Timing Chain Slack - Team Camaro Tech
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post #1 of 16 (permalink) Old Sep 7th, 16, 08:32 PM Thread Starter
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Timing Chain Slack

Hi guys, my first post. I know this topic has been discussed on various threads, but my details are a bit different. I have a 68 with a 350. I bought it about 20 years ago and it has been in the garage apart most of this time. When I bought it, it had a replacement motor, all stock, 1974 block, and I think it had been very recently installed/new. I have probably driven it only a few miles over the 20 years. Now I am starting my restore in earnest.

I had the timing cover off & noticed quite a bit of slack in the timing chain (have video but wasn't able to upload it).

I read about how to measure timing chain slack. Attached wire pointer to crank, rotated crank ccw until tight, set pointer to zero on timing tab, & rotated CW until slack taken up against cam. Got 1-2 degrees crank rotation, so thought I was OK. Now have engine buttoned up & decided to check slack at the distro rotor. I am seeing 7 - 8 degrees slack! Does this indicate excessive play in cam/distro gearing, or that I really do have excessive timing chain wear, or ???

Seems puzzling that the timing chain would be worn much with so few miles on the engine.
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post #2 of 16 (permalink) Old Sep 8th, 16, 04:36 AM
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Re: Timing Chain Slack

Welcome, Bob
You did good on measuring slack, but you should have gone both CCW & CW until resistance was felt.
And apparently you did after buttoning it up.
Most slack I've seen is about 16 and the chain had almost enough slack to 'skip' a tooth, and at the final 1/4 rpm at shutdown, this is the place the chain will most likely skip.
So my Rule Of Thumb is 10-12 of slack, then replace.

The chain is under tension both ways of rotation - as the lifter goes up the lobe opening ramp and lifter goes down the other side closing ramp.
You can see this while turning engine by hand with cover off.
But, as the chain spins at speed, the chain is always trying to obtain a 'circle' configuration because of centrifugal force.

Double roller chains and sprockets work better.
An old school trick, since the chain only gets splashed oil from the front cam bearing, a 0.010"-0.015" diameter hole is drilled into the main oil galley plug - if it is a pipe plug - to lube chain.
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post #3 of 16 (permalink) Old Sep 8th, 16, 06:06 AM
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Re: Timing Chain Slack

Quote:
Originally Posted by Everett#2390 View Post
Welcome, Bob



An old school trick, since the chain only gets splashed oil from the front cam bearing, a 0.010"-0.015" diameter hole is drilled into the main oil galley plug - if it is a pipe plug - to lube chain.

x2

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post #4 of 16 (permalink) Old Sep 8th, 16, 04:39 PM
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Re: Timing Chain Slack

To install a double roller chain you may need to grind the block so you can clear the double gear and chain. make sure you check the clearance first. Install the gear on the cam and push it back and rotate to ensure it's clearing the block/ It will typically hit at the oil galleries above the cam bearing.

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post #5 of 16 (permalink) Old Sep 8th, 16, 07:01 PM
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Re: Timing Chain Slack

Did you check the vertical play in the distributer shaft? They typically have too much play and need to be shimmed. Remove gear, add shims and reinstall gear. Can't remember the correct play but someone will know. Not much.

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post #6 of 16 (permalink) Old Sep 8th, 16, 09:16 PM
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Re: Timing Chain Slack

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Originally Posted by scblucam View Post
Did you check the vertical play in the distributer shaft? They typically have too much play and need to be shimmed. Remove gear, add shims and reinstall gear. Can't remember the correct play but someone will know. Not much.
If I remember correctly, a cast iron distributor should ideally be .006" to .007" and an aluminum should be .014" to .015" (difference due to the different expansion rates between cast iron and aluminum). Most of the distributors I've found are anywhere from .060" to .080" which is a lot of vertical play (read slop).

