Hi Folks,

I have a 68 OHC-6 with a TH-350 (chevy) trans behind it. When I get the car going about 35 mph, I get a resonating sound inside the car. It almost sounds like the bass from a boom box in a car next to me. The sound goes away at speeds above or below approx 35 mph. I do have sound deadening material on the entire floor pan.

Any ideas or anyone else seen or heard this problem?

Thanx

It sounds like a balance issue, possibly driveshaft, a bad u-joint, or perhaps torque converter? What is the converter from?

The converter came with the chevy trans. I had to use some spacers (washers) between the flywheel and the converter and then bolts with nuts to hold the converter in place. This converter did not have threads like the original. However, I did not want to use that converter, since it was in a ST-300 trans. I'm not sure why I had the spacing issue, but it may have been due to the BOP to Chevy adapter plate.

Is this a sound you can hear or feel more?
If you hear it more than feel it it could be exhaust, Does it resonate if you shift to neutral at 35 mph?

My sister once had a rustbucket '70s Firebird. The car had a cracked flexplate. At low speeds it created its own sonic boom. At idle it rattled. On the highway it felt lopey, out of balance.

I'd venture to guess that your bolt/nut/washer solution is a contributor to the problem.

Jim's test method is sure and simple to separate engine noise and exhaust noise from driveshaft and axle noise.

I haven't tried to put it in neutral while driving yet...I will check that out. There doesn't seem to be any vibration, but it is one of those quirky things that bugs the heck out of me. I suppose if bad goes to worse I will have to re-check the spacing on the flexplate to converter.

Are the converters and driveshafts typically balanced to be installed in any orientation, or is there some alignment that I am not aware of?

How much space did you have between the flexplate and the convertor? What does it have now?

I can't remember exactly why I needed the spacers to begin with, but I think the spacing was approx 5/16 - 3/8. I thought it had something to do with the adapter plate. Also, the BOP converter was threaded and the chevy converter wasn't. I installed the trans about 5 months ago and haven't driven much since then. I will have to get under there to measure it and check it out. Maybe the converter is too far away from the flex plate? If I take the spacers out, will it matter if there is that extra sapce on the spline of the trans?

I'd have to side up with Vikki. I'm not fond of that word 'adapter' when it comes to drive train parts.

If I remove the spacers between the flexplate and the converter, is the 5/16 - 3/8 extra space okay between the converter and the trans, or will it start leaking fluid? If I remenber right, the trans side of the converter fits on the spline and inside the fron trans seal. Does that sound right?

Without pics it's hard to say. Personally, if I were in your shoes I'd be looking into the correct replacement trans. In my past experience with trying to adapt what I have on hand, it cost me time and money that could have been spent to buy the right stuff.

Don't get me wrong. I'm not the guy over your shoulder sayin' 'I told ya so'. Some adaptations work, some not. It's all trial and error. And being on a budget forces one to put it to the test. I was once a grasshopper.

The converter needs some space to expand when doing its thing. It depends how accurate your 5/16 or 3/8" is. If the converter is pulled too far forward it could become disengaged from the front pump.