have nailed it. John Romano stuck his so hard the piston broke at the wrist pin. The Comet, being a single cylinder, will not dissipate heat well. Riding up an incline will send the temperature of the exhaust side of the piston soaring and if you do not have the clearance it is unfortunate.

In racing, we see the same problem with the blue on the wrist pin with the singles. I had Carillo take another half thou out of the wrist pin bushing and the problem seemed to have gone away.

David
 

passenger0_0

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You're quite right David to also point out the problem of having a tight gudgeon pin which can lead to many problems including piston scuffing. The gudgeon pin should always be a finger sliding fit in the piston and small end bush when both are at body temperature.

Carillo rods used to use aluminium bonze for the small end bush and these can really grip and blue the gudgeon pin if too tight. I once experienced a Carillo rod snapping at 9,000 rpm on a triumph triple (not my build) while full throttle cornering - I heard the 'ping' sound before crashing due to the subsequent engine lock up. Later examination found all three gudgeon pins blued and in expressing my inner feelings to the sponsor/owner 'lost' my ride. :) Happy days!
 

greg brillus

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I would pump some oil with an oil can into the timing side mainshaft to see if oil oozes around the big end, if not then that is where I would start looking, perhaps the oilways through the crank are blocked. Maybe the original owner knew something about the bike that he did not want to pass on..............
 

mercurycrest

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I've stuck more Comet pistons in the last 15ish years than you can count. Never had much of a problem before then. A Vintage airplane guy (Merlins, Pratt & Whitney, etc.) said they use .002 per inch these days. So By what he said .006" is what you want. I'm game for anything, my A Comet stuck while idling in a queue a few years ago after zillion miles of trouble free service. It must be todays petrol/gasoline.
 

Bill Thomas

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What make of piston was it ?, There have been some bad ones, Not good for Vin's.
Always run a bit on the rich side, To keep cool.
Run bigger clearance than the makers say.
Use the highest octane petrol you can get.
Poor old Comets , Have a hard life !.
Been out 7 times this year on mine, Love it to bits, But it's Knackered, Like me.
Cheers Bill.
 
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