Just finishing up my L98 rebuild plus T5 install. I got it in and running. Everything seems great and I am working out the bugs. One bug is the engine has power but seems to have a harshness in general. I thought I would first check about the flywheel and harmonic damper. I used a newer AC Delco off of my pretty good running LB9. I'm pretty sure I checked that it was the same part but I am questioning everything right now.

Then there is the flywheel. L98's never came with the T5. I bought a new stock clutch kit from Summit and had the flywheel surfaced at a machine shop. I got the T5 setup off of a stock donor car. I think is was a 90 TBI with WC T5. Everything works really well except for that RPM harshness. I did not have any of the rotating parts rebalanced as this was pretty much a stock rebuild reusing the stock parts (new stock replacment HE flat top pistons though). The L98 probably came from a TH700 but I'm not sure. It's a 1988 L98.

 

Should all be the same, flywheel for sure if 1pc RMS like your '88 L98 is.

What is " RPM harshness?"

 

Kinda like bad motor mounts. Engine revs and a vibration kinda flows through the car. My motor mounts are fresh though and they are fine, I checked them.

 

I for one am a huge proprietor of having an engine balanced during a rebuild. I’m not sure what factory specs are during this process but I like knowing it’s as close to zero as as possible. Easier on rotating parts and bearings and just overall smoother operation and revs.

If it’s a slight shake or shimmy at idle it’s probably not a big deal. If it has a horrible vibration at engine speed you may want to diagnose that sooner than later.

 

It's not horrible but it's not as smooth as I think it should be either.

 

A few things I'd like to ask since you say it a vibration.

Sorry-must ask- you did install a pilot bearing in the crank?

Did you use a clutch alignment tool before putting in the tranny?

And, it's a long shot, but should be checked, was the bell housing indexed? Not usually an issue, but enough mis-alignment will cause issues.

All assuming all the parts you used for the install are in good shape.

 

Thanks. Yes, all of those items were done except for the bell housing. There were two large alignment pins in there so I didn't see a way to adjust it. The parts were all in excellent shape.

 

You check concentricity with a dial indicator and then install offset pins if needed to adjust.

 

Excellent. I will do that and report back. Thanks.

 

I haven't done a runout check yet but I was under there and noticed the larger diameter crossover pipe of my new MAC Y-pipe is very close to the crossmember and oil pan. Maybe too close. That could be causing the waves of "rpm" harshness. I will investigate that as well.

 

Well I checked clearance, it's fine. Harshness seems to be getting better with each tune cycle. Huh.

 

From what I've seen over the years a flywheel out of balance a bit, maybe due to resurfacing, will be worst at about 2500 rpm or so. Might also double check the motor and trans. mounts, exhaust routing etc. some times get a bit jammed or twisted and have some metal to metal contact.

 

Any chance you used poly mounts in the swap?

 

this vibration Is worst at 2300-ish RPM. Sure acts like a flywheel. I had this problem on a new 5.0 mustang back in the late 80s. Could not figure it out until i put in a new Mc Cloud flyweel then it was super smooth. I am thinking this is the same thing. Even though i had the factory flywheel resurfaced I’m pretty sure it wasn’t balanced. I wonder if i should bother indexing it and try that? I still will do the runout check.

and no i do not have poly bushings. New oem mounts.

ugh. Not looking forward to pulling the Trans and messing with it so soon.