Costly engine timing mistake.....

Vipergrün · 06-04-2002, 01:41 PM

I tell my tale that others may benefit and not make the same mistake I did.

Somehow, in the process of removing/reinstalling tensioners, idler arms, and replacing chain ramps in my SC motor (71K original miles), the timing got screwed up and I had the #6 piston hit a valve.

I took all precautions, wedged the cam sprockets so they could not move, tied off chain on both sides so it could not move, or come off the sprockets, and put the motor at TDC on #1 before starting.

The ONLY thing I did was take slack from one side to the other when replacing the chain ramps. All slack was removed from chain when putting idler arms and tensioners back in and everything *appeared* to be ok.

If I were to do it all over, I'd purchase a dial indicator gauge and check the cam timing AFTER turning the motor over by hand to verify nothing was binding.

Luckily, my mechanic had a 43K original mile SC motor and gave me a decent trade on my motor. While in the long run I probably got a good deal and a low mileage motor, it was a very expensive mistake, to the tune of $4500.00 cash and my old motor.

Please folks, be careful, check your work.....then check it again. DO NOT turn that key until you are sure you are ready.

BTW, this project was warranted due to chain rattle and a slight knocking noise which turned out to be a clogged oil feed line to the passenger side tensioner, allowing movement of the idler arm.

Cheers
-Brad
80SC with new(er) motor.

Early_S_Man · 06-04-2002, 02:00 PM

Sorry to hear of your misfortune, Brad!

Yes, given the infrequent visits to the chain cases ... checking the cam timing afterwards is a good idea, anyway. I did it in '79 when I replaced my tensioners the first time, and I guess it should be recommended as a part of the tensioner procedure. Some things that are just intuitive doing certain procedures tend to get left out of the articles!

Doug Zielke · 06-04-2002, 02:35 PM

I was very new to the 911 motor when I did my tensioner upgrade. I thought of the same horror described by Brad.

Back then, I felt I didn't have enough know-how to check cam timing, so before I "turned the key" I turned the motor over several times slowly by hand to feel for any binding.

Tinker · 06-04-2002, 04:40 PM

Guys,

A dial gauge and holder cost less than $100. Seems like a small investment as compared to the cost of getting the cam timing wrong.

Tinker

Wayne 962 · Wayne 962's Garage

Quote:

Originally posted by bb80sc

The ONLY thing I did was take slack from one side to the other when replacing the chain ramps. All slack was removed from chain when putting idler arms and tensioners back in and everything *appeared* to be ok.

I'm not really sure what this means? For the SC to crash like this, you must have slipped a tooth somewhere along the line, and not known it...

It's not terribly easy to check the timing with the engine in the car, however if it gives you piece of mind...

-Wayne

Vipergrün · 06-04-2002, 08:24 PM

Hi Wayne, when I said this....

"The ONLY thing I did was take slack from one side to the other when replacing the chain ramps. All slack was removed from chain when putting idler arms and tensioners back in and everything *appeared* to be ok"

I meant that as I removed the tensioner/idler arm from one side,
I was able to get a bit of slack on the opposite side by gently pulling on the chain. Is there any way this could have turned the crank via the intermediate shaft, but not the cams, leaving the pistons in different positions?

Thanks for the replies all!

cheers
-Brad
80SC, purring like a kitten

Wayne 962 · Wayne 962's Garage

Okay, I've read this five times, and I still don't get it?

More explaination please?

-Wayne

IAN · 06-04-2002, 08:51 PM

Wouldn't the chain have to had slipped a tooth for the timing to change???

I don't get it....

I would think that pulling slack from one side to the other would only change timing if the chain slipped a tooth.

