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BMC 1.5 chain tensioner


GBW

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On reassembling my re-conned engine, I find that the spring in the chain tensioner has gone missing.  The tensioner itself, looks fine.

As they are only cheap, I ordered one from MG Owners Club spares (I am a member).

Unfortunately, they are out of stock.

I have a box full of springs.  Which should I select?

(I am intrigued by the notches in the "piston").

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No idea about the spring tension but the notches form a ratchet mechanism that prevents the tensioner "collapsing" with no oil pressure.

 

There is a pin inside the tube (hope I get this correct from memory) fitted to the rubber block and the little bit with the notches and a hexagon hole in the bottom fits inside the tube so the pin engages with the helical slot and notches. The spring fits between the two.

 

Having said that i now realise the actual spring tension is not that vital so use one that looks and feels correct.

 

Right,back to the tensioner. The steel block that screws to the engine front plate has a small hexagon screw in the bottom plus lock tab (from memory).  Before refitting the tensioner remove this screw and use an Alan key to wind the notched bit up the pin until it's at the top of the helix. At that point the notch is, I think, slightly larger so when the notched bit is in that position it locks.

 

You can now fit the tensioner and then I always used the Alan key to release the notched bit. The spring pushes the tensions into contact with the chain and the pin "locks" it into an appropriate notch so you an recheck the timing. Refit the screw and lock tab.

 

When the engine is running oil pressure is fed into the "tube" so the tensioner is pushed against the chain. As the chain  wears the oil pressure pushes the tensioner further out of the mounting block and the spring pushes the notched bit down until another notch lines up with the pin. In that way the pin and notch hold the tensioner in position when there is no oil pressure. Its a bit like automatic rear brake adjusters where when the movement of normal operating becomes sufficient a spring loaded ratchet moves to take up the excess movement.

 

I always used to take the mounting bock off the engine to make it  easier to get the chain and gears back on and to make it easier to use the Alan key for the initial adjustment.

 

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The spring needs to be short enough for the notched inner to full go into the tube else you can't fit the tensioner and long enough to fully push the inner all the way out. Difficult to give you a poundage for the spring but you should be able to fully compress it twixt finger and thumb with some effort.

 

ASAP should have new ones.  Or try Calcutt, who may find you a used spring.

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Thanks all (especially Tony).  I thought it probable that the force on the chain derived from oil pressure but the notches baffled me.  Now it all makes sense.  I have a spring and I'm sure all will be well.

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28 minutes ago, GBW said:

Thanks all (especially Tony).  I thought it probable that the force on the chain derived from oil pressure but the notches baffled me.  Now it all makes sense.  I have a spring and I'm sure all will be well.

The notches are to stop it going back in again when the chain thrashes.

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