Douglasmotorcycles.net
General => Douglas Motorcycles - General Discussion => Topic started by: richson on 31 Mar 2007 at 19:38
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The barrels on my engine have a wear lip around the top which I guess is about 0.05mm uniformly around the bore. Both barrels have been rebored at some time, and 30 thou oversize pistons fitted. The bores measure 61.4mm at the widest point, but I can't measure immediately below the wear lip which must be a bit wider. At the bottom of the cylinders they again measure 61.4mm. I found the oil scraper rings on both pistons were broken, but the bores are quite unmarked.
I'm not keen on futher rebore, and I've decided to fit new rings, reassemble and see how it goes. I read in the maintenance guide that it's recommended to fit card gaskets to the case/barrel joint instead of the original brass ones. My worry is that using the thinner material will result in the top ring hitting the wear lip. Should I fit a couple of the card ones?
I also noticed that the replacement scraper rings are different to the old ones. The originals were a 'C' section sort of ring with a slotted centre, but the new ones have a relief on one side with small facets all the way around. Which way up do they go? All help appreciated.
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It is generally good practice after disturbing the piston or cylinder, to remove the ridge from the top of the cylinder bore. Even when putting everything back as it was, you can never be sure the piston will stop exactly where it did before. Since there is a 50-50 chance it could travel slightly further, it will of course do so 99% of the time! It only takes the piston to stop just a few thousands of an inch too far, for the ridge to beat the rings to death. They make ridge reamers for cars to do this task, but I have not seen a smaller version for motorcycles, though I imagine such are made. You can ease the ridge down with a fine file and finish of with some emery cloth, with much the same results.
However a 0.05mm step (0.002") is not all that much to worry about; 0.012" might be the point of thinking about a re-bore. 61.4mm is 0.024" over standard (60.8mm), so I think the +30 over pistons have been turned down, there has been a error in measuring the bore, or the pistons are out of another bike using a 60.5mm standard bore.
-Doug
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You can also fit thicker gaskets to bottom of barrells so the ridge wont come into play
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Thanks Doug. I measured the bores again at 61.4mm, definitely no measurement error. The pistons measure 60.0mm accross the crown down to the reduced skirt which is 60.4mm. Does that sound right?
Also, do you know which way up the scraper rings go?
I am going to try reducing the ridge as you describe, and to start with anyway, fit both the brass and card gaskets to be sure the new rings don't get trashed straight away.
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You can also fit thicker gaskets to bottom of barrells so the ridge wont come into play
Hi Trevor that's what I was thinking. What's the problem with the brass gaskets? Should I fit this plus the card gasket or two card ones?
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Thanks Doug. I measured the bores again at 61.4mm, definitely no measurement error. The pistons measure 60.0mm accross the crown down to the reduced skirt which is 60.4mm. ......
Correction, the piston accross the crown measures 61.0mm!
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i think the brass headgaskets are better than the other type the gasket im referring too is the gasket between crankcase and barrell this lifts the cylinder up thus clearing your ridges but also reduces compression ratio
if the ridge is to big u may still have problems with carbon building up there and piston may nick it but worth a try if u want a quick fix
good second hand barrels may be better in the long run