SOHC.co.uk Forums > CB350/400

Piston and ring size markings

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paulbaker1954:
Something very odd here, the standard spec for bore size for the 500 four is 56-56.01mm with a servicable limit of 56.1mm

Youer measurements indicate 51mm which makes no sense (assuming it is a CB500 four.

Chris400F:
First line of the first post says it is a 400/4, not sure where the 500 four came from ....
51mm would mean the bore size is standard.

paulbaker1954:
Ahh silly me I didn't spot that  ;) ;) ;)

Laverda Dave:
Well, the rebuild still gathers momentum although another evening spent in the garage following the glaze bust has revealed a nasty surprise on the piston and bore front.
I thought I'd measure the gaps on the old standard rings after inserting them in the bore about 25mm from the top of the individual bores and before ordering some new ones from DS.  Shock and horror  :o, I appear to have some heavy wear on the bores. 
The measurements I took going from number 1 to number 4 are: 0.66, 0.90, 0.96, 0.92.  These gaps appear big enough to drive a bus through  ::)!

I am further confused by the wear limit measurements given in the Haynes manual as being between 0.15-0.35(mm) whilst the Clymer manual gives a wear limit of 0.15(mm) with no upper or lower limit? I'm not sure what measurements I should be taking notice of, fair to say though it would appear I need a re-bore. 

When I collected the barrels from the engineer who did the glaze bust he advised the piston to bore clearance when measured was 25 thou, he thought that was a bit slack but OK (old Triumph's apparently run 40 thou clearance as standard).
In view of the gaps I measured it would appear as though I need 0.75 oversize pistons, nobody appears to make this size, DS, Yamiya or Cruzinimage, I might have to go to 1mm oversize.  The Yamiya parts appear to be cheaper than DS although I will likely be stung for import tax making them more expensive.  Has anyone any recommendations?

Dave

Bryanj:
You have just used an old worn ring to measure a bore THAT DON'T WORK, you need to use an internal micrometer or a cylinder bore gauge

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