Just Curious about Timing Chains and Nylon Gears

S

SteveW

Active member
Nov 3, 2012
567
44
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Greetings from Southern California and Belated Happy Holidays !!

I just completed some work on the front end of my engine (455). Timing chain, timing gears, water pump, fuel pump, engine mounting cushion, new oil
lines, new hoses and belts, and the new Sprinter radiator fan setup. Thanks to Tom C, Bill K, and Dave G for their help and support. It's not a
trivial task for a first timer - but when I do it again - it'll be lots faster and easier.

I complete understand that the nylon gears are categorized as "when they fail" not as "if they fail...". I know quite a few of use have replaced
them preventatively as I just did with 98,000 miles.

Having finished the job - it now makes me wonder - do we know of any (original) timing gear / chain failures ?? A few ?? A lot ?? Enquiring minds
want to know.

Thank you all for the wealth of information and advice posted here. I've certainly been a beneficiary and sincerely appreciate it.

Steve W
1973 23. Yellow
Southern California
 
The nylon cam gears fail fairly often. Sometimes with catastrophic results.
The metal ones wear along with the chain. When the accumulated wear is
sufficient, timing will jump, usually retarding cam timing to the point
that the engine will run so poorly that repairs need to be made.
Time to failure depends upon use and oil change and maintenance
intervals.
Best to change them before they fail.
Jim Hupy
Salem, Or
78 GMC ROYALE 403, with original nylon cam sprocket and 130,000 + original
miles.

On Wed, Dec 26, 2018, 7:56 PM Steve Weinstock Greetings from Southern California and Belated Happy Holidays !!
>
> I just completed some work on the front end of my engine (455). Timing
> chain, timing gears, water pump, fuel pump, engine mounting cushion, new oil
> lines, new hoses and belts, and the new Sprinter radiator fan setup.
> Thanks to Tom C, Bill K, and Dave G for their help and support. It's not a
> trivial task for a first timer - but when I do it again - it'll be lots
> faster and easier.
>
> I complete understand that the nylon gears are categorized as "when they
> fail" not as "if they fail...". I know quite a few of use have replaced
> them preventatively as I just did with 98,000 miles.
>
> Having finished the job - it now makes me wonder - do we know of any
> (original) timing gear / chain failures ?? A few ?? A lot ?? Enquiring
> minds
> want to know.
>
> Thank you all for the wealth of information and advice posted here. I've
> certainly been a beneficiary and sincerely appreciate it.
>
> Steve W
> 1973 23. Yellow
> Southern California
>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Steve,

In the mechanical world, fail doesn't always mean "break". The nylon timing gears will do that, but it is rare. In passcar service, catastrophic
failure is rare. In all my durability time, I have seen but a few.

In the past, more than a few coach owners have reported that the ignition and/or cam timing (harder to measure)had drifted late. One way to find this
is to set the distributor and then see how much you can move the crank without the distributor moving.

Now let's look at motorhome service. The average cylinder pressures are going to be higher so load on the entire valve gear is going to be higher
also. This last year, I broke down (disassemble) two 455s. One had the OE chain and the other had been replaced. The OE chain was probably still in
service limit and that may have been the only part of that engine that was. It had been abused. If it had been in a coach, that would most likely
have been scrapped before it got that far.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
I know there has been a couple failures over the years, but most are just very loose when they go to change them out. I do know of at least one
owner who had zero slop in a 80k mile 403. I always wondered if the 455 was harder on the gear/chain then a 455, but others have told me they have
seen slop in the 403.

Chris Giles had his chain jump a tooth last summer. His engine/coach is very meticulously maintained, and I don't think he pushing any mileage
limit.

here was my chain at about 80k miles:

https://youtu.be/2mqm6l3EmnI

--
Jon Roche
75 palm beach
St. Cloud, MN
http://lqqkatjon.blogspot.com/
 
Steve,

When I got my coach in 2008, with 66,000 miles on it, both Jim Bounds and my local mechanic, Ron Murphy, recommended that I replace the timing chain,
saying that the nylon tipped cam gear is subject to failure more due to age than mileage. Ron had done one for a GMCer who was passing through town
and the teeth had failed and the oil pan had ended up with a bunch of plastic in it. Ron told the guy that he really needed to remove the oil pan to
clean the debris out, but we all know what a hassle that is, so the owner opted out. He spun a bearing some time after he left Tucson due to a
clogged oil pick-up. Should have listened to Ron and spent the money.

I did pretty much the same thing you just did, minus the fan, at about 70,000 miles and the chain and gears seemed to be fine. The chain was a bit
loose, but the nylon tips on the cam gear were all in tact. It did give me some peace of mind to have it done. Yes, it was a lot of work and a lot
of money, but worth every penny to no longer have to worry about that.
--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member
 
It is heat and time that causes the nylon gear to age

> I know there has been a couple failures over the years, but most are just
> very loose when they go to change them out. I do know of at least one
> owner who had zero slop in a 80k mile 403. I always wondered if the 455
> was harder on the gear/chain then a 455, but others have told me they have
> seen slop in the 403.
>
> Chris Giles had his chain jump a tooth last summer. His engine/coach is
> very meticulously maintained, and I don't think he pushing any mileage
> limit.
>
>
> here was my chain at about 80k miles:
>
> https://youtu.be/2mqm6l3EmnI
>
>
>
>
>
> --
> Jon Roche
> 75 palm beach
> St. Cloud, MN
> http://lqqkatjon.blogspot.com/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
In the seventies I worked in a Pontiac dealership service department. We replaced the timing chain assemblies regularly. My job (being the new guy)
was to remove the pan and get the trash out. We had a special engine jack that attached to the corners of the front of the engine to raise it enough
to slip the pan out. I would clean the pan out and put it back and tuck a shop towel in the front and a more experienced person would do the rest. I
don't ever remember one having skipped time BUT that was lots of years ago. I have not gotten to that job yet on my GMC.

Melbo
--
Albuquerque NM Bus Conversion 1978 MCI 1973 GMC
 
Dealers are taught to force cutomers to do these things to keep the shop
busy.
Not a warrenty work.

> In the seventies I worked in a Pontiac dealership service department. We
> replaced the timing chain assemblies regularly. My job (being the new guy)
> was to remove the pan and get the trash out. We had a special engine jack
> that attached to the corners of the front of the engine to raise it enough
> to slip the pan out. I would clean the pan out and put it back and tuck a
> shop towel in the front and a more experienced person would do the rest. I
> don't ever remember one having skipped time BUT that was lots of years
> ago. I have not gotten to that job yet on my GMC.
>
> Melbo
> --
> Albuquerque NM Bus Conversion 1978 MCI 1973 GMC
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
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