Engine Seize

Hello everyone. This is my first post on this board. About 15 years ago, I restored a 72 Commando (combat motor) from the ground up. I bought a basket case but all the parts were assembled. The motor was seized up. I had the bottom end rebuilt by Phil at Fair Spares. After boring out the cylinders, Phil advised me on the pistons to install. I assembled the top end, and got the bike running. I put on about 3,000 miles and went for a long trip to Southern Cal from Northern Cal and while heading up a long grade on the 101 before Santa Barbara, one piston seized. I was at the top of the grade when it happened and of course stopped and let the bike cool down. I was able to start the bike and ride all the way to LA and back up to Sacramento. I then called Kenny Dreer and bought new pistons and if I rememeber, I had the cylinders bored out again. I reassembled everything and started riding the bike again. A few thousand miles later I was heading up I80 to Lake Tahoe, going up a long grade chasing some more modern bikes. The bike seized again. Same cylinder. Same results, made it back home and rebuilt the pistons again.

I ended up selling the bike because I couldnt afford owning more than one at that time. I sold the bike to a fellow (a fireman). Two years later he sold the bike to my best friend growing up (I kinda lost track of him). Recently, I ran into my old best friend and found he had my Norton! He wants to sell it back to me. I can now afford it, but I am leary about it seizing again. I will ride it if I buy it and I dont want to go through this whole affair again.

Anyone here know what can be done to permantely fix the seizing issues? It only does it going up a long grade. Both times I was doing about 80 mph up the hill. Other than that the bike runs very well and strong.

I would really like to get the bike back, but must have confidence that I can fix it before buying it back.
thanks much.
Mike

Any idea what piston clearance you were running? I've run into a couple cases of Norton owners installing forged pistons, like Kenny's JE design, at the same clearance as the stock cast pistons, and having seizures. My experience, and that of several others who run them, is that you need to hone the cylinders to .005" clearance for the forged pistons. It's really important with non-stock pistons to have the pistons in hand when boring and honing the cylinders, so they can be sized to match the pistons. Other than that, there are all kinds of possibilites, in particular lean mixture and/or too much advance, especially if you were running a high compression ratio. Fuel starvation at extended WOT, air leaks at intake, an on and on. I'd say you should buy it back anyway, and sort it out, but then I think you can never own too many Commandos.

Ken

Up a log grade at fewer than 5000 RPM that is the way to hole or seize a piston for sure. We often hear that the RPM is 3500 or so (old man syndrome). You almost never hear that a person letting the motor rev well into the 5000 plus range holing or seizing a piston. This fourth gear work is the hardest for our motors to do riding on the torque band and out of the HP band. If there is any tendency for detonation in the motor this is the time you will find it. A short list of questions to help get you tracking it down follows. Are you running points? or E.I. How are the slides on your Amal’s ? Or what type of other carb or carbs? Have you cleaned the bottom of your oil tank after each of these times? But most important what did the rings look like? Did they properly bed in? Rings carry the heat away from the pistons to the jug they can't do that well without fully seating. So on the part of the rings not scuffed by the incident is there evidence of a full bed in? Was there much carbon build up each time? How did the plugs read? Is there a small crack in the head that opens wider with heat? Did it burn oil at all? This will lower your octane rating many points. Detonation is what caused it but why?

Its been so long ago but I know the clearances were what was recommended by Kenny Dreer and Phil. I remember the piston was scuffed below the rings on one side near the back but off to the side just a bit. The plugs looked the same on both sides. I didnt try going with a bigger jets. Again, we are talking 15 years ago, and I dont have the bike now so I dont have much info. I do know that all the settings were what were recommended by the experts at the time. When I pulled the bike apart after it seized both times, it didnt look like it was running hot as the pistons looked good except for the scuff marks on the one side of the seized one.

I see there are more parts now available for the bike than there was 15 years ago. LIghtweight pistons, etc. All the modern bikes have nikasil coatings on the cylinders. Wonder if just upgrading to new stuff would solve the problem.
Mike

As a normal motor should not seize underthose conditions throwing new parts at it will not cure the underlying problem. As you were going up a hill at a constant speed you were running on the needle not the main jet so needle postion would be the first area to look at on carb tuning. The other area to look at is the problem with oil wet sumping that the 72 cases have, the oil return stops working at high engine revs and the oil builds up in the cases and the oil tank runs low. You say its always the same piston that seizes but not which one, if its the driveside one then this is a possibility as the oil pressure is lowest on that side and will drop here first.

The fix is here

http://www.oldbritts.com/n_c_case.html

Buy it back! It has come back into your life for a reason. You want to be laying awake at night wondering where it is?

(In the movies this is the scene where you go get the girl and kiss her...now off with ya!)

Russ

kommando said:

As a normal motor should not seize underthose conditions throwing new parts at it will not cure the underlying problem.

Click to expand...

good advice

It's common for Commandos to run lean in the mid range. That and a long hill coupled with a tad too much advance and pistons that are nice and tight . . . . .

As heat is added to them, forged pistons grow more than cast pistons. As LCRKen said, they need more clearance. Folks like to set them up too tight because they make too much noise when they are cold, before they expand to their "running temperature" size. That's why forgings need more warm up time than cast pistons, and one of the reasons that OEMs use castings instead of forgings.

I'll bet a dollar to a doughnut it's sucking air on the side that repeatedly seizes. Could be a fuel delivery issue or poor seal at the carb-to-intake or intake-to-head junctures. Your right petcock is normally the main and the left is the reserve. Would it by any chance always be seizing on the left?

Thanks for all the feedback guys. I thought the piece on the oil return modification was very interesting. I am currently dealing with a cracked case on my Ducati ST4 so after I fix this, I will consider the Norton acquisition again.

Mike