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Big bore or not

Home Forums TR3OC Members Forum Big bore or not

This topic contains 8 replies, has 5 voices, and was last updated by  James Herbert 4 months, 2 weeks ago.

  • Author
  • #9068

    francois besson

    hello fellow members, after a long troubleshooting, looking

    for oil leaks and smoke culprits, I dismantled the engine and

    found the cylinders oval though it looked like a bore job was performed

    recently, so the question is : try to rebuild the engine , rebore, new pistons,

    the cylinders are reading  2” and 41/64,  or 67 mm, or get a new 850 kit

    from LP Williams?

    Any advice would be deeply appreciated,  François.

  • #9069
    timothy rendall
    timothy rendall

    Don’t do it ! i’ve had head sealing problems with the 850cc kit. So i sold it and went back to 750. The only 850cc kits i know to be good are very expensive.

    Also your revs should be lower and you will only find a small increase in torque and not much more. Unless you are going the full hog and fitting big carbs & cams. I’d stick with the 750cc just get some one to do the job who knows how to do it rather than just any old car machine shop.

    • #9076

      Andy Salter


      As opposed to the 850 kit as Timothy was, I would be right at the other end, absolutely no issues with head sealing, and no reason you should have, not certain what his issues were that refitting the 750 sorted.

      Different bike, though as a caveat to this I did do the recommended valve timing at the same time, however oil burning issues are a distant memory. The improvement in the new blocks mechanical strength is beyond belief.

      I did fit the Nova big bore kit, though I did have an issue with the starter cover fouling.

      Standard block seems to have strength issues, every time I pulled my 160 down, scored, oval and tapered bores, re-bore always done by trident specialist. (Hyde)



  • #9071
    Kerrin Gautrey
    Kerrin Gautrey

    If you decide to go the 850 route it is cheaper to buy it directly from Neil Beadling rather than from LP Williams.

  • #9073

    James Herbert

    In times fraught with overwrought Brexit wrangling how pleasing to see a European using our traditional measures.   Not the Napoleonic!   I make that bore +3 thou (0.003″) which means it is only a little worn from standard, or new.   If they are oval, conical or not exactly perpendicular to the crankshaft then that must be coreected.  The base flange of cylinders could be not square to the existing bores.   We assume the crankcase top is parallel to shaft.  It does not matter if the bores lie slanting in the plane of connecting rod swing but is imperative it does not lie sideways.   If it does wear is rapid and assymetrical.

    I would first check that base flange is square to bore, if not then the case for a new cylinder block becomes strong.   The boring needed to be done square to base as opposed head face.

    Plenty of 71mm bore engines up to 974cc have been troublefree.   Obviously with head faces and gasket all in proper order.

  • #9074

    James Herbert

    I ought also to have said check the connecting rods are not bent or twisted.   The gudgeon pin must rise and fall parallel with the crankpin.

  • #9077

    James Herbert

    ‘…. any old car machine shop.’



    ‘…. Trident specialist. ( Hyde)



    Coventry Boring & Metalling.



    Very few cars nowadays ever have any engine reconditioning work done.   Whereas nearly every bike shop had a piffling little boring bar which was profitably used as an adjunct to piston selling.   Where there were well known machine shops are now posh houses.   The last time I visited a decent survivor it had a De Havilland crankshaft on the counter, I felt confident they could cope with my brother’s Honda.



    It is sad that too many motorbike owners believe in cult religions and magic.

    • #9079

      Andy Salter

      I think I may have missed the point of this post, when I said Hyde, I did mean Norman, though I do now know better, at the time it did seem perfectly reasonable to expect a good job to be done.

  • #9082

    James Herbert

    If you want pretty bend handlebar go to bike shop.


    For replacing chain and sprockets, bike workshop.


    If you took a crankshaft for regrind to either place and proprietor was business minded he said “Will be done in ten days, Sir.”.  Then he sent in van to place with Prince crank machine. You pay: grinding plus mark up, van plus mark up, handling charge or shopkeeper used his knowledge where to send it.


    I make recce visit to machine shop, see aeroplane crank: pleased because I like insides of engines, reassured because to fly again some licenced ground engineer will sign that all inside that engine is as it should be – if a crack starts from lack of fillet radius at end of pins or journals the AAIB will find it in the wreckage, the ground engineer will lose his licence and livelihood.   If ‘Special Old Bike Shop’s’ organised reground crank breaks because of missing fillet radius you bet it will be v plausible to layman explanation that in his 20 year experience this type of failure results from improper use by customer and lack of regular dealer servicing.   Will customer pay fee to Ricardo for expert analysis and witness?


    At school I took G3 cylinder to bike shop for rebore, returned to me with piston up bore and paper instructions about running in stuffed underneath.   I paid the £4+, took it home on pushbike.   Why piston not in Hepolite box?   I was too young and green to smell rat.   The gudgeon pin was rusty…


    It is worth reading Tim Smithells’ experience of Trident spares and ‘rebulds’.   Also wonder why people who were Trident riders chose to give up the day job in favour of Trident overhauling commercially.   I choose to buy tools, discover how to do my own work unless it needs a really huge expensive machine, in that case I like to see the DH crank entrusted to my chosen machine shop.


    One day I will write a chapter of staggering doings by shops known as Ron Devious or A N Other.   These old bikes are now seriously expensive but there are plenty of people who buy and sell kippers, when a customer complains they were unfit for eating the reply is that they were not kippers for eating but for buying and selling.  Remember English law states Caveat Emptor.   The best bike I ever bought was a Moto Guzzi, I rode it 15 miles with vendor on his BMW, stopped on concrete, lay down to look underneath clutch etc for oil leaks.   The vendor was delighted, said I had passed and could buy it as obviously able to look after it.  Go thou and do likewise then you will have a good Trident.


    It takes time, effort and disappointment to learn but is well worthwhile and you can then do anything with pleasure and certainty.

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