Locked up Lightweight

Ulrich,

I share the agony having followed your thorough rebuild. I hope the engine shop might admit they left the fit of pistons in the bore too close. Better still if they can correct things at no cost to you.

You might consider a light cleaning of the pistons with a very fine file. Just to take off the high bits. Clean the ring grooves so the rings are free again. Hone out the bores and then run in with care.

Depends how bad the bores are of course.

How were the first 25 Km? Good I hope.

Peter

Ulrich, what a shame. I hope you can get it sorted. 

Looks like the pistons 'picked up' in the cylinders

Was it a tight fit getting the pistons in the bore?

Did anyone check the ring gap with the piston in the cylinder before assembling the cylinder to the crankcase. 

Did you lubricate the pistons and cylinders generously on assembly.

Is the oil pump working well, for example did you see oil returning to the oil tank at a good flow rate.

Did you run in the engine gently before you first run?

Sorry if I am stating the obvious.

Dennis

Although it looks bad, the main damage would appear to be in the ring lands which should have more clearance than the skirt . If new rings can be made to work its just possible that you could save the pistons after a good clean up. I would be looking at clearances ,timing, and carburation. Its also possible that the pistons were not made correctly.

Hi Uli

That's bad news indeed, they look like heptolite pistons, while its unlikely with new ones they did suffer from cracking usually from the gudgeon pin up to the rings, this allowed the piston to expand more and seize, I have one in my hand that did just that in 1977!

Hopefully you can clean them up satisfactorily but it does look like like the piston clearance was too tight or they were running very hot.  I doubt a wide valve clearance would could that problem.

Fingers crossed.

Dan 

When similar happened to my Dommie many years ago, it was due to one carb needle being lower than the other - and one overheated on a long hill climb.

That led me to a rebore etc etc.  The reason for the rebore was the state of the cylinder walls.

But in retrospect, I think most of what looked like severe damage was in fact aluminium from the piston, and I suspect it could have been removed using caustic soda with no loss to the bore size.  I think I could have got away with new pistons, and saved myself a lot of trouble and time.  Maybe others have similar tales to tell?

HinUli

I agree the best way to time the engine is with a disc and a strobe, that’s what I did on the jubilee.

The Boyer gives a very steady spark, as you would expect.

dan

I don't have the Boyer system so the detail may difer Uli but yes the engine rotates anti clockwise as you look at it so once you have established TDC accurately set up a marker the way you have and then turn the engine back clockwise to the fully advanced position of 24 degrees (beware, you have the pointer at 30 deg in your photo which is for Electra timing!).   Then follow the Boyer instruction manual to set up the rotor to time the engine statically.   Once you have done that connect up your timing strobe, start the engine and point the strobe at your timing marker and increase the engine revs and you will see the ignition start advancing.   Slowly increase the revs until the electronic ignition gets to fully advanced and make a note of how many degrees that is.  Stop the engine and adjust the base plate accordingly, start the engine again, repeat the procedure until you have got the timing spot on.   With my Pazon system the static timing is generally within 1 or 2 degrees so it only takes a very small adjustment to fine tune it.   I have found that transforms the way the engine performs but do take care - with the engine running keep well clear of the primary drive and clutch.   Good luck!         

That’s right Uli, it’s important to set the timing with a strobe. Note to self, must check mine! 

Uli, 

I think I am correct in saying that the timing should be 24 degrees BEFORE TDC.

If the timing disc you show is in the correct position it looks to be AFTER TDC.

I am happy to be corrected if I am wrong.

With the Pazon you just set the 24 degree setting and the electronics take care of the rest. Not sure if that is the same with the Boyer.

Dennis

Correct Uli and your latest picture is spot on!  As a suggestion so I could use the big (and therefore more easy to be accurate) timing disc I put several large washers behind it on the alternator retaining stud on the end of the crank shaft so it was clear of rotor and wiring.   Good luck with getting the barrels honed, the rebuild and careful running in!    Nick   

Did they use when reboring? I've found many workshops are far too optimistic and bore to clearances more appropriate for modern water cooled engines.

