Nylon Timing Gear & chain replacement

[Joe L.]

Re: Nylon Timing Gear & chain replacement

Isn't it possible, even likely, that the valve train was designed with a designed failure path? It's a common engineering practice to design a "weak link" that predictably breaks first during system failure in a way that minimizes damage.

For example, what if the cam bearings fail, and the camshaft seizes? With an all steel timing set, isn't there a good chance the cam will grenade in the block causing a big mess? But what if the timing set was the designed failure path? The gear breaks instead of the cam.

I don't know if this is why the timing gear was designed with nylon. But it seems curious Chevrolet would under design a critical part of its engine just so it would run a bit quieter. Is it possible the engineers specified nylon teeth for another reason?

Mark-------


I am very confident that noise control was the primary, if not exclusive, reason that the nylon-toothed cam sprocket and powder metal crankshaft sprocket were used. These were not only used by GM but also by many other automotive manufacturers.

You will note that for MD/HD truck engines, they did not use the nylon and powder metal sprockets. That's because noise control is not an issue in these applications but low maintenance and durability is.

[Joe R.]

Re: Nylon Timing Gear & chain replacement

Dan------


That's exactly where the interference, if there is any, occurs with the Tru Roller timing set. Like I say, I've never had it occur but I've heard of it. Your solution for relieving it is exactly what I spoke of.

Hi Joe:

I'm posting this just for anyone reading this thread who is considering a double roller timing set.

I've only installed the wider, double-roller sets on two engines (both were 1967 '657 small block castings), but in both cases the cam gear interfered slightly with the boss on the 12 o'clock oil gallery plug.

The first time I was not expecting the problem, but I'm glad that I detected it and fixed it with some minor grinding on the block. I think it would be very easy for someone to miss this and have unexpected problems.

I think the reason I noticed the problem is that the double roller is noticeably wider than the the OEM set. I was concerned about it hitting timing cover, but my measurements showed that the timing cover clearance was the same as the OEM set, and the extra width was all placed on the block side.

In any event, the problem is easy to fix if you detect it during installation, but I think it's also easy to miss. I'm surprised that Cloyes did not have a warning about this possible problem in their installation instructions. It's pretty important to check the clearance.

[Domenic T.]

Re: Nylon Timing Gear & chain replacement

GM also had another bad timing gear that was on the 235 6 cylinder. It was a replacement gear and I don't know if it was aftermarket. I rebuilt m,y 235 truck engine in 1970 and the cam gear was fiber. That was all that I could get. It was also for noise reduction. The truck made it from Chicago Heights to LA, and on the way back the gear lost most of it's teeth and I was stranded in Oklahoma. A fella stopped and said he could help me but I needed a new gear. I bought what they had and that was another fiber gear. It also failed on my return trip with about 5K on it. I then left the truck and searched for a iron gear and when I found one the parts guy said I was going to make NOISE. I told him the noise would be acceptable as the noise of pistons hitting valves was much worse.

dom

[Kenneth B.]

Re: Nylon Timing Gear & chain replacement

GM also had another bad timing gear that was on the 235 6 cylinder. It was a replacement gear and I don't know if it was aftermarket. I rebuilt m,y 235 truck engine in 1970 and the cam gear was fiber. That was all that I could get. It was also for noise reduction. The truck made it from Chicago Heights to LA, and on the way back the gear lost most of it's teeth and I was stranded in Oklahoma. A fella stopped and said he could help me but I needed a new gear. I bought what they had and that was another fiber gear. It also failed on my return trip with about 5K on it. I then left the truck and searched for a iron gear and when I found one the parts guy said I was going to make NOISE. I told him the noise would be acceptable as the noise of pistons hitting valves was much worse.

dom

Now that's what I'm saying. I towed my brother home off the interstate when his GTO judge put the piston through the side of the block. He was doing about 90MPH & he said it sounded like a mortar round when he was in VN 6 mounts before that.

[Joe L.]

