How much tension should mechanical tensioners have??

[CamB]

Trying to find why my engine rattles when cold from, I think, the drivers side, but that is also the side I sit next to so it could be either...

It sounds like chain rattling to my uneducated ear. I DO NOT have pressure fed tensioners, and as I am thinking about going for a newly rebuild 3.0 engine early next year, I am not keen to spend the NZ$1000 or so the pressure fed ones cost.

So I have (keep the gasps of shock to a minimum), acquired a pair of second hand mechanical tensioners known to be ok (as best as can be determined, ie they hadn't failed).

In the process of installing them, I am basically confused. The tensioner on the driver's side of the car has some spring for the first few millimetres then is much harder. The two tensioners I have acquired are the same. The tensioner on the passenger's side is rock hard and possibly a bit shorter.

So I look at that in isolation and would assume there is a problem with the RHS tensioner. But the timing chain is tight on that side against the "upper ramp". See http://www.pelicanparts.com/techarticles/911_carrera_chain_tensioners/pic25.jpg to see what I mean by "upper ramp".

Conversely, on the driver's side, the chain is loose on the "lower" ramp. See http://www.pelicanparts.com/techarticles/911_carrera_chain_tensioners/pic4.jpg.

So, which tensioner is doing its job properly - the one which has two freshly arrived ones the same but with a loose chain, or the one with the tight chain but which is possibly partially collapsed. Oh, I haven't tried moving the engine a few degrees in either direction to see if this is causing the tightness or looseness in chain tension which may be confusing me - do I need to try this?

Thanks, I am so confused (and not a little bit worried!)


------------------
Cameron Baudinet
1975 911S (in bits)
1969 911T (chain covers off, so immobile for the moment )


[This message has been edited by CamB (edited 11-03-2001).]

[martinh]

The mechanical tensioners should not move at all by hand - they can only be compressed slowly in a vice. If they move a few millimetres they have air in them and need refilling and bleeding, which is easy enough to do - the Haynes manual decribes it. The difference in chain tension is, as you guessed, due to engine position.

I had a temporary failure of my right-hand tensioner on my 1977 2.7 which cleared after a few minutes driving. Not wanting to spend the money on the hydraulic upgrade, I removed and stripped the mechanical tensioners and found nothing wrong with them. So I put them back together and reinstalled them only to have the same failure (that noise is unmistakable, and there are fresh marks on the housing from the chain) a few weeks on.

I've come to the conclusion that some of these old tensioners can fail for no logical reason (at least not logical when you look at them on a cold, stationary engine, if we could see them operating on a running engine it might explain more), or can somebody explain this?

If your chain is just rattling at idle though and hasn't failed, the chances are you could get away with just refilling and bleeding the RH tensioner.

I've got the engine out at the moment to do some work on the gearbox, and I think I'm going to order the hydraulic kit even though I'd rather spend the money on something else.

[john walker's workshop]

the tensioners fail for a reason. either dirt is between the check ball and seat, or the rubber o ring seals have blown out, allowing air to get in. some earlier styles with the thick upper mounting ring were even thought to be porous. there are rebuild kits available to fix them. just bleeding the air out is usually only a temporary fix. if you rebuild, do not look directly down on the spring loaded plunger when removing or installing the snap ring that holds it in the tensioner body. serious eye damage could result if it shot out into your face.

[911pcars]

Some readers might be confused by the interchangeable terms used in the first couple of posts.

The "mechanical" tensioners they refer to appear to be the self-contained hydraulic units supplied by the factory. They fail due to a massive hydraulic leak which results in a loss of chain tension and consequent engine damage from sprocket jumping, etc. As Mr. Walker states, these can be rebuilt, replaced with new units or replaced with the pressure-fed system.

True mechanical tensioners do not incorporate any hydraulics in their construction. There are two types; custom-manufactured units or modified factory tensioners. The chain tension is adjusted by hand. Because these have no provisions to automatically compensate for heat expansion nor chain stretch, they are not recommended for a street-driven vehicle.

Sherwood Lee
http://members.rennlist.org/911pcars

[CamB]

Sherwood is correct - I have the non-pressure fed hydraulic variety, but not true mechanical ones.

So it appears I need to check, refill and bleed them. I have the Haynes manual but if anyone has any tips on the methodology, I would be grateful.

Thanks, Cam

[This message has been edited by CamB (edited 11-04-2001).]

[911pcars]

Cam,
There might be easier methods of doing this, but when I had them, I submerged them in a quart can of clean engine oil. Work the shaft/piston up and down until all bubbles are bled out. You'll find it very difficult to compress the piston once it's bled. Wipe off excess oil, then install.

Sherwood Lee
http://members.rennlist.org/911pcars