Drive side piston

Hi David,  Its possible that after a clean up you may be able to re-use , check the clearance  5 thou may still be ok.  You are right to check the  mixture settings and the timing. I had a big overheat recently and I am a sure that  the fully synthetic oil  I use prevented a sieze up.  The Drive side has a lower supply of oil and always shows its displeasure first if all is not well. If you are going for new pistons try to avoid the split skirt type ( as fitted)  ,they are a bit weak apparently  but did not give me any trouble on my tuned 88. If you want me to look at anything , I'm not far away .  Bob.

That oil looks good.   Don't use it in the primary case , not good for our clutch .I use ordinary 10/40 in there.  I had a similar seize up on my 88  and the pistons were too loose and clattered well. Happily the dealer  felt sorry for me and supplied me replacements FOC.  A light emery rub in the bores and I was fixed.  No more  95 mph races with  3 litre cars. 

Hi David,

As we know from previous postings on oil, it is a very subjective subject.

You may want to look also at Penrite Classic Light 20W-60 for your application as it is formulated for air cooled engines.

I have used Penrite oils now for over 40 years in all my vehicles with no issues (no commercial interest in Penrite either).

Here, I use HPR 40 due to our warmer?? (sometime soon I hope) in all my motorcycles as well as my classic car and TEA 20 tractor. 

Car and tractor have oil pressure gauges fitted and this oil holds constant pressure over a long period of use.

regards Ian

Changing the timing side seals also a good move. While in there I would strip and clean the OPRV  and check the pump gears for play etc. Won't cost anything except time.

That is insufficient clearance, was the bore checked with the piston that will actually go in it. Also wise to look for the things you can't see, there is no sign of oil failure or overheating, so it can only be incorrect clearance. 

Plug chop is pretty pointless these days as the fuel burns very clean, exhaust analysis is the best way and not expensive to have done. If you are not burning oil and have a sooty plug then you are not even close, if you are in the band, from rich end to lean end of the band a plug appearance will look very much the same.

My first 88 had been used  all round Europe for 10 years by the previous owner with no issues .I used it every day  for 3/4 years including runs to North Wales Rallies fully loaded and cruised it at an indicated 80 mph. It was fast and reliable and was regularly revved to 7000 rpm. Its downfall came  when at the end of a long  fast run accross England to the lakes  and a flat out down to London it overheated (fairing/ hot day), 95 mph  and seized the drive side piston . I have worked on a couple more that did the same thing .  No way did it have insufficient clearance from the factory and after all those years hard use.  Overheated, with  too hot  oil it found the weakest point ,a piston the farthest from the pump. Solve the overheating and improve the oiling (and the oil) and it wont happen again. I do agree with Ashley that its really difficult to get carburation right without leaded fuel,and that adding clearance will make seizure less likely,but it will also give you piston slap,although the 88 seems to tolerate more clearance that the 99.

It would be good to see the piston from all sides including the inside including the pin, it has the some signs of a 4 corner seizure. When a piston with the right clearance warms up from pre detonation or weak mixture the parts of the piston with the greatest cross section of alloy expand the most and these area's seize. These areas are normally front and back of the gudgeon pin hence the term 4 corner seizure. The amount of smearing of alloy on that piston is more than you would expect from a slightly tight clearance. 

Should have been plenty of clearance.  Some build with 31/2 thou and get away with that . I would not build with less than 41/2  . That is assuming the piston is of the right alloy and correctly machined. I don't think they are  correct. My suspicion is that they are pattern  The finish is a bit crude. possibly the alloy is not low ex  or the internal casting too heavy . Not good enough to really withstand the rigors of a Norton  drive side. I would also check the fit of the pin. If very tight can cause problems for the piston. Should float when warm.  Do you have twin carbs.? If all else correct the pistons should not sieze with your clearances although the motor would not be quiet. I have always preferred old stock or even SH orriginal pistons by  a Brit manufacturer.If you decide to clean up and re-use I would set up with a slightly richer mixture on the drive side ,a slightly worn needle jet may provide that or one notch on the needle. Ever played Russian roulette !!

If the OPRV is stuck open  you could lose pressure ?. I have often thought  why do  Nortons suffer drive side lube  issues ?.  Leakage between crank parts ?, centrifuge action  sucking the oil back at certain revs? ,  Big ends seem to survive well . Perhaps  a slightly higher pump output and a rod bleed hole on the DS would  help.

Rarely is it the main jet too small on old classics (because we are rarely on the main jet!)  The Issue is more often  the needle jet  being too small (dud pattern part) ,too big (wear)  or the needle position (or wrong needle). It could also be a supply restriction to the petrol, A blocked petrol cap (polish!)  on my 250 gave loads of stress. And a  (T)  two stroke? needle jet nearly did mischief to my 99. No cross drillings and the jet internal  height is wrong too. Thanks carb specialist !!. Why am I so trusting?.

 The extra compression came from the head on after 59 bikes .  (expect you knew that). 

Even with 5% ethanol, it's worth increasing the main jet by a size. 

A new piston is going to need some running in (unless you can use the old rings?) , Probably have to bust the glaze a little  and the especially slippy oil will make running in  tricky.  I think you are better off  selling me the whole (hole?) problem.  £ 3K  OK ? !!.  

The piston does not even look like it has overheated. If it was oil failure to the DS piston I would expect to see big end damage as well. Best bet is to measure the bore with 3 point mics throughout the bore, if it is good then junk the piston. 

Not had a dodgy thick  powdercoat finish to the barrel ?.