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Oversize Chain Idler Arms? Chain Stetch

I have a mystery motor, as I understand it it has 10,000 miles since a rebuild.

My question is whether when it was rebuilt if it should of hand either oversize idler arm sprockets (tensioner sprockets) or a shorter chain, if either is possible. I ask this as i have compared the chain that was in it (left side only) to a new one, and there is almost no difference in length - could the IMS be trashed, or did the rebuilder start with a chain set up which was too long?

------

As you can see, the problem is both the left and right tensioners are very close to the end/cam housing. The right side was rubbing against housing (3.2). The car was running on hydraulic tensioners, but I bought the engine partially diassembled (the left bank stripped down to pistons), so i have never seen/heard it run. The engine bay had a fire, so while the cam oil lines were burnt I presume it cut out quickly.

The chain is not overtight just a small bit of flap with the stomski tensioners. There are no cracks in the sprocket or arm on left side. Left side top of idler arm to chain housing is 2-3mm with a tight chain.

I have found the brown chain ramp on the right side was in the wrong place, it was top right, and the black was bottom right, rather than the brown, on the right side.

I have put a new chain on the left side just to see (split chain - as per the old one). I understand that i should replace the chain and sprockets at the same together, but for the moment, the spockets are showing minimal wear, and i am wondering if the engine builder should have used either oversize idler sprockets, or a shorter chain if that is possible, as surely there must have been minimal wiggle room from the off for the idler arm to chain housing clearance.

Has anyone ever shortened the chain by 1 link?

The engine was a 3.2 taken to 3.5, if that is a factor. The cam housing to cam tower seem to line up okay, so its unlikely to have been a major machining issue.

What is the best course of action? Shorten chain? oversize idler sprockets, and new cam sprokets? split the case and inspect IMS?
?

See pics.



Last edited by strictly; 09-05-2017 at 11:18 AM..
Old 09-05-2017, 08:19 AM
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Old 09-05-2017, 08:22 AM
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What caused this situation? Others may blame worn timing chains. On new chains?

I experienced this situation when modding a 2.0 to 2.7 some years ago. The heads were milled, cylinder heights equalized and crankcase spigots enlarged which reduced the deck height/shortened the "stack" (cam housing, cylinder heads, cylinder), reducing the distance between the camshafts and the intermediate shaft drive sprockets. That resulted in more slack in the chain as well as the tensioner shaft extending beyond specs.

To restore the geometry, I sourced slightly larger idler arm gears from German Precision in Sunnyvale, CA. Here's a related thread that also lists other sources:
Porsche356registry.org • View topic - recommendations wanted for machine shop in San Jose area

Larger idle arm gears are noted in Bruce Anderson's book (pge 140).

If the chain slack is due to machining processes described above, confirm the cams (and cam seal) are centered in their respective chain boxes, otherwise oil will exit (leak) here. If needed, machine each chain box mount surface to compensate.

Sherwood
Old 09-05-2017, 11:42 AM
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Hard to believe that some machining would cause that much slack. if the thrust plates on the cam shaft bolt up, then that should cancel out the machine work theory.. the thrust plates don't have that kind of tolerance's to effect that amount of chain play... JMHO
Old 09-05-2017, 12:47 PM
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Quote:
Originally Posted by scootergt3 View Post
Hard to believe that some machining would cause that much slack. if the thrust plates on the cam shaft bolt up, then that should cancel out the machine work theory.. the thrust plates don't have that kind of tolerance's to effect that amount of chain play... JMHO
Believe me. The engine I acquired was in such a condition. The cam plates were bolted to the cam, but the O-ring was distorted. There are 3-4 sealing surfaces that can be machined, especially when installing larger pistons/cylinders. Better to inspect and confirm than to guess wrong, yes?

You're correct in that the thrust plates don't have that much tolerance, but all it takes is some machining of the chain box gasket-to-crankcase surface to restore that concentricity.

