Author Topic: Mark III cylinders  (Read 4290 times)

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Offline prentii

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Mark III cylinders
« on: 08 Mar 2007 at 17:20 »
The barrels on my Mark III have been rebored to +0.030, one of which is good at this size, the other needs to be bored out further to make it true (there is extra wear at the top).  They are currently at about 61.47mm, and the worst part is about 61.72mm - so this barrel would need to go to 62mm to be completely true.

Can the barrel stand being bored out so big?

Also, is it better to use the JP pistons as supplied by the Douglas club, or convert to Honda 250 pistons?

Thanks..

Offline trevorp

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Re: Mark III cylinders
« Reply #1 on: 11 Mar 2007 at 04:54 »
From what i have seen douglas cylinders dont like to be overbored as they tend to crack on the barrel section that protrudes below the fins and sleeving seems to have the same effect
mk1 where even worse for this problem as no rebore was available
best fix may another second hand barrell thats within specs
im not sure why they crack there and im wondering if it is vibration or or flexing wether or not a band fitted around this section of barrell may help
anyone tried the same

Offline prentii

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Re: Mark III cylinders
« Reply #2 on: 12 Mar 2007 at 11:29 »
I've now been to see another engineer, and he seems to think that it will take +0.060.  His idea is to bore the bad barrel out to +0.060, leave the other are +0.030.  Fit a +0.060 JP piston, and balance the +0.030 and +0.060 pistons, by removing some meat from the +0.060.

It's the second engineer who thinks there is enough meat - so that's the route I'll go.  At least I will keep one barrel at a sensible size, and can carry on looking for another - just in case...