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Re: Piston failure

Pinking can be tollerated but full blown detonation can blow a hole in a piston in as little as 2 combustion cycles
Ask race tuners about how quick it can happen.

Re: Piston failure

Thanks for the thoughts.
The engine is currently a standard 600 with an alloy head. Only deviation is that the barrel is one of the linerless types, recently fitted with a standard size liner & piston.
The carby is a brass Allparts 276 replica in good condition. It has a 170 main jet with the needle in the #2 position. It's been like this for a fair while.
Plug colour is a milk chocolate brown, hard to read plugs these days.
I'd given the timing a quick check recently & it was a shade advanced, about 1/16". Will do so again.
The hole in the piston is a neat arc equidistant from both valves. A dainty bite size. Most of the scoring's there.
I hadn't heard any pinking noises.
Compression was a bit down lately & I was thinking of investigating- should've done so sooner.

Re: Piston failure

Well I have to eat humble pie regarding the ignition timing. Measuring with the head off, the points are starting to open at about 9/16". I've no idea how I got that so wrong, it doesn't do much for one's self confidence.

Re: Piston failure

It happens Bob, your a brave man to admit it. Best of luck with the rebuild.

email (option): pes.sales@btconnect.com

Re: Piston failure

Bob, I have had a partial seizure of a new piston and bore installation. I am new to these bikes and learning. I am still scratching my head on the cause of this as I was very careful with the assembly and timing. The first short run I took on the bike (about 1 mile) the mag timing pinion had slipped. (This may have happen in your case) This was pretty evident when I got home it was running retarded. The timing was setup with a dial gauge with the head off and a buzz box so it was correct.
After removing the head the timing was retarded. (full advance = near TDC) The piston and bore were in perfect condition at this stage.

Contributing factors
Not lapping the pinion gear to the mag correctly.
Using an old nut and not tightening correctly.
(On removing the pinion nut with the extractor tool was pretty easy)

Learnings
Follow the manual, use a fine paste and lap the pinion gear onto the mag until you get a nice even dull grey finish. When I performed this I rotated in the pinion gear in one direction, comments?
Use a new nut or run a tap through the second hand nut so the thread is true.
Ensure taper joints are free from oil on assembly.
Tighten the nut sufficiently.
I did use a very small amount of Loctite 263 on installation. Not sure if this is a good idea. Would like to hear others opinions on this.

Results
The bike ran very well. I ran the bike for about 10 miles.
Removed the timing chest and the pinion lash was a tad tight.
Removing the pinion nut with the extractor this time was what you would expect. Took a bit of effort to remove and came off with a bit of a cracking sound.
Corrected the pinion lash and reassembled and did the timing again.
I did make up a small 4mm rod with marks on it to check the timing via the removable bolt on the LHS of the head. The timing stayed correct.
Easy to check for peace of mind.

The Piston damage
Not likely caused by the timing in my case. Happened after the introduction of the 626 carb and some other factors. Will post some questions on this later. I’ve had enough pie for today.

Cheers

Brian

email (option): zola.industries@bigpond.com

Re: Piston failure

When lapping the taper I move the gear around as you would when lapping in a valve...I don't think the method is critical though as both ways produce the desired result...I don't use any form of Loctite or similar as a taper is a mechanical lock and doesn't require it when functioning correctly..The thread on the armature is 3/8"BSF...Always check the setting after you have completed the timing procedure as movement of the relative positions is common when tightening everything up...

Personally, I like to time the bike with the head removed, using a depth micrometer to determine the correct piston position and a 'buzz box' for determining the point at which the contact breakers open...With practice this method ensures accurate timing every time...Using the 'stick down the timing hole' method will produce very variable results...I once timed the engine 5 times on the bench and got 5 different results...Of course the bike would have run but the timing wasn't 'accurate' to the degree I prefer...

Regarding the piston failure, weak mixture setting, mixture weakness caused by air leaks, retarded timing, incorrect bore clearance or driving too hard in the initial stages of running in are the most likely causes...Ian

email (option): ian@wright52.plus.com

Re: Piston failure

Thanks for your reply and advice Ian. Very much appreciated.
The initial timing was set up with the head off with a dial gauge and buzz box. I used the 4mm rod just to verify while running the bike it had not shifted appreciably.
I checked it with the buzz box and dial gauge after I found the damage and it was correct.
I drained the oil after 300 miles and found metallic fragments in the oil tank. This prompted the removal of the head. Disappointing as the bike was running quite well.

Probable Cause – combination of incorrect bore fit and lean mixture

Incorrect bore fit
The piston and barrel were bought as a second hand refurbished set. Not sure if they were matched correctly.

