150 miles on a new motor and this

This looks to me like the edge of the piston failed. I have not checked the end play on the crankshaft. I bought Hepolite pistons this time

That's a little unnerving as I just installed a set in my Combat Project.
Looks like the pin pushed on the clip which blew out the piston.
Or maybe the pin moved after it blew out.
Either way it's ugly.

Really interested to hear to what the experts have to say.

I was wrong- here's the circlip.

Pete Komarek said:

This looks to me like the edge of the piston failed. I have not checked the end play on the crankshaft. I bought Hepolite pistons this time

Click to expand...

It sure does.
It looks like the circlip was trying to do it's job but had nothing left to work with.

Glen

MexicoMike said:

I wouldn't think piston speed has much to do with the load on a circlip since the force involved is up/down on the pin, not side load but I admit I'm no expert on the particular dynamics involved.

OTOH, depending on any 'out of square' alignment of the piston/piston pin/connecting rod/crankshaft and the bearings involved, there could be considerable side load. Hmmmm....

Click to expand...

Piston speed has everything to do with it. The momentum of the clip is considerable when it has to change direction at the top of the stroke.
If everything is straight [barrel, case and crank machining and of course the rod], then the pin side loading will not exist.

I have seen a few failures from using JCC/Emgo piston clips. The failure looks just like this. [and the JCC/Emgo piston looks a lot like yours].

Can you provide a picture of the inside of the piston? Does it have JCC cast into it?
On the inside of one skirt. It can be hard to see.

Curious to see what piston speed some modern machinery runs at ( bikes that last, not race bikes)
The 955i Daytona has a 65 mm stroke and is rev limited at 12,000 .
The piston speed calculator says this is 26mps.
So we are maybe no so bad with the old Norton at running at 6500 rpm and 19.3mps.

Some difference in crank rigidity and machining tolerances may be present in this comparison

Glen

Pete Komarek said:

This looks to me like the edge of the piston failed. I have not checked the end play on the crankshaft. I bought Hepolite pistons this time

Click to expand...

My interpertation is the circlip popped out of it's groove and it acted like a hockey puck hammering away at the wrist pin hole until it wedged itself into the piston skirt and exploded...


As far as the "I don't see how piston speed has anything to do with it" goes, It's just physics. If the circlip rotates horizontally in it's groove, then up and down motions are going to exhert force on the circlip, (based on it's speed and it's mass) It's the same kind of force that a jet fighter pilot endures when he pulls "G's" changing the direction in his jet. So long as the circlip is rigid enough maintain it's shape under the force of the piston's speed, then it resists compressing and popping out. If that force significantly lowers the pressure it exherts on the groove, there's a risk that a small side load force from the wrist pin could push it free...

Having had this happen to me... the moral of the story is don't skimp on a 5 cent part who's failure can result in a few thousand dollar repair...

Thanks Jim for your contribution here. Without your comments, the result of this thread would be a completely opposite impression of why this happens to commandos occasionally.

To Glenn, Perhaps by the time the Daytona was being built the manufacturer knew that they needed a significantly better circlip to handle those piston speeds... That just makes sense... right?

Back in the '70s I helped a friend rebuild a 1608cc Fiat engine, he was supplied with Teflon buttons to retain the wrist pins, has this fallen out of favor, or are they inappropriate for a Norton?

Best

Well at least all of the clip is accounted for just leaving bits of piston to find. This may turn out to be a smidge less catastrophic than anticipated. Of course providing there's no greater underlying cause than piston/clip failure.
I hope for your sake you get by with cleaning & dressing things up with some new pistons..... even a little boring would be on the nicer side of things rather than the bigger bits.

Just got a call from the shop. The valves are fine, no leakage. The head gasket is blown. I used a copper gasket and it did not compensate for the surface irregularities of the cylinder. I measured it and where there is blow by there's .0025" gap. They will check for flat on the head. Looks like head and cylinder will be milled flat. They suggested a composite head gasket.
Be boring the cylinders out .040"

RoadScholar said:

Back in the '70s I helped a friend rebuild a 1608cc Fiat engine, he was supplied with Teflon buttons to retain the wrist pins, has this fallen out of favor, or are they inappropriate for a Norton?

