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Location: Denver, CO
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M97 Rebuild - Connecting Rod Orientation?

Hi Guys,

Unfortunately I am in the middle of an M97 rebuild. After waiting for what seemed like an eternity for the cases to come back from LN Engineering, I'm starting my rebuild and hit my first snag. My engine is an M97.21 from an '08 Cayman S, but I would expect the rod orientation to be the same for any M97 (or an M96 for that matter).

I'm trying to determine what is the correct orientation of the connecting rods. A friend of mine who races spec boxsters, has a manual for the M96 and it indicates that on bank 1 (cyl 1-3), the locking tabs (that prevent the rod bearings from spinning should point towards the bottom of the engine. For whatever reason, his version of the manual doesn't say anything about bank 2 (cyl 4-6).

To me it makes sense from a rotation/loading perspective that the locking tabs would face up on bank 2. He has asked some of his racing buddies and the consensus is coming back that all the tabs should point down (towards the oil pan) which doesn't make sense to me. Adding addition confusion, I'm pretty sure when I disassembled the engine, all the rods faced the same way, i.e. the markings on the rods were such that the same side was facing forward on all cylinders, but I didn't take enough pictures to confirm that so I'm not 100% positive.

Does anyone know the correct orientation or have a factory manual for the M97.21 that explicitly defines the orientation?

Thanks!
Old 06-02-2018, 06:41 PM
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Puny Bird
 
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Location: Port Hope (near Toronto) On, Canada
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I'd have to check to be 100% but I'm 99% sure the other side will be bottom as well.
VW T1 rods are directional they have a bump on one rod side, they all face up. The rods are all the same.
A directional rod will have an offset, usually on the big end, to center the rod in the jug.

I have a 3.6 M96 stock rod and a M96/M97 Carrillo rod in my hands and I don't see an offset on either.
The Carrillo rod spec card, which tells you every measurement on the rod, makes no mention of an offset.
Also took a straight edge to both rods, if there's an offset you should be able to see it, I don't.

The M96 3.6 rod is the same rod as the M97, at least the Carrillo rod is, it says so on the box on both Carrillo's and LN's labels.
It does say 3.6/3.8 non turbo, so turbo rods are a different spec, likely a hair shorter for less CR/deck.
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Last edited by Mark Henry; 06-02-2018 at 08:35 PM..
Old 06-02-2018, 08:00 PM
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Thanks for your reply Mark. I agree the rods are symmetrical and fit identically in either orientation. I'm just trying to understand why the correct orientation would put the tabs on the inside of the journal (closer to the crank rotational axis) on one bank and on the outside of the journal on the other bank. The only thing I can think of is that it has something to do with when the engine is not running, i.e. when gravity would have the rods rest on the side without the tab in both banks.
Old 06-03-2018, 04:28 AM
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Puny Bird
 
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I can't really see it, but if you find out let me know, I'm starting 4.0 (3.6) M96 build in a few weeks.
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Old 06-03-2018, 04:34 AM
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The consensus is that in the M96/M97 all of the rods should be installed with the bearing locking tabs facing the bottom (oil pan). I did get some input from folks that installed all the rods with the same feature facing forward.

I didn't do a good job of explaining why pointing all of locking tabs towards the bottom of the engine doesn't make sense to me. Most of the information you find about connecting rod orientation talks about loading during the power stroke. In a flat 6 that would mean that the banks would have the rods installed with the locking tabs down in one bank and up in the other. By installing them with the locking tabs always facing down, the orientation of the locking tabs during the power stroke is reversed from one bank to the other. That is what doesn't make sense to me.

This thread on the forums has a discussion about it. (although the picture in one of the posts that tries to show forces on the connecting rod is wrong)

I'm going to install the rods with all the locking tabs facing down because that seems to be what most of the M96/M97 builders do. Good luck with your build...
Old 06-06-2018, 05:05 AM
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Puny Bird
 
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I've read several threads in the VW aircooled forums, they use a lot of aftermarket H beam rods and the consensus is bumps up (if applicable) and tangs down.
Stock VW type one rods are also bumps up and tangs down.

Tangs down was also given by SCAT Enterprises, a major manufacturer of aftermarket cranks and rods.
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Old 06-06-2018, 03:31 PM
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porque?
Old 06-06-2018, 07:04 PM
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I sent an email to Carrillo.

"Hi I have a porsche 996 and your rods from LN engineering I can't find any info on stock or aftermarket rod orientation, I don't believe they have an offset. Flat 6 boxer engine.
I'm actually am building two engines a 3.4 and a 3.6 your rod numbers PR-996>65708H-00 and the second set is PR-99603>65590H-00

Rod orientation do you reccomend bearing tangs down?
Or does it matter? "


Below is the reply.

Quote:
Hi Mark,
With these rods that have no offset, the orientation does not matter.
Best Regards, Richard Batchelor, Rod sales Manager, CP-Carrillo Inc.
So I can't speak about the stock rods, but with Carrillo's they say it doesn't matter.
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'74 Porsche 914, 3.0/6
'67 Bug, 2600cc T4
'67 Bus, 2.0 T1
Not putting miles on your car is like not having sex with your girlfriend, so she'll be more desirable to her next boyfriend.

Last edited by Mark Henry; 06-07-2018 at 08:38 AM..
Old 06-07-2018, 08:33 AM
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Quote:
Originally Posted by Mark Henry View Post
I sent an email to Carrillo.

"Hi I have a porsche 996 and your rods from LN engineering I can't find any info on stock or aftermarket rod orientation, I don't believe they have an offset. Flat 6 boxer engine.
I'm actually am building two engines a 3.4 and a 3.6 your rod numbers PR-996>65708H-00 and the second set is PR-99603>65590H-00

Rod orientation do you reccomend bearing tangs down?
Or does it matter? "


Below is the reply.



So I can't speak about the stock rods, but with Carrillo's they say it doesn't matter.
It doesn't matter. The rods can go either way. Just make sure you put them all the same way so that the next person who rebuilds the engine that has OCD doesn't have a heart attack :-)
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Old 07-04-2018, 03:09 AM
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