I prefer to allow a thou or two more than standard - let's face it, who wants to spend the rest of their life running the engine in? I know nothing about the lightweights but .007" or so for the singles is reasonable in my book.

Phew, glad Uli corrected me! 

Dennis

I see JP do Navigator pistons, they don’t have a very good reputation but must be better than nothing! 

https://www.ebay.co.uk/itm/Norton-Navigator-Motorcycle-JP-Piston-Ring-Set-STD-349cc-Twin-Cyl-OHV-1949/273248628405?hash=item3f9ee32eb5:g:

Dan

JP make a full range of over sizes too Dan, but like you I have seen some very mixed reviews on line, particularly over their pistons being significantly over weight and prone to seizure.   That said I have also seen very positive reviews that suggest most of the problems are because the engineer reboring the barrels don't use the manufacturers tolerance guidance which is different from the original specifications.   Interestingly when I enquired about Navigator pistons at Hamlins in Bridgewater they said they offer JP pistons and having experienced their engineering excellence I can't believe they would use sub standard pistons.   I wonder if anyone in the Lightweight world has first hand experience to enlighten us?

Nick       

hello from my engineering experiance  they is a number of things that could of caused this  failure  one is the oil the 40 mono grade,as it gets hot it breaks down and its viscosity then get thinner,as I am seening, black burnt oil on the sides of the pistons,      a 20/50 mulit-grade like Duckhams Classic  will be better for small twins  as the oil ways are smaller to the big twins  and to time a twin  you time from the left hand side cylinder  and it runs anti-clock wise so you  turn your timing disc back clock wise to 24 degrees, BTC and the piston to cylinder clearances are 3,thou and end gaps sholud be 6thou to8 thou  and bronze valve guides are far better then cast iron ones  and you reduse the timing by 2 degrees with electronic igintion systems  So you have a number of issues to look at  But I do feel your pain and angish  after trying to do the right things in your rebuild,  hope you sort things out this tie around,   Yours  Anna J 

Brings back bad memories of juggling to get the barrels on the pistons on my Jubilee in the past without damaging the rings Uli, good luck, it looks like you are almost there!   As for ring gap the Norton manual quotes it should be between 0.006" and 0.008" so 0.2mm is safely at the top end of that range.   I'm sure you know but I thought I would mention in that last picture it looks like you haven't replaced the gudgeon pin circlip - not something you want to discover after you have lowered the barrels into place or worse still after you have started the engine with catastrophic results!    Keep up the good work and hope the rest of the rebuild goes well.  

Nick     

I share your pain Uli, trying to slip them into place without them slipping from the grip of the circlip pliers to ricocheting across the workshop to fall in to a dark and impossible corner or as you found into the welcoming crankcases can take hours.   But cursing in any language helps!   Keep up the good work, it will be worth it.

Well done, almost there Uli, by the time you have finished it should run perfectly!   My advice is to take it very steady running it in and re-torque the heads in accordance with the workshop manual order and torque settings after running the engine up to normal running temperature and again after 100 miles and 500 miles.   That may seem over cautious but it should ensure an oil and gas tight head.   For the first few runs I keep them short and take it very steady, not over reving the engine or letting it labour going up hills with several stops to check that the mixture is correct by checking the plugs (avoid a weak mixture, I let my newly rebuilt engines run a little rich to aid cooling until they are nicely run in), checking the engine isn't overheating and the oil is circulating to the top end nicely.   If you think it is running hot again stop, recheck the timing, the tappets, oil circulation and the mixture - I have had something as simple as a poorly seated inlet manifold gasket cause an air leak and a weak mixture which over heated a freshly rebuilt engine causing a partial seizure.    But all that said now you have the timing spot on and the engine idling steadily and not overheating all should be well.   Let us know how the test runs go and good luck!   Nick