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Drive side little end - advice sought


Dear Members,

I seized my 1965 650SS about 2 months back and upon stripping it I found that I had holed the drive side piston. The machine had been performing a bit oddly for some weeks beforehand being particularly slow on long uphill climbs. I figured it was petrol starvation so increased the main jet size and raised the needle. On the day of the seizure it all happened very quickly just a mile from home and on the flat.

When stripped the piston was ok in the bore but the gudgeon pin was seized solid in the con rod small end.

I tried various ways to pull the gudgeon pin out but failed until gently sawing the piston off and then constructing a substantial pulling tool to finally extract the pin from the con rod. The little end bore of the con rod has a few minor scores in it.

There is no little end bush on these models so I would appreciate any advice members may have in rectifying the very minor scoring. I have a full set of expanding reamers but thought polishing the bore might be a better idea even if it takes ages to eliminate the scoring.

Timing side was OK but even there I noticed a slight depression in the piston which is a prelude to it burning out.

I have now changed the petrol tap to brass on brass so that fuel flow is no longer encumbered.

Clearly I shall be fitting new pistons. Bores and big ends are perfect.

Any other members experienced this problem? Advice would be much appreciated.

Anthony Bolton



There are two solutions for your scored small ends.

First - you could bore out the small ends and fit the same bushes as used in the 99 conrods.

Second - You should be able to obtain Gudgeon pins that are slightly larger than standard and have the pistons and small ends machined to fit.

The first is cheaper and easier by far.


hello well let's go through all this  your small end has sized  and the piston top has overheated causing this to melt a hole in this top,  well that drive side has been running over weak or the ignition timing was out, the small end seizing is the lack of oil to the small end  and I would of strip the engine with the piston in place  and parted the cases and removed the connecting rod and piston in situ and removed the piston afterwards  under a press to remove the gudgeon pin  has for the bore they just need a hone with fine honing stones and get back the crosshatch in the bore and investigate the lack of oil to the small ends    and fit a pair of correct pistons like BHB  23270 and 23271  there not easy to get hold of but they are out there on sale you just have look for them  now I do hope you get your motor back to fine health  Yours  Anna J


Anna is making some useful pointers. I would also check  that the big end oil bleed hole in the rod is the right way round - and that the oil hole in the shell lines up.If you have twin carbs the drive side should be set up the richer of the two carbs . I have found the tolerances on new piston pins may be tight in the rods  .I am assuming the crank sludge trap is clear. 


First of all thanks to Phillip, Anna and Robert for their advice.

I polished out the defective con rod little end with Brasso and although it took some time the results and fit were very comparable with the non defective timing side piston so I have now fitted two new pistons and gudgeon pins and the engine rotates very nicely.

I suspect I have been running with a weak mixture and both the valve and ignition timing may be out or somehow there is a permutation of all three of these faults.

I shall reset the ignition timing, forensically check the valve timing and revert to twin carbs, setting the drive side to be a bit richer.

The big end bleed holes are correctly positioned and there is zero play in the big end bearings.

I shall fit a refurbished oil pump to ensure the correct oil flow. The sludge trap was spotlessly cleaned when I refurbished the bike some years ago and the oil is changed every 1000 miles.

Watch this space but it may be 10 days or so before I complete the work. Thanks for the help.

Anthony Bolton 


This mystery is starting to unravel a little bit.

Fitted the head, forensically checked the valve timing and ignition timing; satisfied that it is absolutely correct.

I'm moving now to fit the original standard Amal Monobloc and chopped Monobloc which are respectively models 376/288 and 376/289 both being currently fitted with 250 main jets as per one of my Norton Plumstead Manuals (Publication P106/P), printed on older imperial sized paper.

However my Haynes manual (published 1974) quotes 250 or 270 (not sure why it states both) but my more detailed Norton (Plumstead), A4 sized (and probably later) manual quotes 270.

Recognising that weak mixture is probably the source of my woes I am minded to fit both carbs with new 270 main jets. However, recognising Robert Tuck's advice about the drive side should be richer, should I fit a 280 jet  to the standard Monobloc on the drive side or should I up the clip position on the needle  to slot 4, richening the mixture that way but retaining the 270 jet?

Apologies for sounding really pedantic on this but the devil is in the detail and other opinions/expertise will be most welcome.

Anthony Bolton


You would really have to be throttle happy on a very  long ride to get to the point where the main jets are cooking the pistons. Your problem is more likely to be the consequence of the rubbish fuel we are all having to put up with and/or some dodgy Ignition Timing.  Running with 270 jets in both carbs should be OK for normal riding. Do regular plug checks to confirm the mixture is correct.

Generally when the manuals show two sets of main jet numbers the lower number is for a carb wearing a filter. I have just looked up the recommended settings for your bike in the Dominator Service Notes 2020 Edition and it says.

650SS (1962 – 66)  Carbs 376 / 288 & 289  Choke Size 1 -1/16    Main Jet  250    Air Slide 376 / 3-1/2  Needle Position 3    Needle Jet 0.106      Pilot Jet 25


Sid Lawton,builder of the SS that won the three , 500 mile races ,used that trick to avoid the possibilty of drive side seizures  as the drive side always ran hotter due to getting less oil.  A plug chop once the bike is fully run in , was the way it was done. Nowadays a session on a dyno would be worth it.  I doubt you were fully on the main when it overheated,  I would check the fuel supply is not restricted somewhere.  You may have had a newer needle jet (weaker) on the drive side. A new jet really overheated my 99. While running in it won't hurt to be a bit rich. I have worked on 3 bikes that seized on the drive side only. As Phil says the fuel is a diferent thing now. I stick to Esso top grade, (no eth), In the IOM I was running the cheap local stuff,(also no eth) and the motor ran even better !!, Its a bit of a lottery. Stick to one fuel. 


I restored my late '66 650SS 10 years ago and have had the motor apart twice since first problem turned out to be a magneto cam ring not being 180 degrees apart there for the ignition timing was different between cylinders by8degrees and this seems to be a common problem, after this i had a piston seizure at about 55mph on a up hill run, which turned out to be too lean on the needles which had been masked by a too rich main jet, I ended up getting some leaded race fuel so i could get an accurate plug reading.


After reading the tales of piston troubles it seems amazing that Phill Read and Brian  were able to take the SS round thruxton flat out for 500 miles  with no problem!.  The SS had been delivered by a factory tester who had managed to partially seize the motor on the way. Carefull preparation by Lawton made the difference between a lemon and a winner. Simples!!, ---possibly!


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