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Just about wrapped up with a huge project on my 71 olds cutlass. Had it’s numbers matching 350 rocket rebuilt, bored .030 over, 30 degree backcut valves, mondello flat top pistons, custom cam with lots of duration but not much lift, Holley street demon carb on Edelbrock 7111 intake, plus some more. Had TH350 rebuilt and 2300 stall torque converter installed. New motor mounts, new driveshaft and u joints made by a local power train shop. Replaced rear control arms with tubular arms and polyurethane bushings (libricated well, torque applied with axle weighted to ensure control arms are seated well).
First time I went to drive it, had a crazy knocking sound from the trans tunnel area and could feel the floor shaking under braking and acceleration. Also, the fan hits the bottom of the shroud which it did not do before the rebuild. I’m wondering if I need a trans mount or if I’m looking in the wrong place? I bought an 18” flex fan temporarily to clear the fan shroud as the five blade 19” fan would not rotate in the shroud without dragging on the bottom of it.
I’m sure this has been asked and answered but I would appreciate any help you can provide me, I miss driving the cutlass it’s been almost a year since the project started.
You need to find the source of your knocking in the trans tunnel. Is the shroud mounted correctly? Were the engine and trans mounts changed? Are the body bushings in poor shape and perhaps the sound is just resonating through the floor? Exhaust hitting somewhere?
Knocking is a rather vague term - none-the-less, it sounds like an unbalanced propeller shaft &/or faulty incorrect U-Joints; or, possibly, the motor mounts are incorrect and your entire engine & transmission are tilted upwards from the normal pitch. There shouldn't be any reason the 19" fan should now be hitting - sounds like the motor mounts are incorrect (pitched upwards) causing the blades to hit the shroud. If that is the case, this upwards pitch would carry all the way to the rear of the vehicle through the transmission, propeller shaft and into the rear end.
If the fan is hitting the bottom of the shroud, then either the engine mounts are collapsed (causing the front of the engine to droop) or the trans mount is too tall (pushing the rear of the engine up so the front tilts down). I would bet on the trans mount area unless the engine mounts were replaced along with the other work performed.
OK then, that is what I would suspect as a possible cause for the fan hitting the bottom of the shroud.
As for the knocking, the only time I have experienced that is when I lost a u-joint cap on the driveshaft. I would say correct the fan-shroud contact / possible engine mount first then reassess the situation.
When I was referring to pitched upwards, I thought the OP was referring to the fan was hitting the bottom of the 'top' of the fan shroud. You're right, it could be it's hitting the bottom but I couldn't tell which area of the shroud the OP was referring to.
Well now that you say it that way, I am wondering the same thing.
Fan pitched upwards could mean the engine mounts are too high.
Fan pitched downwards could mean the engine mounts are too low or the trans mount is too high.
We need clarification on where the fan is contacting the shroud.
crazy knocking sound from the trans tunnel area and could feel the floor shaking under braking and acceleration
OK, I just re-read this and now I wonder if the trans mount is installed and properly bolted to both the cross member and the transmission. The reported sounds and knocking could be due to the trans not being secured at the rear mounting point. I had the same issues on my Jeep when the trans mount failed.
I read this the very first time & I could not determine what this statement referred to. Of course you need a transmission mount. Why would you not have a transmission mount? The transmission must be mounted to the cross-member rail of the chassis. The vehicle came w/ a transmission mount. I'm still not clear what you mean by this question.
Thank you all for the valuable input. Tomorrow I will be checking the trans mount in the car to make sure it is correct. The driveshaft exits the trans with a slight downward angle at the front yoke which makes me think the trans is sitting too high and/or not connected properly. The fan hits the very bottom of the fan shroud which makes me think the engine is tilting downward. I replaced the motor mounts with ones from rock auto I don’t remember the part number but they were for a 350. I have heard that the 455 mounts are taller but I doubt that if I had taller motor mounts I would be having this exact issue. I will check the trans mount in the morning and report back. And thank you again!
I had a driveshaft made for it because the shop could not balance the old one. The new shaft is larger diameter and constant, it does not taper or have a smaller tail section like the old one. They used Spicer U-joints. I was told it was perfect but I’ve really only driven a few miles so not sure if it is. The problem was there before I replaced the driveshaft and is still there afterwards.
Last edited by markvii1; June 6th, 2020 at 08:18 PM.
I do not know the exact difference between the height (sizing) of the motor mounts between the 455 bbo and the 350 sbo for the 1971 model year. However, if you know (or did know previously) there was a difference, that is most likely going to be an issue, if not the single most important issue. Think of the notion of being off 0.5" in height at the corner of a building and running that 0.5" height down 1', or 5', or 20', or 50'. That 0.5" is enormous by the time it gets to 50'. The same applies to motor mounts. The motor mounts establish the entire pitch of the engine, transmission & propeller shaft as it is carried to the fan and into the carrier at the rear axle. That's why they make motor mounts for a 455 and a 350.
I do not know the exact difference between the height (sizing) of the motor mounts between the 455 bbo and the 350 sbo for the 1971 model year. However, if you know (or did know previously) there was a difference, that is most likely going to be an issue, if not the single most important issue. Think of the notion of being off 0.5" in height at the corner of a building and running that 0.5" height down 1', or 5', or 20', or 50'. That 0.5" is enormous by the time it gets to 50'. The same applies to motor mounts. The motor mounts establish the entire pitch of the engine, transmission & propeller shaft as it is carried to the fan and into the carrier at the rear axle. That's why they make motor mounts for a 455 and a 350.