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post #7 of 16 (permalink) Old Sep 9th, 16, 05:12 PM
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Re: Timing Chain Slack

Slop's not good. I'd just replace the gears and chain with a quality double roller like Crowler or equivalent. I did mine many years ago and it was an easy task. Glad I did as mine were a bit sloppy too.
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post #8 of 16 (permalink) Old Sep 13th, 16, 08:14 PM Thread Starter
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Re: Timing Chain Slack

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Originally Posted by Everett#2390 View Post
Welcome, Bob
You did good on measuring slack, but you should have gone both CCW & CW until resistance was felt.
Hey guys thanks for the quick replies and good info. Sorry for my delay in responding, I've been offline for a while. I may not have described what I did too well. I did turn the crank CCW until the cam was about to turn and then turned CW until the cam turned and measured degrees rotation (1-2) between the two points. Jake, I think I did what you described. ??

I will check distro shaft end-play. That sounds like a possible source of the relatively large discrepancy between the measurement at the crank vs at the distro.

Thanks
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post #9 of 16 (permalink) Old Sep 14th, 16, 04:25 AM
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Re: Timing Chain Slack

You did good, Bob, 1-2 of slack is almost like new
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post #10 of 16 (permalink) Old Sep 14th, 16, 03:33 PM Thread Starter
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Re: Timing Chain Slack

Sorry Everett, I thought it was Jake who was working on the car!

Bob H
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post #11 of 16 (permalink) Old Sep 14th, 16, 05:47 PM
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Re: Timing Chain Slack

You were more correct than you might think Bob. I hear Jake is Everett's, supervisor, even if in spirit only.
So, guys, first post with the old computer moved out to the shop via 200 foot Ethernet cable. OK so far?

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post #12 of 16 (permalink) Old Sep 23rd, 16, 10:38 PM Thread Starter
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Re: Timing Chain Slack

I checked the distro end play - got about .060 gap. Got shim washers (had to order from Amazon) & shimmed it down to about 0.20 gap. Measured slack based on distro movement -- still getting about 8 deg crank rotation! So I'm wondering if: 1) this is indicative of excessive cam/distro gear wear, or 2) is simply due to the fact that when basing the measurement on distributor motion, the cam/distro gear set (naturally) add more play and the difference between the two measurements (2 deg vs 8 deg) is normal?

Anybody out there ever do this comparison in the past?
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post #13 of 16 (permalink) Old Sep 24th, 16, 01:49 PM
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Re: Timing Chain Slack

Greetings Bob: All things considered, if you are in the engine that far and have that much slack , which is to some tolerable and acceptable, for the low cost of a good set of gears and double row chain, make it like new again. That removes all bout.

Just my opinion here so don't take it wrong but I suspect your 20 year old "new" engine was actually a shortblock with parts from the old engine hung onto the new block. The rest was what used to be referred as an "IMRON REBUILD" referring to IMRON paint. Again, nothing disparaging in my thoughts, just years of experience.

Regardless, I'm confident you will end up with a first rate engine repair. Your diagnostics are top tier. Good job. That kind of explanation of diagnostics is a great teaching aid for the up and coming generations of owners.

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post #14 of 16 (permalink) Old Sep 24th, 16, 03:51 PM Thread Starter
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Re: Timing Chain Slack

I agree, I should have put the new parts in when I had it apart. Prob is that now its all together, coolant is in, etc. I am just a few work hours away from turning the key to start it & having the "moment of truth" --after 2+ years of having it apart. Just need to finish reworking the exhaust to fit the new headers. So ... I think I will live with it for now. My question above is mostly curiosity vs deciding to change out the timing set.

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post #15 of 16 (permalink) Old Oct 4th, 16, 10:35 PM Thread Starter
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Re: Timing Chain Slack

Back again, I know I'm slow. I am retired so I'm busy. I finally fired it up, set the timing, and it runs good. I don't hear any unusual noise from the timing chain area, and when checking with timing light, timing doesn't seem to bounce around at all. So all is good. Thanks for the advice.

Once again, maybe what I was seeing is just a normal difference between the slack measurements made the 2 different ways.
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