Need advice on replacing timing chains

justinp71 · 04-05-2007, 08:36 AM

I pulled my motor (75 2.7s) out to replace some oil seals, I decided to take the front chain covers off and inspect. I found that my left side chain is stretched quite a bit, but not so much on the right side. I don't have the funds to get into a rebuild right now, but I would not mind putting on new chains.

I remember last summer I would get a little detonation if I revved the motor up past 5500 Rpm's when it was warmed up.

Should I put on two new chains or one? Or should I just put on the fail safe chain tensioner collars for now? The motor has low hot idle oil pressure, so I figure that I will be rebuilding it in the near future.

My suggestion to myself, would be to put on two new chains, tensioner fail safe blocks and run it for another 20k miles or so. (I don't really drive it that often.)

Candelaria · 04-05-2007, 10:23 AM

Install both, it is cheap insurance, Candy.

justinp71 · 04-05-2007, 11:16 AM

If you install new chains, it is a must to redo your timing?

To me it would sound like to big of a risk not to, but if I do I need to get $300 in tools

Candelaria · 04-05-2007, 11:52 AM

Yes. you will have to redo your timing.,Candy.

justinp71 · 04-06-2007, 09:48 AM

I just read another post that talks about replacing your timing chain and not your sprockets will cause the sprocket on the intermediate shaft to fail.

Any opinions on this?

spuggy · 04-06-2007, 01:58 PM

Quote:

Originally posted by justinp71
I just read another post that talks about replacing your timing chain and not your sprockets will cause the sprocket on the intermediate shaft to fail.

Any opinions on this?

It's a very bad idea to fit a new chain to a badly worn sprocket, it will wear the chain prematurely.

With experience, you can gauge the wear on the sprocket and chain by picking the chain up by a link and moving it around, eyeballing how much room there is for it to move.

Not very scientific, and difficult to explain. If you see a bad one, you'll probably be in no doubt - but that leaves a lot of uncertainty if the first one you look at is part-worn or even good.

As the sprocket wears, the teeth get sharper and "hook". In extreme cases, the valleys will wear to the point where the tips of the teeth become sharper and unsupported - whereupon they will break off.

It can be hard to see without a reference - much easier to compare your part to a new one...

Once the sprocket is worn, the tooth then doesn't have as much vertical component to keep the chain controlled - even if the tip doesn't break off, the wear in the valleys allows the chain much more room to move around.

You have opened the door for slack in the chain to jump a tooth, which you really don't want to have happen to your valve timing.

I must've replaced quite a few chains over the years without replacing sprockets, it's entirely valid in some applications - but it is a judgement call.

Sometimes you can just accept slightly accelerated chain wear, and sometimes there simply isn't sufficient wear on the sprockets to make it worthwhile replacing them - especially if you know you'll be in there again for more work in a fixed mileage.

A shop may well advise replacing both, if the sprocket is worn - even if you fully intend to do some work in 20,000 miles and the sprockets would comfortably last that long, they don't know if you will actually do it or not - and what happens to their reputation if you sell the car without, and it later goes bang with their invoice as the last work done?

Given the consequences of failure (and the difficulty of checking/replacing), I'd say to err on the side of caution - any doubt, replace it while the motor is out.

The cam drive chains also go through quite a tight radius, and I'm given to understand that Porsche tensioners aren't as rock-solid as you might like to think when the motor is running. In fact, most chain drives - even correctly setup ones - can be quite alarming to watch without the cover.

That said, it occurs to me that I'm pretty sure my shop just fitted new chains and tensioners to my existing sprockets on my 930 rebuild - and I'm pretty comfortable with their doing so.

Their standing instructions for the rebuild are "if there's a question mark or a doubt, don't even ask, just replace".

Just my 0.01 GBP.

justinp71 · 04-06-2007, 02:19 PM

Thanks for the info, Spuggy. I did some more analysis and found that my left side cam sprocket face is 56.3mm from the front of the intermediate shaft, according to Wayne's book it is only suppose to be 54.8mm. That is 3 shims off and could possibly be why this chain is considerably stretched more than the right side chain.

I also checked all of my head studs, they were all snug. But only about half of them were at the proper torq value. I know I definitely will not be pulling the motor apart down to the case right now.

When you talk about the sprockets, are you concerned with the intermediate sprockets and the cam sprockets?

At this point I am thinking about putting the motor in, as is, for about 5k miles or so (which will probably take a year) and then pull it back out. At this point check the head studs and possibly do a rebuild then.

But then again if I replace the chains and I add an oil cooler, I believe I have pretty good chances on getting 30-50k more miles out of the motor...

What to do...

racing97 · 04:00 PM

Also
As the picture shows the gear wears and the chain drops further in to the gear pitch, as you can see in the picture. When this occurs the chain does not roll it gets tugged by the contact changing from radial to linear and it will pull the tensioner due to the extra load, and the viscious cycle will continue due to the gear both on the cam and the intermeadiate gear in the case. Look for the deep flat lines between and at the bottom.

regards

justinp71 · 05:20 PM

So should I replace the Chain and the cam sprocket?

Or leave the motor for now and later tear the motor down and replace all of the cam sprockets, intermediate shaft sprockets and chains?

edit: I probably wouldn't drive more than another 3k miles before removing it again.

Eagledriver · 04-06-2007, 08:25 PM

I'd install guards on the tensioners and drive it for another year while you get a plan together for overhauling the engine. The sprokets don't look that bad (the ones you can see from the pictures). There have been some pictures of some ugly sprokets on here that were still working. Yes the chain has some wear but there have been others on here that were so worn that they rubbed on the case.

