Soft Seize?

comnoz · Sep 2, 2014

The only Commando pistons that would run at .0033 clearance was the old Hepolite piston with the slot under the oil ring. That slot was a dam that kept the heat in the top of the piston from traveling to the skirt and making it expand.

Any of the non-slotted cast pistons regardless of the manufacturer are going to need somewhere around .0045 clearance to avoid "tightening up" {seizing} when hot. Always better too loose than too tight when your talking piston fit. Jim

Fast Eddie · Sep 2, 2014

comnoz said:
The only Commando pistons that would run at .0033 clearance was the old Hepolite piston with the slot under the oil ring. That slot was a dam that kept the heat in the top of the piston from traveling to the skirt and making it expand.

Any of the non-slotted cast pistons regardless of the manufacturer are going to need somewhere around .0045 clearance to avoid "tightening up" {seizing} when hot. Always better too loose than too tight when your talking piston fit. Jim

Regarding 'better too loose han too tight' ... prior to strip down, my motor was very mechanically quiet, no audible piston slap, no smoking, good compression... when measured there was circa .010" clearence. Im not saying this is what we should aim for of course, but it did make me question the quest for super tight clearences.

ashman · Sep 2, 2014

comnoz said:
The only Commando pistons that would run at .0033 clearance was the old Hepolite piston with the slot under the oil ring. That slot was a dam that kept the heat in the top of the piston from traveling to the skirt and making it expand.

When I did my first rebore on my Norton many many years ago, we did the final hone to bore clearance at less than .0033 and of course they were the old Hepolite pistons, it started up first kick it was in the middle of winter so the first ride was very early in the monring before sun rise and very cold, I did a 100 mile ride taking it very easy, I remember it well as I was so cold and I never had any problems with it nipping up and I still have that same bore and pistons in the bike now, it doesn't blow any smoke or use any oil at all and I run a Lochart oil cooler all year round, my motor still pulls like a train.

So I don't think he had a soft seize at all, if his motor nipped up it would not start up straight away and would have to wait for sometime to cool down, so I think his problem is somewhere else.

Ashley

comnoz · Sep 2, 2014

ashman said:
comnoz said:

The only Commando pistons that would run at .0033 clearance was the old Hepolite piston with the slot under the oil ring. That slot was a dam that kept the heat in the top of the piston from traveling to the skirt and making it expand.

When I did my first rebore on my Norton many many years ago, we did the final hone to bore clearance at less than .0033 and of course they were the old Hepolite pistons, it started up first kick it was in the middle of winter so the first ride was very early in the monring before sun rise and very cold, I did a 100 mile ride taking it very easy, I remember it well as I was so cold and I never had any problems with it nipping up and I still have that same bore and pistons in the bike now, it doesn't blow any smoke or use any oil at all and I run a Lochart oil cooler all year round, my motor still pulls like a train.

So I don't think he had a soft seize at all, if his motor nipped up it would not start up straight away and would have to wait for sometime to cool down, so I think his problem is somewhere else.

Ashley

Click to expand...

Less than .033 clearance? -I guess it depends on who is measuring it. It might be possible if the motor is never run very hard before the bore wear gives enough clearance -many thousand miles later.

I have certainly had my share of "soft seizes" over the years that started again within seconds -including the mild seize last year that ended up with a collapsed piston 3500 miles later. The only indication I had of the seizure was a loss of power and then when I pulled the clutch it stalled. A few seconds later I released the clutch and continued on with no noticeable problem. A couple hundred miles later you could hear the tapping at idle but not at running speed. After 2000 miles it was tapping all the time. 3500 miles later the piston skirt broke off because of the excess clearance. Jim

PS -I knew what was coming because that was not the first time.

Biscuit · Sep 2, 2014

To answer your question exactly, "Has anybody else had this happen" I can say "yes, yes I have". Years ago I replaced piston with +.020 and did the exact same math as you did regarding the smallest bores compared to the largest piston. Set my final clearance at .0035" looking for that extra tight engine. Got around 1000 miles out of it before the same exact symptoms of a light seizure. I was pulling a long hill in the White Mountains of NH in top gear at around 4000-4500 RPM. Weather had changed from warmish at the bottom to pretty damn cool at the summit. I think the engine had generated a pretty fair amount of heat and was happy enough until the temp dropped. I believe at that point the cold air shrunk the hot barrels fairly rapidly and the pistons not being able to respond got squeezed. I too could let out the clutch and have the engine run again. It was a bit louder though. The bore had aluminum transfer on it from the pistons but not any real damage. New +.020 pistons and the bore honed to give .0045" proved to be all I needed.

Hobot · Sep 3, 2014

How can we-me make sense out of such a range of clearance experiences, ugh. Some do, some don't and some its just can't tell.
I've lost sleep on Peel clearance decisions with JMS kit so with piston skirt thermal swelling and cooler less expanded lower bores in mind I had Left Coast Racing do em ancient aircraft style, tapper bore top .0055 to .0065" bottom. May be a noisy loose as a 19911 but should keep on firing hot.

motorson · Sep 3, 2014

That seems like a cool idea, Hobot. Sort of a wedge effect compression! Should make the power stroke more effective too since the piston is racing toward loose after being wedged in tight. :lol:

I'm still processing all the input from everyone. I have a call in to the shop that fit the pistons to see what they say they fit them to. I had left a paper in the box that asked for .0035 but my measurements said less when I got them back. I am chasing up a possible ignition problem and looking into fuel delivery as well. If the shop claims that they fit them to the above dimension and my measuring equipment is questionable then I will opt for a very long break-in after making sure the ignition and mixture are good.

