Explain the engineering behind Rollmaster oversize chain/sprocket sets....

[Blazer406]

Maybe I am missing something.....

First let me lay the groundwork.....

Rollmaster makes timing sets with "oversized" gears/sprockets for the people that have had their engines line bored and now the stock timing chain/gear setup leaves too much slack.... because the crank centerline has been mooved up a little as a result of the line boring.......

TA's site says the chains are the same between the sets.....and that the sprockets are made slightly oversized.... to accomidate the slack.

OK... here is where I am getting lost. First lets agree that the number of teeth (per sprocket) is fixed... and cannot change. The crank gear will have 1/2 as many teeth as the cam gear... therefore spinning the cam at 1/2 crank speed..... which cannot be any different.... it has to be that way.

Now.... if the chain is the same.... then the pitch or distance between each roller is the same.....and if you make the sprocket oversize....... how do you accomplish this without changing the pitch of the chain? It would seem to me if the sprocket grew in diameter......that if the tooth count was the same..... the pitch would have to grow for the chain to mesh properly with the sproket.

If they said.... each chain is matched to each different spocket "set" I would agree it is possible to manufacture a slightly longer pitch chain to accomidate the slightly longer pitch on the sprocket.... and it would work fine...... but that is not what they say...... they say the chains are the same.

Can someone explain this? - Sorry... it is just the Mechanical Engineer coming out in me..... I need to know why stuff works the way it does..... We (ME's) aren't wired to just accept an explanation and go about our way..... just doesn't happen..... LOL

 

[VIN7]

I'm not an engineer but I see where you are coming from. I see two diameters on a sprocket, diameter 1 is tip of tooth to tip of opposite tooth, and diameter 2 is bottom of tooth to opposite bottom of tooth.

For these oversized cam sprockets, do they actually increase diameter 1? Is this what you are measuring? I can see it working if they increase diameter 2 and make tooth depth shorter. How much gap needs to be made up from a line bore?

Just throwing ideas out there.

Mike

 

[Blazer406]

I'm not an engineer but I see where you are coming from. I see two diameters on a sprocket, diameter 1 is tip of tooth to tip of opposite tooth, and diameter 2 is bottom of tooth to opposite bottom of tooth.

For these oversized cam sprockets, do they actually increase diameter 1? Is this what you are measuring? I can see it working if they increase diameter 2 and make tooth depth shorter. How much gap needs to be made up from a line bore?

Just throwing ideas out there.

Mike

If you make diameter one bigger..... the pitch would also be bigger would it not? ..... the arc length would have to increase if the tooth count remained the same.

As far as how much they can move? .... the sell sets from +.002" to +.010" ..... and the best I can tell.... this is the actual amount the crank has "moved up"....... which can be measured I guess.....

 

[ijames]

I'm guessing it is a combination of two things. First, I'm sure the chain can accommodate a small range of tooth widths, and this may explain it all. Second, they could make the teeth a little narrower along with being on a slightly larger base circle, if they needed more room.

 

[Sam Colalillo]

You refuse to believe the size is of the gears makes the determination of the slack.

Here is the thread where I explained it

 

[Alky V6]

From the research I've done, the chains are the same for std. and undersize. I don't believe the undersize number designation corresponds to the change in cam to crank centerline change. At least, that is what I've found. I'm sure the difference between the sets is in the root circle of one of the gears. I would guess the cam gear. When you think about it, it wouldn't take much change in the root circle of either one of the gears to make up quite a bit of change in the cam to crank centerline. I'm sure we're talking about a very small number. A smaller amount applied to the cam gear would make for the largest change.

 

[Blazer406]

You refuse to believe the size is of the gears makes the determination of the slack.

Here is the thread where I explained it

LOL I forgot I started that thread as well....... way back..... but the issue is still puzzling to me..... and yes... you had a very thourogh dissertation......

FWIW... it still is perplexing me..... I think the manufacturing tolerances of chains... and CNC sprockets are close enough that they need to be identical pitch between sprockets and chains.... or you will have problems at some point.

 

[Blazer406]

From the research I've done, the chains are the same for std. and undersize. I don't believe the undersize number designation corresponds to the change in cam to crank centerline change. At least, that is what I've found. I'm sure the difference between the sets is in the root circle of one of the gears. I would guess the cam gear. When you think about it, it wouldn't take much change in the root circle of either one of the gears to make up quite a bit of change in the cam to crank centerline. I'm sure we're talking about a very small number. A smaller amount applied to the cam gear would make for the largest change.

From the Romac website:

Tunnel Bore Sizes
How to Determine Tunnel Bore Sizes


After fitting your standard Rollmaster Timing Chain Set to the engine you need to pinch the chain with two fingers halfway between the crank and the cam sprocket on the right hand side. Without forcing or pushing hard move the chain back and forth and note the amount of movement.

Use this formula:

1/4" = .002" to .004"
5/16" = .004" to .006"
3/8" = .006" to .008"
1/2" = .008" to .010"

These sizes are the amount of the oversize you need to order to obtain the best possible results for chain fitment.

Currently I have a near new .002 set.... and about 3/8" slack...... looks like I need a .008 or .010 set.....

 

[Alky V6]

From the Romac website:

Tunnel Bore Sizes
How to Determine Tunnel Bore Sizes


After fitting your standard Rollmaster Timing Chain Set to the engine you need to pinch the chain with two fingers halfway between the crank and the cam sprocket on the right hand side. Without forcing or pushing hard move the chain back and forth and note the amount of movement.

Use this formula:

1/4" = .002" to .004"
5/16" = .004" to .006"
3/8" = .006" to .008"
1/2" = .008" to .010"

These sizes are the amount of the oversize you need to order to obtain the best possible results for chain fitment.

Currently I have a near new .002 set.... and about 3/8" slack...... looks like I need a .008 or .010 set.....

Yeah, I used the same site.