Best

Click to expand...

I used teflon buttons in my racebikes.
Regular Norton circlips wouldn't stay in when I was running up to 8500 with a long stroke motor.

I used to run them in my streetbike also but I had one get loose and end up inside the wrist pin once so I went back to circlips.

Pete Komarek said:

Just got a call from the shop. The valves are fine, no leakage. The head gasket is blown. I used a copper gasket and it did not compensate for the surface irregularities of the cylinder. I measured it and where there is blow by there's .0025" gap. They will check for flat on the head. Looks like head and cylinder will be milled flat. They suggested a composite head gasket.
Be boring the cylinders out .040"

Click to expand...

A copper gasket should have conformed to .0025 if it was annealed before use. But yes, I prefer composite head gaskets -and lots or re-torquing with either type. At 150 miles I would be doing my third re-torque.

As Comnoz pointed out, the flat-ended wrist pin and flat clips are simply an old inferior design relative to the plain wire circlip with tapered wrist pin end as used on modern engines. All the modern motocross, 600cc crotch rockets and large displacement super bikes routinely run to 21+ m/sec average piston speed (with full warranty) and of course don’t have issues with wrist pin keepers coming loose. For this set-up to fail would require that the circlip be pushed over the wrist pin (how could that ever happen) and for the resulting galled/seized assembly to completely destroy the piston as it pushed out. If the pin/keeper/groove look like the image below it's pretty much going to stay where it was when assembled.

WZ507 said:

As Comnoz pointed out, the flat-ended wrist pin and flat clips are simply an old inferior design relative to the plain wire circlip with tapered wrist pin end as used on modern engines. All the modern motocross, 600cc crotch rockets and large displacement super bikes routinely run to 21+ m/sec average piston speed (with full warranty) and of course don’t have issues with wrist pin keepers coming loose. For this set-up to fail would require that the circlip be pushed over the wrist pin (how could that ever happen) and for the resulting galled/seized assembly to completely destroy the piston as it pushed out. If the pin/keeper/groove look like the image below it's pretty much going to stay where it was when assembled.

View attachment 13333

Click to expand...

When the pin slams against a wire clip the bevel of the pin end actually forces the clip tighter into the circlip groove.

.040" ain't a bad punch. I dusted a sporty and had to do .060" from stock. This shall be in the past none too soon for you I imagine..... I hope fortune smiles this time.

I am not a Norton expert but for high performance engines this is the retainer of choice for the last 20 years..


The retainer used by the OP was common years ago and looking at it you'll see one side has a sharp edge and the other a rounded edge formed when it was stamped out...The flat edge always faces away from the pin so it "grips" the aluminum goove..But as mentioned they are not reliable at high piston speeds.

Hillbilly Bike said:

I am not a Norton expert but for high performance engines this is the retainer of choice for the last 20 years..

The retainer used by the OP was common years ago and looking at it you'll see one side has a sharp edge and the other a rounded edge formed when it was stamped out...The flat edge always faces away from the pin so it "grips" the aluminum goove..But as mentioned they are not reliable at high piston speeds.

Click to expand...

And Spirolocks do work but I have never found one that would fit a Norton size groove.

I have seen Teflon buttons as piston pin retainers (although not a Norton engine) the situation we see here in Pete Komarek's engine could have been prevented this way imo.
Did anyone ever used this method on a Commando engine ?

Sorry to see this Pete, hope it doesn't work as bad as first thought.

Judging by the shield shape and the raised edges around the damaged piston I reckon the circlip could have snapped in half, and then each piece been hammering on their respective sides of the piston until they vacated the premises.

As this doesn't seem to be an uncommon occurrence Is it possible that the gudgeon (wrist) pin is slightly too long and thermal expansion is forcing the circlips out of thier groove ?

Cheers,

cliffa.