had some issues putting together our m90 bottom end, looking for experienced advice

[kronus]

We're putting together a refreshed M90 out of my old 635csi for our lemons car. Last night, the pistons went in and we had to work through some issues. I'm looking for maybe some confirmation/denial of our conclusions from what we found.

The crank got checked out at a machine shop and all the rod journals are round and in spec, so we got standard, non-oversize rod bearings. 4/6 rods plastigauged dead in the middle of spec (.025mm - .055mm). One other one was loose, and I went about replacing the rod with a used one out of a B35, and found something interesting!

The M90 rods have the bearing shell notches on opposing sides, judging by residual dirt on various rod surfaces. The B35 rod I was replacing it with has them on the same side, and this was consistent with all the other B35 rods I have lying around.
- Which is more correct?
- Is there a production difference or any specific reason this change would've been made?
- Should the M90 rods be installed only in the direction that buts up the bearing shells against the notches to prevent them from spinning?
- Does direction matter for B35 rods?

The other not-good one was way too tight (like, motor wouldn't spin with it torqued down), and the cause was pretty confusing. Despite thorough, repeated cleaning, the bearing shell was too long, and sat proud of the split between rod and cap wherever it was inserted. I'd assume there is some decent quality control on bearing shells - is this not completely unheard of? We replaced that shell with the best used one I had lying around and it sat flat and plastigauged perfectly, but I'm concerned about mixing new and old shells.

Anyway, advice on what to look out for in assembly is appreciated. I've done my share of heads, but not as experienced with block assembly and looking to learn more. Sadly I haven't been able to find great, in-depth documentation on things like this.

[LJ851]

We're putting together a refreshed M90 out of my old 635csi for our lemons car. Last night, the pistons went in and we had to work through some issues. I'm looking for maybe some confirmation/denial of our conclusions from what we found.

The crank got checked out at a machine shop and all the rod journals are round and in spec, so we got standard, non-oversize rod bearings. 4/6 rods plastigauged dead in the middle of spec (.025mm - .055mm). One other one was loose, and I went about replacing the rod with a used one out of a B35, and found something interesting!

The M90 rods have the bearing shell notches on opposing sides, judging by residual dirt on various rod surfaces. The B35 rod I was replacing it with has them on the same side, and this was consistent with all the other B35 rods I have lying around.
- Which is more correct?
- Is there a production difference or any specific reason this change would've been made?
- Should the M90 rods be installed only in the direction that buts up the bearing shells against the notches to prevent them from spinning?
- Does direction matter for B35 rods?

The other not-good one was way too tight (like, motor wouldn't spin with it torqued down), and the cause was pretty confusing. Despite thorough, repeated cleaning, the bearing shell was too long, and sat proud of the split between rod and cap wherever it was inserted. I'd assume there is some decent quality control on bearing shells - is this not completely unheard of? We replaced that shell with the best used one I had lying around and it sat flat and plastigauged perfectly, but I'm concerned about mixing new and old shells.

Anyway, advice on what to look out for in assembly is appreciated. I've done my share of heads, but not as experienced with block assembly and looking to learn more. Sadly I haven't been able to find great, in-depth documentation on things like this.

For starters, if the rod and crank are in spec and the clearance is not then the bearing must be bad.

The rods need to be balanced between themselves (all big ends the same and all small ends the same), are the B35 rods the same weight as the M90 pieces?

I would try to run a matched set of rods. I would put a set of bearings that measured good in another rod in the rod that measured loose and see what happens.

There are multiple ways that rod bearings are located on rods, some racing rods have no locating notches at all. I wouldn't worry about that, just make sure the rod caps go on the way they were machined.

Sounds you may have gotten some bad bearings, i have seen this more in the last 5-10 years than ever before. Some porsche guys are reusing good original bearings instead of risking using the new, poor quality OEM ones.

[kronus]

yeah, rod weights are a concern. I'm going to bring a scale along this weekend and check the rod that got swapped out.

we swapped the shells on the loose one with another good one and it was still exactly as loose, on two different journals. Is it possible that the old bearing was loose inside and the bore got ovaled out?

in a fit of dumb, the rods and caps got mixed and matched when the engine was pulled apart. I think we got them all back together right, though, they felt good.

[LJ851]

I think your chances of having the right caps on the right rods are slim.

You really need to torque the rod big ends and measure for roundness with a bore gauge. This is a spec that is in the ten thousanths accuracy, feeling good is not enough.

Do you need me to send you some rods, too? lol.

[kronus]

Going cap to cap, there was always 5/6 that clearly didn't fit well. We torqued them before checking. Yes, I would feel more confident with rods that were for sure the original ones that were bored together, but they weren't machined so precisely that you can't tell one from another.

And yes, I've learned my lesson. We paint pen everything now.

[Good & Tight]

in a fit of dumb, the rods and caps got mixed and matched when the engine was pulled apart. I think we got them all back together right, though, they felt good.

That's your issue right there. Only way to make sure 100% is to have the block line honed and the rods resized.
This is one area of engine building you cannot guess, the bearing clearances are critical and if improperly assembled the bearings will not last 2 miles.

[Doughnut]

That's your issue right there. Only way to make sure 100% is to have the block line honed and the rods resized.
This is one area of engine building you cannot guess, the bearing clearances are critical and if improperly assembled the bearings will not last 2 miles.

Ouch, looks like your up for some $$$ Kronus,

Very informative thread btw.