Lean mixture
The bike was run initially for a few very short gentle runs on its original carb. The head was removed after this and the bore was fine. The carb was worn out and leaked so I had to solve this for a roadworthy. The 626 was introduced settings –
Pilot = 19 (Premier)
Needle = 106
Cut Away = No 4
Needle was in initially in position 1 (lowest position)
Gave the bike a test run and using the air lever found this to be to lean. The bike did seem to get quite hot.
Needle position 2 (Middle)
On testing this was an improvement, use of the air level indicated a lean mixture as the bike would run much better when you close it. The bike seems to get quite hot.

Needle position 3 (Highest position)
Bike ran very well. Not getting hot. Introducing the air lever resulted in an indication of rich mixture now.

I did remove the exhaust between the carb set up and getting the roadworthy. I do remember seeing some white flecks in the exhaust port. This is probably when the damage has occurred.

Learnings
If you buy a piston and bore that as supposed to be matched – get them checked.
Probably not a good idea tuning a new carb an a fresh top end rebuild. Use a proven set up.

I hope this may help someone else avoid this situation down the track.
Cheers
Brian

email (option): zola.industries@bigpond.com

Re: Piston failure

I'm pleased to see this thread resurface.
The damage to my engine wasn't as great as feared. The bore cleaned up well, what looked like scoring was piston fragments adhering to the cylinder.
So a washout, new piston & careful running in & it's done several hundred miles now & feels OK. But I'm not running it hard.
I tried a #180 main jet which the engine didn't like at all. Sluggish with a hunting sensation at low throttle openings. I haven't moved the needle from its #2 position.
Re reading my first posts, the information I put up there was double checked by others, nothing seemed so seriously out as to cause the failure so I still don't know what went wrong.

BW
Do you mind telling me what part of the world you're in?

Re: Piston failure

G'Day Bob.... Australia mate...

Ian has posted some settings for his 626. Will be using the same when I get going again.

Hopefully will yield a different outcome.

Cheers

Brian

email (option): zola.industries@bigpond.com

Re: Piston failure

Thanks for the quick reply, Brian.
I live out the back of Adelaide.
At one stage I had a Concentric on my bike. It was a lot less hassle. I can't remember the size, it was fitted as part of an unsuccessful big bore experiment. The carb was about the only good part of the whole thing.
Bob

Re: Piston failure


Know how you feel Bob.

The carb is probably the only good thing on mine....

email (option): zola.industries@bigpond.com

Re: Piston failure

In the old days we used to time the old field bikes T D C on full retard and advance as needed until the engine ran sweet

Re: Piston failure

Yes it's quite easy to adjust the timing on the road with a distributer. Not so with an M20 though:thinking_face: Ron

email (option): ronpier@talk21.com

Re: Piston failure

My WM20 doesn't have the timing cover access plug for the magneto pinion, which is probably correct. In practice, how easy do these make "in the field" timing adjustments?

Re: Piston failure

Far better to get it correct in the shed before you enter the field Bob. Ron

email (option): ronpier@talk21.com

Re: Piston failure

Quite so Ron. Attention in the workshop saves many a problem on the road. I may've been lucky but I've never had slippage of the pinion on the road. Or at all, come to that.
Without experience of the plugs, I've always thought they looked like both a good idea & an inconvenience. It saves having to remove & replace the timing cover but looks fiddly & it would be hard to keep oil off the taper.

Re: Piston failure

I am right in thinking that the spring washer fitted into the mag pinion is a non standard thickness? Could some of the pinion slipage mentioned in this thread perhaps be attributed to an incorrect spring washer being used? Just a thought.

email (option): fuzzyempire@hotmail.com

Re: Piston failure

I never fit a spring washer and have never experienced 'slippage'...
Regarding the timing cover with the plug I find them very fiddly to use for resetting timing and would rather take the cover off...I do carry a timing cover gasket in my 'touring spares' package but have never actually used it...As long as the tapers are good the timing setting is very unlikely to move...Well it hasn't for the last 45 years!:laughing: ....Ian

email (option): ian@wright52.plus.com

Re: Piston failure

Same story as Ian.
Because there is no advance mechanism with bob weights hammering ( well tapping any way )on the taper as you get with the auto advance models there is little to cause the pinion to walk on the tapper
I carry a pinion puller inmy kit as well and like Ian have never used it roadside.
I have been known to cheat on occasions on the side of the road and alter the points gap when I suspected the mag on some one elses bike was off time a bit.
The only time I get s slip is tightening the pinion bolt but a LIGHT tap with a lead or leather mallet will usually lock the taper before you tighten the bolt
Note the use of the word LIGHT amd it has to be a dead blow, rubber mallets just don't do the job

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