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Dommi 88 engine rebuild questions

Submitted by wayne_rudolph on Sun, 10/12/2023
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Heavy Twins

Well I`ve had some good advice so far from you blokes so here`s a couple of more questions . I`ve got the whole engine in bits now so will be rebuilding after a long awaited holiday in Australia. I`m pretty sure I should but is it necessary to replace the Conn Rod bolts and nuts And all the bolts/nuts and tab washer plates on the Crankshaft Assy ? The journals and Shells look fine but I will get a second opinion anyway. After assy should the Crankshaft assy be `Balance factored` ? ( not sure of the percentage either ? ) . That brings me to Main bearings which actually seem ok but I feel I should put new one`s in having gone so deep. One is a Ball race and the other looks like a `Superblend`. I`ve read the articles on here regarding `superblend/Ball race mixing and found it quite confusing. My bike isn`t for racing but the odd fast ride shouldn`t be unheard of so whats the answer ? As for getting it back together what are the best Gasket sets And is Wellseal jointing compound the best thing to use ? Thanks for your time people.

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Submitted by philip_hannam1 on Mon, 11/12/2023

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The late John Hudson was an advocate of re-using as many of the original engine parts as possibel. He had a deep mis-trust of pattern replacement parts and recommended new old stock as often better than commercially sold new bits and pieces.

If you check out his old  Norton Twin Engine rebuild videos, he demonstrates how to check for thread wear on the connecting rod bolts,  crankshaft bolts and studs.  Generally looking for stretched or torn  threads.   For the bottom end he stated that it was good engineering practice to replace big-end nuts and those on the crankshaft as well but on the 500cc engines nuts in good condition could be re-used.

With regard to main bearings, the mix of one roller with one ball is fine and prevents the issue of having to shim up two roller bearings. John liked to fit an uprated ball bearing on the Timing Side using an RHP M306.   I believe the modern equivalent is  FAG 6306 C3. John said that fitting twin roller bearings to a 500 twin engine just increased the internal friction and thus actually slowed the turning of the motor.

 

Submitted by robert_tuck on Mon, 11/12/2023

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I agree with everything Phil says and would add the following . I would replace the big end shells as measuring the wear is not easy. Unless the crank is badly worn i would not get re-machined. .As you have removed the mains you might as well replace ,but if you find the bearings spin silently with a little oil then I would re-use happily. Uprated bearings serve little purpose in this motor which can even run on all ballraces.Might be worth getting the crankshafts  checked for alignment and also  balance if pattern pistons are used . This is a revvy motor and anything you do to make sure its all free and well set up will pay dividends . Watch out for new big end shells that can run tight even on a used crank due to rods going a little oval. This issue often made worse by having the crank ground.

Submitted by wayne_rudolph on Mon, 11/12/2023

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Thanks for the replies Phillip and Robert. Had the crank checked today. The result was that the journals need a polish and the shells are good condition  Bi-metal one`s made by King. The one concern was the timing side shaft end is a little uneaven and pitted which I`m told could cause an oil feed problem if it doesn`t fit well in the Oil seal in the timing chest cover. This is being looked at. The bloke also suggested that the `vastness ` of the `sludge trap` doesn`t lend itself to giving good oil pressure as on starting the engine the it has to fill this void all the way through before it can circulate. He also wants the Conn rods to check for roundness( as you mentioned Robert) . Sadly it needs reboring too out to +60 so i`m hoping to get some good quality pistons which he has also requested on assembling the crankshaft / conn rods.

Off on a longish break abroad from this Wednesday so cant do much more until I get back but at least the crank is being sorted !

 

 

 

 

 

 

 

Submitted by david_cooper on Mon, 11/12/2023

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Can our experts advise?  Wouldn't it be better to sleeve back to standard rather than try to find +60 pistons?  Easier to find standard ones, and less chance of upsetting the balance?

You are probably right David. Been searching all over for +60`s all evening . RGM have some but not sure what the make is .Awaiting an email response from them at present. My pistons are ok (proper Hepolite) so may look out for another barrel even ?

Submitted by robert_tuck on Tue, 12/12/2023

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Its also possible to overbore the engine and fit NOS std  size or  Secondhand orriginal 650 SS 68mm pistons.And finish up with about 525 cc.  . Might be preferable than liners.

 

 

 



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