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Old 09-05-2017, 02:01 PM
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good point about machining of the chain boxes.. I guess anything is possible, to me it looks like a full link needs to be removed to fit correct..

are you sure the chains are riding on the IMS gears correctly?
Old 09-05-2017, 03:41 PM
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Thanks for the replies =)

I had added some more pics. I am going to try taking a chain link out of the old chain, just to see what it looks like, with regard the tensioner to idler arm distance and idler arm top to chain housing. I'll post pics when Im done (if it works), and see what we can decide is the best course of action should be (i.e short chain, german precision oversize sprockets, etc). Im completely open to the best course of action. It might be I accept the engine has to come apart, and i then just run it to see what its like first, as the rest of the car (engine, trans, suspension, is a complete unknown).

P.s I forgot to add, engine turns over smoothly, no binding or tight spots, and while the block is 3.2, the cam towers are 3 bearing (early), with solex cams. I presume the engine is upped to 3.5, the cylinder bores measured to be 100mm, I havent measured the actual pistons.








Last edited by strictly; 09-06-2017 at 02:55 AM..
Old 09-06-2017, 02:43 AM
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tensioner too extended

I have seen lots of these conditions.Usually caused by improper machining with the cylinder height below 85.5.Simple cure is to use a custom idler sprocket that Peter Dawe from Dawes Motorsport makes.Phone is 917-913-8853.He has them in one tooth more than stock or one tooth less than stock.They are well made and superior quality over the factory idler.Tell him Fred sent you.Cost is about $300-350.00 per sprocket.
Old 09-06-2017, 03:24 AM
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cam plate

A[so you should machine the bottom of the chain housing where it meets the case so the O-ring seats evenly.Ciao Fred
Old 09-06-2017, 03:28 AM
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Fred, thanks for the info, it looks like that is the route i'm going to take. How much affect can deck height have here? i havent checked what this engine was built with, but presumably not its not going to make much difference.

Here are some pics of the old chain, with 1 link removed. Yeah thats really not going to work!!




Last edited by strictly; 09-06-2017 at 07:32 AM..
Old 09-06-2017, 07:30 AM
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I had a similar problem with my last motor. The heads were machined too much.

Luckily I did not have to resort to different sprockets.

However I had to enlarge the holes in the cam thrust plates to allow them to fit. That worked for me. Easier than machining the chain boxes.

I'm guessing that whoever built that motor made sure to check the piston to head clearance, compression ratio, and piston to valve clearance.
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Old 09-06-2017, 08:44 AM
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Quote:
Originally Posted by Trackrash View Post

I'm guessing that whoever built that motor made sure to check the piston to head clearance, compression ratio, and piston to valve clearance.
I had complete faith in the guy until I saw the brown ramp in the wrong place!

---------------

I am going to check all of those, that was actually the purpose of the mock up (left bank was disassembled when i got it). I got stuck at the tensioner/idler problem.

Should i strip down the top end, and measure the cylinder height? Is that going to help the problem solving?

My cam housing to cam tower/cam thrust plate all lines up, so unless i can shim space the barrels and cam housing out, i presume i need to chase the oversize idler sprocket route
Old 09-06-2017, 10:30 AM
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Quote:
Originally Posted by strictly View Post
I had complete faith in the guy until I saw the brown ramp in the wrong place!

---------------

I am going to check all of those, that was actually the purpose of the mock up (left bank was disassembled when i got it). I got stuck at the tensioner/idler problem.

Should i strip down the top end, and measure the cylinder height? Is that going to help the problem solving?

My cam housing to cam tower/cam thrust plate all lines up, so unless i can shim space the barrels and cam housing out, i presume i need to chase the oversize idler sprocket route
Basically you are answering you own question, I agree the heads have been flycut too deep.
Yes tear it down to the shortblock and measure everything. If you're lucky it has too much CR and you can shim the barrel bottoms. If not new heads or you will have to get creative like the Dawes MS sprocket.
BTW also make sure the cylinder wall isn't bottoming out on the head before the barrel seats in the flycut.
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Old 09-09-2017, 10:33 AM
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Hi Mark,

Thanks for your help. With head flycutting, do you know off the top of your head, the difference between the head to the cylinder depth that must always be maintained for good sealing?. I.e cylinder rim to cylinder fin, must exceed head flycut depth by XXXX" or etc.

