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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Kyle10 on July 26, 2013, 06:56:24 PM
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Started reading this write up on timing chain rattle: http://www.ibmwr.org/ktech/timing-chain-rattle.shtml (http://www.ibmwr.org/ktech/timing-chain-rattle.shtml)
Especially this part: "...[The] part is the long curved piece of flat section bar that is pivoted at one end and is "pressed on " (from the underside) by the hydraulic, ratchet tensioner. This part has a plastic wear strip fitted to the top for the chain to run in / over, but the part that was causing the trouble was a small piece of black plastic that is bonded to the underside of this strip, to avoid "metal to metal" contact with the face of the hydraulic tensioner..."
"Close inspection of the small black plastic square showed that some of it was missing. At first I thought it had "chipped off", but then I realised that it had probably been eroded away by the speed of the oil coming out of the hole. It could have been a faulty part, but it seemed to me that the most likely scenario was that the erosion had been caused by the velocity of the fully synthetic oil that was used for the first 33,000 miles. Of course, no piston back pressure = no piston force = no chain tensioning = noisy timing chain. A new part solved the problem, and the bike runs like a sewing machine all the time, now."
The hydraulic tensioner works by engine oil pressure expanding the piston when the engine runs, thus tensioning the chain. However, the piston is not a "closed piston". To ensure lubrication of the faces between the faces of the tensioner and the flat section chain guide, there is a small hole drilled through the face to enable some oil to flow. The small piece of black plastic acts as a "back pressure device" - by pressing against the small oil hole, it causes the piston to expand properly and tension the chain - but in my case it didn't.
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The piece of black plastic on my bike does seem to have a 'ragged' edge, and it's rectangular in shape - not square. When new was this piece square? I can't find any decent image of a new one and I'd like to know before I (again) squeeze the wallet and buy and new one.
*Notice the last picture: you can see the line (looks like a miniature boot heel imprint) where the piston hole is not covered by the black plastic square. This would mean that the intended oil pressurization is compromised, eh? This would mean inadequate tensioning pressure if indeed its supposed to be covered by the plastic square bonded to the tensioner rail.
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