Somebody tell me if I'm wrong.

pnzr911 · 09:07 PM

Hmmmm I just did this a few months ago on my 83SC. I followed Waynes instructions to the letter and it all came out perfect. The only "special tool" I bought was a small adjustable sprocket holding tool that would prevent the cams from moving. I also used small blocks of wood to support the chains so they would not move one bit when I replaced the chain guides. One thing I didn't do that you did was mess with the idle arms. My 83 idle arms are fine. The only scary moment was when I dropped my guide down into the crankcase. Luckly I only had to remove the sump screen to retrieve it(Wayne put that in your book will ya!) Sorry about your misfortune. Bye the way does your "new" engine have the Carrera chain tensioners on it ??

Bye the way I have to agree with Wayne and Ian sounds like you slipped a tooth.

Vipergrün · 06-04-2002, 10:35 PM

Quote:

Originally posted by Wayne at Pelican Parts
Okay, I've read this five times, and I still don't get it?

More explaination please?

Hmmm, lets see if I can splain it better.

I could not remove the left side outboard or lower inboard ramps due to the chain being tight per the picture below. Since I had removed the right side tensioner and idler arm, there was slack in the chain at that location. To give myself a bit of space to work, I slightly lifted the chain and brought a small bit of slack from the lower right side to the lower left side. The cam sprockets did not turn as they were wedged in 4 locations. My assumption is that I turned the sprocket on the intermediate shaft, which turned something else and threw the timing off.

Is this any better

Adam · Adam's Garage

I think I get it...

The tensioner is supposed to take up the slack, which wasn't there as you pulled it over to the other lower side. When it started, it started heaving on the chain straight away and that's probably what caused the valve-bendy incident!

Have I got it right?

911pcars · 06-04-2002, 10:58 PM

bb80SC,
Is that image your engine? If so, I notice the cam nut is not there. If you removed the sprocket pin and something moved, there is a very likely chance you lost the proper cam/sprocket relationship. Removing the nut and sprocket pin isn't normally done when changing just the tensioner.

If the cam and sprocket relationship is okay, then the only other way you could have lost timing was by skipping a tooth somewhere. This could happen if the idler sprocket engages a loose chain link. The "extra" link could have transferred around the camshaft sprocket as the engine rotated.

Just my guess,
Sherwood Lee
http://members.rennlist.org/911pcars

Wayne 962 · Wayne 962's Garage

No, that's a pic from 101 Projects...

Sounds like you skipped a tooth... on the intermediate shaft sprocket. You can never let the chain run loose, ever!

So the right side was just hanging down? That's a no no... I specifically state in the 101 Book that you shouldn't let any tension off of the chain...

Sorry you had to learn the hard way, let's at least hope that a whole bunch of people read this and learn from your mistake...

Sherwood, I disagree with your guess there. The idler sprocket is indeed just that - an idler. There is no way that it can cause the cam sprocket to skip a tooth - it just spins freely on the shaft...

Adam, that doesn't sound exactly right to me. I would have guessed that he would have had tension on the chain when he started the engine (from the tensioner).

Keep in mind a very important fact. You can't let tension go off of the chain because the camshaft is under spring tension from the valve springs. Not so much with CIS and Motronic engines, but especially with the earlier engines. If you let go of that thing, it will spring out like you're opening up a watch! Keep tension on the chain at all times...

-Wayne

911pcars · 06-05-2002, 08:41 AM

"I disagree with your guess there. The idler sprocket is indeed just that - an idler. There is no way that it can cause the cam sprocket to skip a tooth - it just spins freely on the shaft... "

Wayne,
I stand corrected. You're right. It's just an idler to take up chain slack. I was visualizing the wrong direction of rotation. However, I think we all agree that when the chain is left slack (as when the tensioner is removed), valve train tension may cause the cam to suddenly rotate CW or CC and jump a chain tooth.

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

Wayne 962 · Wayne 962's Garage

Yes, that I would agree with. I don't think that there's any way any of us (including bb80sc) will be able to figure out what happened here. The good news is that this very rarely happens, and if you keep the chain tight, then you should have no problems at all.

I just don't want people to shy away from this procedure. Keep the chain tight, and you should have no problems. It's an easy upgrade...

-Wayne