Timing Chain and Spockets Qs

[Tony H.]

When did Chevrolet transition from nylon toothed to steel toothed timing chain sprockets on the small block and big block engines? Did double roller timing chains replace them or are those only after market upgrades?

[Don Barnes]

Re: Timing Chain and Spockets Qs

Tony, i beleive the steel timing gears came before nylon and i don't remember any production engines coming with a double roller maybe someone else can chime in.Nylon gears were used on some low hp engines i know some pontiac and buick v 6 engines had a nylon gear.Hope this helps,Don.

[Joe L.]

Re: Timing Chain and Spockets Qs

Tony------

As far as PASSENGER CAR and LIGHTTRUCK (including Corvette) PRODUCTION engines go, all small block and big block originally used the "Morse" or "early link" type chain. This goes from 1955 through the end of the installation of Gen I small blocks in the above-referenced vehicles. Som MD and HD truck engines used a "double roller" type chain, although the rollers were not a "true" roller design.

The crankshaft sprockets for passenger car and light truck engines were mostly "sintered iron", although some may have been steel.

As far as the camshaft sprockets go, it's confusing. Prior to 1967 the PROUCTION camshaft sprockets MAY have been cast iron. The SERVICE camshaft sprockets for 1955-66 small blocks were definitely cast iron.

Beginning in 1967, if not earlier, the small block PRODUCTION sprockets were changed to aluminum with nylon teeth. Beginning with their introduction with the 1965 model year, the camshaft sprockets for big block passeneger car and light truck engines were mostly of the aluminum/nylon design, although some may have been cast iron. For big blocks, I believe that the nylon camshaft sprockets used for PRODUCTION engines were used right through the last installation of these engines in passenger cars in 1975.

For 67 and later small blocks, I believe that the aluminum/nylon gears were used for most, if not all, PRODUCTION engines for passenger cars and light trucks right through the end of their use a few years ago in PRODUCTION vehicles. This applies to Gen I small blocks only inasmuch as Gen II used a completely different design timing set. However, for SERVICE, the only gears catalogued by GM for 67+ small blocks were cast iron.

Why the difference between PRODUCTION and SERVICE? I believe that it's a function of "engine noise considerations". For a PRODUCTION engine, GM wants them to be as quiet as possible. That's what the vast majority of customers want and that's what may affect their inclination to purchase a particular vehicle. For SERVICE, this is not so important. For SERVICE, customers and mechanics want to see a "steel" gear (especially after they've removed the aluminum/nylon gear with missing teeth). So, for 67+ small block SERVICE, cast iron gears are catalogued and sold. The aluminum/nylon camshaft gear, sintered iron crankshaft gear, and Morse chain are all part of a "quiet-as-possible" engine design strategy.

For MD and HD trucks, where engine noise is not a significant issue and DURABILITY is, double roller type chains are used with cast iron camshaft gears and steel crankshaft sprockets. This is true for both big blocks and small blocks. The pieces can be used for performance applications, too. However, I prefer a Cloyes True Roller timing set to any GM pieces for either big block or small block.

Gen VI big blocks with hydraulic roller camshafts ALL use a single "roller" timing set. This includes a "powder metal" camshaft gear and a steel crankshaft sprocket. It is not possible to use a double roller timing set on a Gen VIbig block with PRODUCTION-type hydraulic roller camshaft.

[Mark Ring]

Re: Timing Chain and Spockets Qs

Joe,
Do you know of any original production applications for P/N 340235? It is a hardened iron cam gear that runs with the HD production Morse type chain P/N 346261. I was expecting to find a nylon gear when I tore the L-82 engine down but instead I found this.

I went looking for a replacement chain a while back and had to shop around a bit to find one in inventory at a GM dealer out of state. Aparently they are discontinued?

-Mark.

[Mike M.]

Re: Timing Chain and Spockets Qs

For what it is worth, I believe Pontiac began using the nylon toothed gear a couple of years before Chevrolet. And for whatever reason they usually failed somewhere between 50 to 60,000 miles (in Pontiac's). If my memory serves me correctly, I think the Federal Government (somewhere in the early 70's) forced Pontiac to recall cars equipped with the nylon toothed gears that were built in the 60's and early 70's. There were too many incidents where the engine would suddenly die (due to cam gear failure) while at speed on the interstates. I think there were even some fatal accidents created by the sudden and unanticipated loss of power. I replaced at least 3 gear and chain sets on friends' GTO's while in college in 60's . Thankfully the timing cover and damper were a lot easier to remove from 389/400's than SBC's.

[Joe L.]

Re: Timing Chain and Spockets Qs

Mark------

I think that the aluminum/nylon cam sprocket was used for most PRODUCTION small block applications. It's possible, though, that the iron sprockets, GM #3896960 or 346235 were used for some. The one you found in your engine could be original or it may be that it was previously replaced. As I mentioned, the SERVICE sprockets for small blocks were iron.

The 346261 chain was used in PRODUCTION beginning about 1975 and it was also available in SERVICE until several years ago. It was replaced by chain #14087014 which remains available.