oversized timing gears for line bored block

[Blazer406]

Background:

Motor gets line bored after caps or girdle install. Center distance between cam and crank gets slightly shorter due to the line bore..... this makes the timing chain appear loose. Presto... there are companies that manufacture timing chans with oversized gears to "take-up" the slack. The gears are machined slightly bigger diameter..... but the chain itself is the same as a comparable stock center distance chain. Now I admit... I am an Engineer.... and it is an attribute of engineers to ask "how did they do that".

The stock type chain has a certain pitch. Stock sprokets have teeth cut to match the pitch of the chain. This is a matched set.

How can a larger diameter sprocket be manufactured that fits the same pitch chain without stepping up 1 tooth or down 1 tooth. i.e. stock sproket has (I'm guessing) 50 teeth.... new one has 51 teeth.... so if the pitch was the same... the diameter would need to increase to accomidate the "extra" tooth. This makes sense.... what doesn't make sense... is that 1 tooth bigger would have moved the center distances a hell of a lot further than .002" (which is the first oversize Rollmaster size from stock). If it is the same number of teeth on the sprocket.... looks like to me.... the only way to make it larger diameter... would be to slighly lengthen the pitch of the teeth in the gear.... which would IMHO... make the chain also need a longer pitch...... but this can't be... cause they say a stock replacement chain will still fit.

So....... How'd they do that?

 

[Blazer406]

To further complicate my question.... since it is a fact that the crank gear must be 1/2 the number of teeth the crank gear... or you won't get the perfect "2:1" ratio required for 1 cam revolution takes 2 crank revolutions. This is a fundamental fact..... therefore it is impossible to just make a cam gear with "one" more tooth..... without actually going 2 teeth bigger on cam gear... and one bigger on crank gear....



So... I really don't know how they compensate for the .002" ......

 

[Lee Thompson]

Brain--Just like some friends of mine

Always trying to complicate simple things. You are only talking .002. That chain will never know the difference.

 

[TURBOTWIN2]

Ask yourself this: how can the big gear (cam) and the small gear (crank) have the same pitch?

 

[Blazer406]

Ask yourself this: how can the big gear (cam) and the small gear (crank) have the same pitch?

No problem.....

They are different diameters..... and have different number of teeth....

Am I missing something.

 

[Blazer406]

Always trying to complicate simple things. You are only talking .002. That chain will never know the difference.

I have been telling myself that.... but.... chain does have a fair amount of slop.

 

[Blazer406]

Always trying to complicate simple things. You are only talking .002. That chain will never know the difference.

Just tried to call you..... if you have a minute to chat...

called from my work cell....

 

[cruzn57]

rollmaster makes ....

020, 040, over size set, ( I SAID SET as in 3 pieces)
I ran a 040 over with align bore and new caps and girdle,
NO SLOP AT ALL!!!!

 

[Blazer406]

020, 040, over size set, ( I SAID SET as in 3 pieces)
I ran a 040 over with align bore and new caps and girdle,
NO SLOP AT ALL!!!!

Yes it is a set.... but they (TA Perf) go on to say that a replacement chain will fit.... unless I misunderstood it..... i.e. one of there std chains would fit one of their .002 sets.... or .004.... or .006..... same chain......

The diameter of the gear itself is what is taking up the slack.... not the length of the chain.

 

[Blazer406]

FWIW... I believe this is only possible if you machine the gear an ever so slightly larger diameter... and ever so slightly longer pitch..... so slight that you cannot really tell a difference when you put the std. pitch chain on it.

At least that is the only reasonable explanation I have at this point. The larger diameter sprocket "takes up" the slack caused by the line hone/bore.

 

[scottk]

i hAD A LINE BORE/ HONE FOR STEEL CAPS AND STILL USE A STANDERED CHAIN:biggrin:

 

[Blown&Injected]

Damn, I wondered why I had not seen this post until just now - thought it was something I could help with but this is a weird one.

I'm thinking that it is such a slight change, they just don't cut the teeth as deep.
?????

 

[Sam Colalillo]

My opinion FWIW

One of the classes I took for my engineering degree was Machine Design. In this class we learned several things including basic design concepts, loads and evaluations, stress cycle diagrams, and many other design aspects of machines.

This was an exciting class, and I remember spending a lot of time on chain drives. I still have my notebook from this class which is packed with several equations. I’ll summarize what I learned.

Several things impact chain drive wear.

Steel links in a chain ride 1/3 to 1/2 engaged on the sprocket. For maximum life and minimum wear a center distance should be chosen so as to provide an even number of links in the chain. When even links in chain are coupled to odd numbered gear teeth wear is minimized. Wear is more rapid on a drive design with a short center distance because the chain has fewer links which forces each link to articulate more often. After laying out several design concepts I learned that a design which has center distance equal to the diameter of the large sprocket plus half the diameter of the small sprocket is a good design in regards to angle of wrap and wear.

A loose chain whips or surges the slack in the chain. This improper whipping cause material fatigue. In my opinion this is why Buick installed a tensioner on the chain from the factory. This allowed them to get long life with larger tolerance on the sprockets.

A chain to tight might carry some unnecessary loading under some special circumstances. In my opinion a loose chain is a lot more of a concern than a tight one, but don’t make it so tight that you are smashing the cam into the cam bearings.

Standard sprocket sizes are listed in a machinerys handbook. I think billet steel sprockets are precision machined to exacting pitch diameters to accurately compensate for line boring. In my opinion when heavy valve spring pressures are present, the timing chain should be installed with extreme attention to detail. By installing custom machined sprockets the chain tension can be set accurately to insure a long living timing chain.

 

[turbodave231]

Sam, Great explanation .................to expand on what you said: Rollmaster makes special chain sets for linebored blocks (and damn nice ones too!). The difference is in the gears.......... the set uses the same standard chain.

The gear set still has the same number of teeth (to maintain the same ratio) only slightly larger pitch diameter to take up the chain slack.

If you go to their website there is an explanation posted.

How do I know this??? My block was line bored and I needed a shorter chain.....The standard set was too sloppy.

 

[TURBOTWIN2]

There you have it, thanks to TurboDave.
http://www.romac.com.au/pdf/Centre Distances.pdf

 

[Blazer406]

There you have it, thanks to TurboDave.
http://www.romac.com.au/pdf/Centre Distances.pdf

Reading that confirms my suspicions..... the slightly larger diameter has to increase the sprockets pitch ever so slightly.... evidently it isn't enough to really wear the chain out any faster..... at some point it would wear the chain and sprockets at a higher rate.... but evidentally... the amount they make it larger....doesn't hurt too bad.