Re: Nylon Timing Gear & chain replacement

GM also had another bad timing gear that was on the 235 6 cylinder. It was a replacement gear and I don't know if it was aftermarket. I rebuilt m,y 235 truck engine in 1970 and the cam gear was fiber. That was all that I could get. It was also for noise reduction. The truck made it from Chicago Heights to LA, and on the way back the gear lost most of it's teeth and I was stranded in Oklahoma. A fella stopped and said he could help me but I needed a new gear. I bought what they had and that was another fiber gear. It also failed on my return trip with about 5K on it. I then left the truck and searched for a iron gear and when I found one the parts guy said I was going to make NOISE. I told him the noise would be acceptable as the noise of pistons hitting valves was much worse.

dom

dom-------


The original GM camshaft gear for MOST 235 cid engines was machined from Micarta or Bakelite. It was GM #3836156. While it is long-since GM-discontinued, it's still available in the aftermarket. The Cloyes part number for the gear is 2500 and it's virtually identical to the original gear.

There was also an aluminum gear originally used for some applications, 1953-55 Corvettes being among those applications. That gear was GM #3835473. This gear is interchangeable with the above-referenced gear. It's long-since GM discontinued, too, but it's also available in the aftermarket. The Cloyes part number is 2514 but it's machined from bronze, not aluminum.

By the way, the crankshaft gear used with the above for all applications except 53-55 Corvette was GM #3836605. This was a steel gear, long-since GM discontinued. It's also available in the aftermarket. The Cloyes part number is 2501. It may or may not be machined from bronze rather than steel. The 53-55 Corvette crankshaft gear was GM #3836073. How it differed from the 3836605 I do not know. It was discontinued without supercession in December, 1973 and I know of no aftermarket equivalent.

[Dan B.]

How it turned out

I thought I would follow up for those that have been following this thread on how things turned out. Fired it up today for the first time and all is well. No more jumping timing marks, and in fact the old chain was so worn I had to reset the timing again but now it's rock steady. One additional thing, I don't really notice any discernable noise difference between the new chain and gears and the old nylon one. I am sure there must be, but I don't notice it. In any case, I could not be more pleased with how great the car ran and very glad I did the job!

[Phil W.]

Re: Nylon Timing Gear & chain replacement

IMG_2730.jpg

This has been a very informative discussion and I thought I'd share my experience.
Pictured above is the result of my stock GM nylon / plastic timing gear / chain set coming apart in my 1971 LS5 454. I was fortunate, it only broke a couple pieces out of one piston skirt (seen in picture) and lightly scared another cylinder. It could have been much worse. I found this mess in the bottom of the oil pan only because we dropped the pan to change the gasket. The engine ran fine with no warning up to that point.
I plan to replace it with the later truck set as suggested by Duke.
I would seriously suggest changing out the stock set before this happens to you - I wish I had many years ago.
Thanks again for all the shared knowledge - it is greatly appreciated.
Phil

[Dan B.]

Re: Nylon Timing Gear & chain replacement

Phil, thanks for posting your actual experience which was my original reason for this thread. I wanted to raise awareness and to help save others with the original 50 + year motor and nylon gear from a catastrophic failure.

[Domenic T.]

Re: Nylon Timing Gear & chain replacement

Joe,
Thanks for that info. Now if I remember back 45 years ago, they were gear to gear.

Dom

[Joe L.]

Re: Nylon Timing Gear & chain replacement

Joe,
Thanks for that info. Now if I remember back 45 years ago, they were gear to gear.

Dom

Dom
-------


Yup. These were actual GEARS, not sprockets. There was no chain used.

[William F.]

Re: Nylon Timing Gear & chain replacement

TMI!. Duke, are those Cloyes chains you recommend 9-3100 and 9-3110 going to cause interference with timing chain cover? Are they same or different from ones from NAPA?
Thanks

[Duke W.]

Re: Nylon Timing Gear & chain replacement

I DID NOT recommend those part numbers... suggest you read my posts in this thread.

Duke

[Joe L.]

Re: Nylon Timing Gear & chain replacement

TMI!. Duke, are those Cloyes chains you recommend 9-3100 and 9-3110 going to cause interference with timing chain cover? Are they same or different from ones from NAPA?
Thanks

William------


Those are the ones that I recommend. They are not the same as the ones you get under a NAPA part number.

I have never had any trouble with interference with the timing cover using the 9-3100 (small block) or 9-3110 (big block). However, there may be very slight interference with the cam sprocket and the block. If it occurs, this can easily be dealt with using a Dremel tool.