E: Engine Partial Single Seizure? Safe to ride?

greg brillus

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The 600 comet I did recently, we gave it 0.006" after we installed a new liner and rings, as over size pistons are not readily available. It is also worth noting on the top end kits supplied from Terry that the liner should be lapped into the head recess, even if the liner/muff has been assembled, and although you cannot rotate it much you still can enough to work. The tooling marks from where the CNC machine has cut out the liner recess in the head leaves an overlapping area that can and will leak over time. I still go to the trouble of running the oil feed to the rear of the skirt on all engines I do, as it cannot do any harm, even if you use a restrictor disc in the timing cover just on the cylinder feeds. On big bore engines, just dremel the oil feed gallery back around the case mouth to center it at the rear again.
 

oexing

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I´d say 5 thou should be plenty of clearance for a 84mm simple piston type in a cast iron linered alu cylinder. That is what I´d go for when installing in an all cast iron cylinder with less heat growth compared to an alu cylinder. Judging from the picture I suspect a poorly honed bore, typical for Sunnen hones without the specified hone oil, very critical. Anyway, I don´t like the Sunnen types with two grinding stones plus two felt wipers, simply prone to chatter marks - and this is what I had with two E-Type pistons. Admittedly I specified way too little clearance, stupid then, but also got an engine block with out of round bores. You cannot measure this with a two point internal mike, only to find out with three point mikes - just. But I honed the block myself with a production hone from then former Zündapp works and could definitely feel the chatter marks in first minutes of hauling out another two or three thou. So since many years the 3.8 E-Type does great with 0.14-0.15mm clearance, old type pistons.

Vic
photo5 174.jpg



Ahmm, maybe I should not publish this photo - but it came out allright and the engine has run many tens of thousand kilometers ever since, engine in car and crank in there while hand honing :

Honen E-Type.jpg
 

vibrac

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We always raced with six thou if I ever built a road only engine I might go for five. Wear with the milage people do nowadays is not a problem. in the scheme of things a rebore and piston is not a big deal, compared to some old makes there are plenty of pistons.
 

erik

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this was my first thought ,too when i saw the Pictures.because the area where the scratches are is so linear.the Piston itself would have left other marks on the liner as we can see above on the jag Pistons.
 

greg brillus

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If the ring gaps have closed up you should see the ends of the rings with a polished look to them. If any of you are running the squish band top end kits with twin plugs you need to be absolutely sure of what ignition system you are running, if it is not right you can be sure of a piston failure. There is a "lot more" involved with some of these bolt on improvements than one might at first think.
 

mercurycrest

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The piston top looks like it was detonating, if it were mine I’d pull it apart for a good look. I’d also recommend .006 inch clearance, especially in Texas. I wouldn’t raise the needle and go to a bigger jet all at once. Make one change at a time, then you’ll know where you’re at. :)
Cheers, John
 

davidd

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John,

That is great advice, mostly because I agree with it!

I think there may be some detonation, but I am not certain that the detonation is causing the seizure. I think it might be the other way around. I think the high friction between the piston and liner is raising the temp of the piston to a pliable stage and the piston itself may be igniting some of the fuel.

I think more and more that Comet owners are running too much advance, but when it was discussed here, the consensus here was the more advance was OK.

David
 

Albervin

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I had a similar event over 30 years ago on my first Comet. Never blew smoke so I thought all was good. I adjusted the timing first. It was too advanced. I then raised the needle and rode it for over 20 years. If you are not blowing smoke or pumping oil then proceed carefully. Put some Xcel plus in the oil maybe. https://www.xcelplus.com.au/products.htm I use 5 thou clearance.
 

Black Flash

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I have a TP 600 cc top end kit and used his figures. I ran the bike a total of maybe 100 miles. One hot [about 32 C] day I made another test ride going up a hill, changed down to let the engine rev with little load and it seized very very badly.
I got a new piston and hat the top ceramic coated, to help the bore hove more time to expand before the piston gets really hot.
also I had the bore rehoned to 0.006 clearance. Also the ring gap was to little. You could see polished points at the faces after dismantling.
I now wait for my 8 inch 4ls brake to be finished and have another go on the bike.
I have less concern with ri g control due to more clearance than seizing the engine again.
in a Vincent the barrel is deeply spigotted and the piston spends a lot of time in the crankcase, rather than being cooled.
 
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