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E: Engine Partial Single Seizure? Safe to ride?

CoreyL

Active Website User
VOC Member
#1
I had another small adventure with my Series B Meteor recently. Now that I have 500 miles on a new piston/barrel, I wanted to do a ride and change the oil and filter afterwards. I went out with two friends and we did about 90 uneventful miles at 35-60 mph. Coming home, I hopped on the highway for the last couple of miles (I was feeling confident). I wound the Meteor up to 65 to keep up with traffic (70 mph speed limit) and, after a few minutes, I felt a loss of power (like I was out of fuel, but I wasn't). I immediately grabbed the clutch and she died. I kept rolling along the shoulder thinking: Farkin'ell, I've seized it again.

Luckily, I was right at an off-ramp and I rolled down the hill and into a gas station. I put her on the center stand and pulled off my helmet, jacket and gloves. I tried pushing the kickstart lever with my hand and she turned over OK, to my great relief. So, maybe she was just starting to tighten up and I caught it in time. I let the bike cool for 30 minutes in the shade. I was only about three miles from home so I decided to see if she would get me there.

She started up first kick and I rode home and into my garage. I drained the oil while it was still warm, replaced the oil filter with a new one, refilled her with oil and then started her up just long enough to verify return from the sump. I shut her down.

Yesterday, I verified that the ignition timing was still 34 BTDC at full advance, I verified correct valve clearance, and verified that the breather hole in the gas cap was clear. I checked compression and got 160 psi which seems pretty high even thoughI have an 8:1 piston. I also took some photos with a cheap borescope that sends images to my iPhone. I could see some light scoring of the cylinder wall (and still some remnants of cross-hatching which puzzles me after 500 miles of riding). The piston looks like it has some deposits on top (what does that look like to you?). I've attached a couple of pics.

At this point, I think I need to richen up the mixture (raise the needle one notch and go back up to a bigger main jet). I'm wrestling with whether I need to pull the head and barrel again for a closer inspection or assume it's OK to ride based on the solid compression data. Thoughts?
 

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vibrac

Well Known and Active Website User
VOC Member
#2
After I rebuilt it, my Trials Comet it seized on the A1 slightly up hill two up with trials sidecar. I caught it before full siezure much as in the above story. I let it cool then carried on the trial (well I had paid the entry fee:rolleyes: )
I used that engine for years with no more problems and with good performance including a solo track day (no not in Trials trim!) I stripped it this last winter for a full rebuild and I could see the marks of that long ago size but no blow by was evident.
 

Sakura

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#3
Had this happen several times whilst testing bought in "rebuilt" bikes for a well known classic bike dealer. If it just tightened and then freed after slight cooling it will almost certainly be ok. Just take it easy for a bit longer and only build up revs slowly and for short periods. If it happens again add a little synthetic oil to the petrol and run in for 2000 miles. Try then without the added oil.
 

greg brillus

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VOC Member
#4
piston to bore clearance not enough ...........Do not over torque the head stud nuts, 30/32 at most and re torque again after a couple of good runs and engine cooled totally first. The singles need plenty of clearance as the piston gets hot quickly, it is working harder than on a twin.
 

Oldhaven

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VOC Member
#5
Hi Greg,

From Terry Prince's instructions for his big bore kit:

"clearance measured at 10mm from the bottom of the piston

should be: .0035. For racing .0045”-.005” must be used"


His recommended spec does not specify twin or single, but do you think the racing clearance recommendation should be used on singles, especially standard displacement Comets? I think you have done a few of these. This is what I have been told by a couple of others who know their business and what I was planning. I certainly don't want to risk partial seizure like Corey did.


 

davidd

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VOC Member
#6
I spoke to one owner of a 600cc Comet who set the clearances at Terry's numbers. I don't think he will do that again. I suggested when he rebuilt the bike that he use 0.006". I don't think many singles will work with less clearance unless they only do a run to the pub.

With Nicasil aluminum bores I set it at 0.0035" for the single racer. They normally recommend 0.001".

David
 

chankly bore

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#7
What carburettor, what jet size and what needle position? Also check for leaks at the inlet stub. I never run less than .005" bottom skirt clearance on a single and I think also that modern "petrol" burns hotter as well as quicker. When going up a long hill in top it is always better to drop down a cog and let the jigger spin faster as I found once to my detriment.
 

CoreyL

Active Website User
VOC Member
#8
piston to bore clearance not enough ...........Do not over torque the head stud nuts, 30/32 at most and re torque again after a couple of good runs and engine cooled totally first. The singles need plenty of clearance as the piston gets hot quickly, it is working harder than on a twin.
I'm running 0.004" clearance and torquing to 32 ft-lbs.
 

CoreyL

Active Website User
VOC Member
#9
What carburettor, what jet size and what needle position? Also check for leaks at the inlet stub. I never run less than .005" bottom skirt clearance on a single and I think also that modern "petrol" burns hotter as well as quicker. When going up a long hill in top it is always better to drop down a cog and let the jigger spin faster as I found once to my detriment.
It's running a 32 mm Mk1 Concentric (K&N filter), #3 cutaway, 0.106 needle jet with the needle clip in the middle groove. I've got a 260 main jet in there now. All the flanges have been made flat and flush (no air leaks). I plan to raise the needle (clip in lower notch) and go up in main jet to 280 or 300 before I take it out again. It's also got a SuperTrapp "muffler". I'm running a low expansion cast Omega piston.
 

davidd

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VOC Member
#10
Corey,

I think that 0.004" is too tight on a Comet with iron liners. If that is the case (and some may disagree) then the clearance is still too tight. The Omega is a fine piston. The carb seems plenty rich. I think that all was well except that the piston got too large from the heat it generated.

I am inclined to say "pull it apart now, get a new set of rings, hone it for 0.006" clearance, clean up the piston a bit and ride it." If you are worried about bedding in the rings run some break in oil from Jegs or Summit. It should run like new and continue to run well (assuming the piston is just a little scuffed).

If you continue as is, another seizure may occur and damage the piston or bore a bit too much and that gets expensive. Whatever you choose, best of luck.

David
 

greg brillus

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#11
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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VOC Member
#12
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

Well Known and Active Website User
VOC Member
#14
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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VOC Member
#15
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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VOC Member
#16
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

Well Known and Active Website User
VOC Member
#17
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

Well Known and Active Website User
VOC Member
#18
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

Well Known and Active Website User
VOC Member
#19
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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VOC Member
#20
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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