The plot thickens on this motor, it has 964 3.6 cylinders (marked 95ZN6, with a fat spacer at the bottom of the cylinder.......

The deck is 0.035" and the valve to piston is barely 1mm (inlet), with the cams significanly retarded from 4.4 to 3.3mm at overlap (solex cams). With the cams correctly timed at overlap, the piston to valve on intake drops to 0.25mm!!

I am going to look at fitting 930 flame rings (enlarged to 100mm) to the head to cylinders, these are 0.5mm and will help both with piston to valve/cam timing, but also head sealing (I still need to check there is a problem, with the cylinder to head depth).

Do you (or anyone know), if there is an off the shelf 964 cylinder base shim/gasket to match the mahle cylinders (95ZN6) so i can increase my deck to something between 0.040 and 0.060?

Does anyone know the chain housing stock dimension (930) i.e thickness of housing where it meets the case, so i can see if they have been machined?

Thanks in advance.
Old 09-15-2017, 11:09 AM
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Are you sure it has actual 964 3.6 cylinders? That 3.6 engine case has different head stud spacing than the earlier engines. However if you have cylinders from a turbo engine, that head stud spacing is the same as the older engines because the 964 turbo engines are based on the older engine, not the 964 naturally aspirated 3.6

You can also use a cam housing spacer. Henry from Supertec has gone this route before. But if the chain housings have been cut, the spacer will create an offset between your chain housings and cam bore. You could stack up gaskets on the chain housing to push it out further.

I had a similar issue on my last engine build. The heads had been machined twice and the alignment in the chain housing was poor. I ended up using no gasket and just went with Curil T green sticky sealant for the chain housing. But I know from speaking with cgarr quite a lot that he likes to cut the heads and the chain boxes the same amount to maintain the cam thrust plate alignment. However that doesn't address the chain length issue. The larger idler sprocket is the way to go for that
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Last edited by KTL; 09-22-2017 at 07:35 AM.. Reason: added comment about chain boxes and machining
Old 09-22-2017, 07:19 AM
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Thanks for the response Kev, appreciate it. I have decided the best thing to do is strip it down and measure and ID everything, including the head cut and chain box dimensions, CC the heads, etc, and build it back from scratch, maybe even change the cams, etc.

I have more pics in this thread : Do they look like 964 turbo cylinders and pistons?

3.2 to 3.5 or 3.6

Is it possible, that the cylinder height and pistons are basically too short, resulting in the fat 2.5mm spacer at the bottom of the cylinder, which should be larger,only a tiny bit of course, but this all affects chain length. The deck on the engine is tight at 0.035, so there is scope to increase it
Old 09-26-2017, 03:15 AM
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I believe there is such a thing as a chain half link - that might do the trick.

The only time my left idler was that close to the top of the housing was when the arm had a crack in it. But my right chain is up closer than I remember to the bottom of the rail. But not this close. I like the cap on the tensioner as a way to keep it within range.

I don't recall if anyone has calculated just how much "stretch" (read "wear")there can be before you get failures or worn through rollers?
Old 09-27-2017, 08:25 AM
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I had the same problem with a 2.7 where everything had been machined to where the chain could no longer be tensioned. I used a pair of oversized idler wheels, from German Precision, which solved that issue. Eventually a piston liquified itself on the track and left me with a broken motor, which was parted out. It is certainly a better choice to try before a complete rebuild.
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Old 10-03-2017, 07:41 AM
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That is, choose the chain wheels, not the broken motor........... See my pictures under "Custom chain wheels" in parts for sale.
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Old 10-03-2017, 07:58 AM
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Thanks for the replies, guys, Walt, i will check for cracks; All okay on the arms. Turbodon1, okay thanks another +1 on the bigger tooth idlers, i will go for those now. Unlucky you blew your motor, but im sure you had fun in the process =P
Old 10-04-2017, 02:45 AM
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