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boring

Submitted by thomas_martin on Sun, 10/09/2017
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Nortons - General

My dominator 500 88 has pistons showing +20 thou, I do not know the rebuild history of the bike and i have never had it running. so i am starting from scratch. So what should the bore measure to refit the good pistons that i have and where should i take the readings. i am thinking of using a dial bore gauge or similar, can anyone point me in the right direction. Tom

Submitted by robert_tuck on Mon, 11/09/2017

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I would measure the thrust faces of the piston and the bore ,if there is 4 or 41/2 thou overall clearance I would take that as ok,check the ring gaps in the bore are not tight and put gaps evenly round the piston. If the bore has been finely honed expect to do a lot of running in,bore finish and ring spec are dependant on each other for getting a proper break in and good seal. What make pistons?.

Submitted by thomas_martin on Mon, 11/09/2017

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Previously robert_tuck wrote:

I would measure the thrust faces of the piston and the bore ,if there is 4 or 41/2 thou overall clearance I would take that as ok,check the ring gaps in the bore are not tight and put gaps evenly round the piston. If the bore has been finely honed expect to do a lot of running in,bore finish and ring spec are dependant on each other for getting a proper break in and good seal. What make pistons?.

they are hepolite old ones and i got some new +20 NOS rings from the norton online shop as the original rings had seized in and needed to go. not ready for the rebuild yet just getting everything in place. the bores are a bit rusty too and will need a gentle hone then i will measure and see what its like. tom

Submitted by robert_tuck on Tue, 12/09/2017

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Pistons with slotted skirts have a poor rep although the never gave me a problem and used for relaxed classic use may be ok.Not all old stock are slotted.

Submitted by robert_tuck on Tue, 12/09/2017

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If using new rings the ridge at the top of the bore needs to be removed,also any carbon at the back of ring grooves,I would also talk to the engineer about honing ,can't afford to do much or the clearances will get too much,I would probably hand scrape the ridge and glaze bust the bore by hand and not worry too much about light rust pitting , but then I am a bit of a pennypinching old bodger. ( and my bikes do keep going for decades!!)

Submitted by thomas_martin on Thu, 14/09/2017

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Previously robert_tuck wrote:

If using new rings the ridge at the top of the bore needs to be removed,also any carbon at the back of ring grooves,I would also talk to the engineer about honing ,can't afford to do much or the clearances will get too much,I would probably hand scrape the ridge and glaze bust the bore by hand and not worry too much about light rust pitting , but then I am a bit of a pennypinching old bodger. ( and my bikes do keep going for decades!!)

thank you so much for the advice on the slight rust in bores i have, maybe a hand clean up an measure will be enough.

Submitted by gordon_johnston on Thu, 14/09/2017

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Some of the old tuners would leave freshly bored barrels out in the rain overnight to form some rust. The idea was that tiny pits would form on the surface which would hold oil. Apparently some of the big factory race teams were not above a similar approach.

 


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