By the way, I really appreciate all the advise from everyone. Thanks!
Dan.

concours · Sep 3, 2014

motorson said:
That seems like a cool idea, Hobot. Sort of a wedge effect compression! Should make the power stroke more effective too since the piston is racing toward loose after being wedged in tight. :lol:

I'm still processing all the input from everyone. I have a call in to the shop that fit the pistons to see what they say they fit them to. I had left a paper in the box that asked for .0035 but my measurements said less when I got them back. I am chasing up a possible ignition problem and looking into fuel delivery as well. If the shop claims that they fit them to the above dimension and my measuring equipment is questionable then I will opt for a very long break-in after making sure the ignition and mixture are good.

By the way, I really appreciate all the advise from everyone. Thanks!
Dan.

Please keep us posted.

tricatcent · Sep 3, 2014

Sometimes this type of power loss can be due to not enough valve clearance. It has only happened to me on a Honda, that I was just riding to hard and it got too hot, and also on a Triumph Bonneville that had very recently been re built. On the Triumph the valve seats had been re cut, and they must have been soft. The valves receeded into the head eliminating the valve clearance. Once the engine cooled it was OK again for a little while. Once I adjusted them again they stayed good for a very long time. Maybe that is your problem also.

Nigel

batrider · Sep 3, 2014

That will be a hell of a long break-in then. I'm still on my original bore at 38K miles. The cylinder wears like iron!!!! I'm on my second set of pistons because the ring lands wore out, but still on the standard cylinder bore.

Russ

Beach · Sep 6, 2014

comnoz said:
The only Commando pistons that would run at .0033 clearance was the old Hepolite piston with the slot under the oil ring. That slot was a dam that kept the heat in the top of the piston from traveling to the skirt and making it expand.

Any of the non-slotted cast pistons regardless of the manufacturer are going to need somewhere around .0045 clearance to avoid "tightening up" {seizing} when hot. Always better too loose than too tight when your talking piston fit. Jim

Jim...I am fitting some of the pistons with the slot under the Oring to my 750 engine ...I was thinking even though clearance could be less, to just do the clearance at .0045 to be on the safe side...What are your thoughts?

Triton Thrasher · Sep 6, 2014

Beach said:
Jim...I am fitting some of the pistons with the slot under the Oring

Aren't they the pistons that break in half?

comnoz · Sep 6, 2014

Beach said:
comnoz said:

The only Commando pistons that would run at .0033 clearance was the old Hepolite piston with the slot under the oil ring. That slot was a dam that kept the heat in the top of the piston from traveling to the skirt and making it expand.

Any of the non-slotted cast pistons regardless of the manufacturer are going to need somewhere around .0045 clearance to avoid "tightening up" {seizing} when hot. Always better too loose than too tight when your talking piston fit. Jim

Click to expand...

Jim...I am fitting some of the pistons with the slot under the Oring to my 750 engine ...I was thinking even though clearance could be less, to just do the clearance at .0045 to be on the safe side...What are your thoughts?

They could be fitted tighter but I would not recommend using them. They will eventually break and may cause major damage when they do. Some broke right away and some lasted quite a few years. How lucky do you feel. Jim

Hobot · Sep 6, 2014

Yep taboo slotted pistons was how I lost a Trixie the 2nd time, after deer strike recovery took couple years for me and her, then on timid return of home of first use after recovery for work commute going 50 about coasting to look at fields of flowers on river valley level hwy, tinkle sound for 1 second then locked tire and a couple more years to recover the Combat from crank shaft, cases and lifter destruction. I now am a loose goosey clearance provider and shudder at runing a pig in a poke w/o full tear down of unknowns, even if running perfectly no smoke noise or leaks. Rebuilt my Kolher 18 hp cast iron barrel/ally piston twin with forced air cooling this year - specs said use between .011 and .014, so had Bore Tech do em at .013". Not a typo on the missing zero.

Beach · Sep 6, 2014

What piston and rings are reccomended?

comnoz · Sep 6, 2014

Beach said:
What piston and rings are reccomended?

For a street bike I would use JCC brand pistons. They are sold by Emgo and others. They are so cheap one would think they are no good but they seem to be every bit as good as the Hepolites. I would not use the JCC rings [use Hastings, Total seal or OEM].

You would want to check the wrist pin clips for a tight fit as some came with undersized clips. Jim

Beach · Sep 6, 2014

And any piston will work as long as it is for 750 Norton? no difference between Commando and Atlas?

comnoz · Sep 6, 2014

Beach said:
And any piston will work as long as it is for 750 Norton? no difference between Commando and Atlas?

The original Atlas pistons were much lower compression. They had a dished top. I do not believe JCC makes a piston for an Atlas but I have not checked lately.

The Commando piston can be used in an Atlas but unless you lower the compression ratio with a spacer you will have to live with more vibration from the higher compression and the carb will need to be jetted leaner.

If you use an Atlas piston in a Commando you will need to go richer with the jetting due to the lower compression but it does make them run smoother. Jim

Beach · Sep 6, 2014

I have a .030 steel base gasket for this motor...Would that work ok to lower the compression?

comnoz · Sep 6, 2014

Beach said:
I have a .030 steel base gasket for this motor...Would that work ok to lower the compression?

It takes about .060 to make up for the difference between an Atlas and a Commando piston. So if you use the .030 shim plus an extra .030 gasket {one gasket under the shim and one on top of the shim] then you will come out about the same when you use a Commando piston in an Atlas [or when you want to go 1.5 points lower compression in a Commando]. Jim

Soft Seize?

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