-Andy

spuggy · 04-07-2007, 03:12 AM

Quote:

Originally posted by justinp71
Thanks for the info, Spuggy. I did some more analysis and found that my left side cam sprocket face is 56.3mm from the front of the intermediate shaft, according to Wayne's book it is only suppose to be 54.8mm. That is 3 shims off and could possibly be why this chain is considerably stretched more than the right side chain.

Seems pretty reasonable - chains don't like being run with the ends mis-aligned.

A chain is a collection of a lot of little roller bearings, they don't (or shouldn't) have much slack in the side-to-side plane.

This is a good tell-tale of a worn chain (but hard to check whilst installed, and again, clearest when compared against a good part). There are published tolerances for chains, in terms of allowable side displacement per foot.

If they're misaligned, they have to conform somehow - which is accelerated wear to the chain, and maybe sprocket damage...

Reading the appropriate section in Wayne's book, he gives the tolerance on sprocket misalignment as 0.25mm.

Quote:

When you talk about the sprockets, are you concerned with the intermediate sprockets and the cam sprockets?

If one sprocket is worn, you should look hard at any others involved. They're not necessarily toast - but do you know they were all installed at the same time?

Quote:

At this point I am thinking about putting the motor in, as is, for about 5k miles or so (which will probably take a year) and then pull it back out. At this point check the head studs and possibly do a rebuild then.

But then again if I replace the chains and I add an oil cooler, I believe I have pretty good chances on getting 30-50k more miles out of the motor...

What to do...

Photographs can be very misleading.

You don't say how long it already ran like that/when the misalignment was probably introduced? Might help you decide whether to run it another 3-5k miles?

I wouldn't be very comfortable with not correcting the sprocket misalignment - unless it's already done some substantial mileage like that?

A new chain would have much less ability to conform and would just knock out the sprockets that much faster.

I'd be inclined to leave the old chain in unless I corrected that - but yours does look kind of stretched...

You already said the motor detonated when revved - that's not good either.

I think I'd probably vote for tensioner guards, sprocket alignment and a new chain as a minimum - if the sprockets looked servicable - but it's your motor.

justinp71 · 04-07-2007, 10:26 PM

The motor has been in the car as is for about 6 yrs and maybe 10k miles or so... we bought the motor before then with the history pretty much unknown.

I don't mind putting some time and money (chains and sprockets) into putting the motor back right. I just don't want to do it and then have the motor fall apart later on, but I guess thats a risk that you normally take.

I also just noticed that both of the chains have master links, so they must have been replaced at one time.

eapcpa · 04-08-2007, 07:52 AM

While the motor is out, you should borrow or buy the tools to check and reset your cam timing. You will need them later if you are going to do a rebuild. I would bet your cams are at the retarded end of the specs and retiming toward advance should give you a little better throttle response. Put the collars on the tensioners and enjoy driving for another year or so. Adjust the valve clearance also.

2.7RACER · 03:36 PM

A The dimension from the face of the case to the face of the left drive sprocket.
Measured through access hole after cap removed.
B The dimension from the straight edge at the face of the case, to the face of
the left cam sprocket.
A and B need to be equal.
C The dimension from the straight edge at the face of the case, to the face of
The right cam sprocket. This dimension needs to be A + 54.8mm.
D This is the design center to center dimension of the left and right drive
sprockets on the intermediate shaft.

Justin, Hard to say if your sprockets are aligned or not. The right side shows alignment wear. The left side does not. I would replace both chains ASAP.
There is a small access hole through which you can measure to the face of the left drive sprocket on the intermadiate shaft, dimension A.
With a straight edge on the same part of the case the dimension to the face of the left cam sprocket, B, should equal the dimension through the hole, A.
The right side, C, will be dimension A plus 54.8mm.

justinp71 · 04-08-2007, 06:38 PM

Quote:

Originally posted by 2.7RACER

A The dimension from the face of the case to the face of the left drive sprocket.
Measured through access hole after cap removed.
B The dimension from the straight edge at the face of the case, to the face of
the left cam sprocket.
A and B need to be equal.
C The dimension from the straight edge at the face of the case, to the face of
The right cam sprocket. This dimension needs to be A + 54.8mm.
D This is the design center to center dimension of the left and right drive
sprockets on the intermediate shaft.

Justin, Hard to say if your sprockets are aligned or not. The right side shows alignment wear. The left side does not. I would replace both chains ASAP.
There is a small access hole through which you can measure to the face of the left drive sprocket on the intermadiate shaft, dimension A.
With a straight edge on the same part of the case the dimension to the face of the left cam sprocket, B, should equal the dimension through the hole, A.
The right side, C, will be dimension A plus 54.8mm.

So your saying that the 2nd picture is showing alignment wear?

From the measurements that I took the spocket in the second is lined up, but the sprocket shown in the first photo is about 1.5mm off (56.3mm)

2.7RACER · 04-08-2007, 07:44 PM

Justin,
I don't understand your statement.

"I did some more analysis and found that my left side cam sprocket face is 56.3mm from the front of the intermediate shaft, according to Wayne's book it is only suppose to be 54.8mm. That is 3 shims off and could possibly be why this chain is considerably stretched more than the right side chain."

You measure the face of the drive sprocket on the intermediate shaft through the access hole. Dimension A.
You then place a straight edge across the face of the case at the access hole and measure to the left cam sprocket face. Dimension B.
The dimension at A is the dimension to set B.
So what did you get when you measured dimension A?

hcoles · 04-13-2007, 07:09 PM

at this point I don't know if my comment will help.... I had quite a time measuring and remeasuring to develop a consistant reading. I made a spread sheet and put the numbers in until I got constant numbers over about 3 measurements.... after all of that the numbers were right on using the original number of shims... I used the Stomski tool and checked the bar on a surface plate...