17 or 18 tooth idler sprocket

[Restore_84]

17 or 18 tooth cam chain tensioner idler sprocket seems to be my best bet if I can find 2, anyone know of any please share.
Im building a 3.0L Turbo actually its a 3.122L Turbo. Recipe starts with a 3.0L block, 3.3L cylinders, custom CP Pistons, standard 3.0L Crank, and heads with custom intake and exhaust dimensions.

Due to the mix of parts when I went to install the solid cam tensioners w/locks, I had to stop due to the chain tension being way too tight. I'm contemplating milling the tensioners and going with Brown ramps all around.

But before I go through the the above experiment, I thought if I could simply replace the 19 tooth sprocket with a smaller one it would save the day.

Your thoughts?

[Steam Driver]

Looking in our host’s parts catalog there is only one idler sprocket listed and it fits all 911’s (I’m sure I may get a correction on that) at least through 1989. So where is a part with a different number of teeth available?

The mixture of parts you have is not unusual. If the chains are too tight I think you have something else going on. I can’t think of what though. Case line boring, for example, “narrows” the engine somewhat but this would result in looser chains, not tighter. I can’t think of what would make them tighter.

I’m sure someone with more knowledge than I can amend this.

[r lane]

The only thing that creates an issue with a chain being too long or short is the distance between the CL of the cams and the CL of the crank. Assuming everything is stock in dimension, then heavy shims under the cylinders will effectively shorten the chain. Make sure your chain tensioners are configured correctly. Bob

[Restore_84]

Hi Bob, your correct. To get the right compression for this recipe and right gaps for piston to valve clearance had to add 2mm cylinder base shims, giving me the tight chain. What do you exactly mean by correct chain tensioner configuration?

What is your Recommendation ? Thank you.

[r lane]

Figure out what you want your piston to head (deck height)clearance to be. .050-.060'' or there abouts. You start with that and then I would machine the pockets on the pistons as apposed to letting that dimension determine the deck height dimension. Tail wagging the dog. You may also need to do some machine work on the piston dome to get the compression where you want it. That of course means disassembling somewhat to get the pistons off, but that I think is the correct way to do it. After that is accomplished and there is still a chain issue, modifying the solid tensioner is easy. Bob

[Restore_84]

Thanks Bob.

[Trackrash]

There used to be someone making smaller sprockets. Are those NLA?

[Walt Fricke]

They definitely were made at one point, and for just this kind of situation. I have a pair somewhere.

So keep looking. I'd contact Steve Weiner, Peter Dawe, Mike Bruns, Henry Schmidt, or any number of experienced engine builders to see if they have a source.

[Restore_84]

Great news Walt! I will start making calls, thanks for the leads....

[r lane]

Jerry Woods and Rothsport has had them in the past. But as I understand the build, that doesn't appear to be the correct solution in this case. Bob

[Restore_84]

Additional details; Due to #8 bearing wear the block was line bored and decked, taking all measurements into account and the mix of parts, achieving proper gaps, resulted in the need to add additional base cylinder shims. To avoid tensioner failure or going with mechanical tensioners (which is an option/last resort). Smaller diameter Idler sprocket puts me closer to my ideal chain tension specs for this Turbo build. I’m waiting on a set of brown chain ramps to replace all the black ones as my first test to see if the thinner ramps gives me enough slack.

I'm still in the market for a smaller set of idler sprockets, waiting on call backs from the leads shared by Walt.

I very much appreciate the interest and support!

[Flat6pac]

Can your ORing seal on the chain box at the cam handle the movement?
Bruce

[r lane]

You have .080'' shim under the cylinders. That is .070'' more than stock per side. What is your deck height dimension (piston to head)and how much was taken off the case halves. How did you handle the chain housings? What compression are you wanting?

[Restore_84]

Negative Deck height is 1mm, compression is roughly 8:1. chain housings along with the rest of the assembly so far, torqued up to spec with no issue and everything turns nice and smooth.

[Restore_84]

I found the sprockets very happy with the fit and chain tension.

[KTL]

Where did you get the sprockets? My last recollection is that German Precision supplied them

[Restore_84]

Jeff from Rothsport came thru for me with the perfect size 16 tooth Sprockets.