View Full Version : Transmission knock at Low RPM?

07-01-2012, 05:20 PM
I've got a 1989 Prostar with the velvet drive transmission and I was noticing today that there's a knocking noise coming from the back at low RPM's.

I searched the forums and found guys posting bad dampener, loose prop, strut bearing worn out, and shaft bent, etc.

I felt the shaft today, and it doesn't feel like it has any play in it. Also checked all the transmission and engine bolts, which are all tight.

Any suggestions on which direction I should chase this? It's similar sounding to the knock you get in reverse and it seems to go away when the RPM increases.

Can you tell if your prop is loose by hand, or will I have to put a torque wrench on it?


07-01-2012, 05:44 PM
I just went back out and checked the play in the strut shaft and on the prop.

The prop is tight as a tick, so I don't think its that, but there is a small amount of lateral play in that strut bushing.

If you grab the shaft with both hands and try to move it from side to side, you can see it move a little (nothing huge - guessing maybe .010-.015" ), but should it not move side to side at all?

07-02-2012, 08:57 AM
"Low RPM" = ???

It is normal for the damper to knock at excessively low rpm's. Your idle should be set to ~650 when in gear. If its much lower than that, it will knock.

07-02-2012, 12:07 PM
"Low RPM" = ???

It is normal for the damper to knock at excessively low rpm's. Your idle should be set to ~650 when in gear. If its much lower than that, it will knock.
It knocks from about 650 to 1000.

I rode in a freind's 94 Prostar 190 this weekend and his doesn't knock at all.

This knock is really noticable.

07-02-2012, 12:44 PM
Prob the damper palte. Mine is doing the same thing at low RPM. I think the knock goes away at higher RPM becuase of the centrifugal force. Stinks.... a bear to replace. I wonder what the damper plate does anyway?

07-02-2012, 03:22 PM
Just found that it is used to reduce vibration to decrease gear rattle in the tranny.

07-02-2012, 04:05 PM
The damper is what connects your engine to the transmission. If its knocking above 650, then something isnt quite right. Either its on its way out, or Ive heard that an engine in poor tune (rough idle, etc) can cause knock.

07-02-2012, 09:31 PM
I talked to Scott at Ski Dim today, and he says from what I told him, its the damper plate.

He says they usually loose a spring or develop cracks in them.

He walked me through the change. First you have to loosen your engine mounts, disconnect the tranny and shaft, then jack up the transmission with a bottle jack.

From there he said you could see the damper in the bell housing.

Next remove the starter and bell housing, then remove damper.

When you reinstall you have to make sure you get your shaft realigned correctly.

No more than .003" gap at any point between tranny and shaft mating surfaces.

I'll let you guys know how difficult this turns out to be.

If anyone has any tips or 'gotcha's' with this job, I'd appreciate those as well!


07-02-2012, 10:51 PM
I just finished doing this job on my PS190 over the weekend.
I bought the heavy duty replacement damper plate for the peace of mind.
I probably read 15hours of damper plate info posted here before doing the job which might have taken around 7-8 hours.
I did not torque the plate through the starter hole while transmission is re-attached like most here seem to be doing. I final-torqued it in place with tranny off then snapped input shaft back in, works out way better.
I stripped the tranny mount bolts on bracket (3/8-24)....watch out, unless yours is wood.
I had to re-tap helicoil's back in, worked out perfect.
I did not have to re-align shaft, and maybe you won't either.
Unless your taking the whole motor out or ''tinkering'' with the mounts, if you raise the tranny up from under the risers and lower it back down, chances are your alignment will not move.
Still check afterwards to make sure, Borg-Warner says .005'' is ok.
I worked without an engine lift or chain block and picked it up myself out of the boat.
This is a Powerslot 1:52:1 on a 351.

07-07-2012, 09:40 PM
I have to do this job as well. How do check the the alignment? How do you raise the tranny up from under the risers?


07-07-2012, 09:51 PM
I have to do this job as well. How do check the the alignment? How do you raise the tranny up from under the risers?

To check the shaft alignment, you loosen the transmission and drive shaft coupling flange. Using a feeler gauge, you should not have more that .003" clearance at any point between those flanges. In other words, if the alignment were perfect, both flanges would mate together with no clearance all the way around.

To lift the back of the engine up from under the risers, disconnect tranny/drive shaft coupling, slide drive shaft aft, remove exhaust hoses, etc, loosen engine and tranny bolts, nuts, and put a bottle jack under each riser. Slowly jack until you have enough room to pull the transmission out.

07-08-2012, 09:44 AM
Thx, I appreciate it. Dang I'm not looking forward to this job.

07-08-2012, 10:08 AM
Will this plate work?
http://www.ebay.com/itm/Ford-302-351-Marine-Drive-Plate-Damper-26-Spline-Gas-Boat-Small-Block-/280909669268?_trksid=p4340.m1850&_trkparms=aid%3D222001%26algo%3DSIC.CURRENT%26ao%3 D1%26asc%3D11%26meid%3D463492965964303038%26pid%3D 100011%26prg%3D1005%26rk%3D5%26