It would be great to think it might be related to a polyurethane trans mount reinforcement plate issue (or similar), yet I'm suspect it's something different. Since the 19" fan which was not hitting the fan shroud is now hitting the fan shroud after this work was performed. I'm assuming the OP is convinced the correct fan shroud is mounted correctly otherwise why go to the expense of purchasing an 18" flex blade fan. Something in the pitch of the drive train angle alignment remains suspicious.
Got the car on stands this morning and got underneath it. There is a clear and fresh gouge across the underside of the floor pan about where the front u-joint is located on the driveshaft. All the bolts are in place and torqued that hold the trans to the mount and the mount to the crossmember. The trans mount seems to be sitting higher on the driver’s side and lower on the passenger side. I’m going to order new motor mounts and transmission mount and replace them to see if it helps with my knocking issue. My running theory is that the trans mount is torn somewhere I cannot see and allowing the trans to bounce when loads are applied. Will post back again once some progress is made, most likely next weekend after parts have arrived.
And again, thank you guys so much for the links this forum has been a godsend for me. Between this and the chassis service manual I ordered (at this forums direction I might add) this project might actually complete successfully.
Last edited by markvii1; June 7th, 2020 at 09:52 AM.
Car looks great must have been terrible to hear/ feel it hitting like that. My fan hit the bottom of the shroud w the factory 350 when i got the car. I changed the original whipped motor mounts for correct new sbo ones and had no more issues. Later i changed the whipped trans mount too.
Unless you know for certain which engine frame mounts are installed, I would definitely validate you have the correct engine frame mounts on that vehicle; and, here is why. You stated in Post #15:
The problem was there before I replaced the driveshaft and is still there afterwards.
Take the additional incentive to validate (1) you have the correct engine frame mounts and you match up (2) the correct motor mounts to the correct engine frame mounts. Without doing so, you risk premature wear on many of the critical pieces of the engine & the entire drive train. The correct angle must be maintained into the differential. Look up on YouTube (or otherwise) the importance of a correct drive train alignment.
Thank you vintage chief I will look into the frame mounts for the engine and make sure I have the right pair. In the meantime I loosened the trans mount and a huge spacer fell out from above it in between the mount and the trans. I removed that and reinstalled and the knocking is gone. The fan is still too low so I will be investing some time into getting the correct motor mounts and frame pads for next weekend. Thanks again!
Thank you vintage chief I will look into the frame mounts for the engine and make sure I have the right pair. In the meantime I loosened the trans mount and a huge spacer fell out from above it in between the mount and the trans. I removed that and reinstalled and the knocking is gone. The fan is still too low so I will be investing some time into getting the correct motor mounts and frame pads for next weekend. Thanks again!
Check the Tech Editors section for a Sticky on motor mounts and frame pads. I "thought" JoeP made a "sticky".
According to rock auto, I ordered Pioneer 2261 motor mounts originally, I ordered another set from them today as well to compare. I will look to see if the frame perches for the motor mounts are incorrect on the car. I am wondering why the PO would go to the trouble of changing the perches if he kept the original engine in the car and it had never been rebuilt before I pulled it and did it myself? Was there a performance or longevity increase for using different engine frame mounts? With the spacer removed from the trans mount I no longer have issues with the driveshaft hitting the trans tunnel, but the fan is still too low to clear the shroud.
Sounds unlikely then that the frame mounts were changed maybe the trans mounts mount is broken allowing the engine to pivot on their mounts.
also the shroud should fit into two sheetmetal tabs at the bottom, sometimes the shroud isnt fit into these correctly, or maybe a non shroud top plate was adapted to a shroud ?
I am wondering why the PO would go to the trouble of changing the perches if he kept the original engine in the car and it had never been rebuilt before I pulled it and did it myself? Was there a performance or longevity increase for using different engine frame mounts? With the spacer removed from the trans mount I no longer have issues with the driveshaft hitting the trans tunnel, but the fan is still too low to clear the shroud.
Honestly, who knows why anyone does the things they do. Or, you might consider looking at this way. Who put that spacer on the trans mount? Surely someone put it there. Did you pay someone to put the spacer on the trans mount, did the previous owner? How did it get there? Unless you know for a 100% complete fact, nothing should be considered impossible. A PO may tell you one thing, but someone else may know for certain they've seen your car in a previous lifetime sporting down a race track running 1/4 miles. Are you 100% positive the fan shroud is correct for the vehicle and are you 100% positive the fan shroud is mounted correctly? I'm willing to wager big $$ the vehicle did not leave the factory with the fan hitting the bottom of the fan shroud. How is it then the fan hits the bottom of the fan shroud?
You are correct, I don't have many answers and plenty of questions. I will continue to investigate. The fan shroud seems to be the correct unit for the car - and the radiator cover on the top has the correct bolt holes for the shroud. The shroud is a plastic one piece unit with the angled feet on the bottom to fit over the metal tabs on the lower core support panel. Initially I slid the shroud in front of the tabs but found that it fit better when the plastic "feet" fit around the tabs and sat behind them. When I removed the shroud (a year ago) I don't remember it being attached to the lower mounting points at all, both of the clips that held the bolts in were broken. I ordered the hardware kit from Inline Tube with shims to try to adjust it and used the new clips that came in the kit to install the shroud correctly.
I never understood why the poly trans mount comes with the huge strap. I tried using it once and it never looked right, never ran it on the car. It's about 3/16 in. thick and increases the overall height of the mount from stock. Maybe some applications need it. Non of my 350 cid cars used it: 67 Camaro, 70 Camaro, 69 Cutlass, or 68 Cutlass.
1971 Cutlass/442 Assembly Manual
I do not see any differentiation between a 455 and a 350 stated anywhere; therefore, I assume the diagram is correct for both. If you need diagrams of the radiator supports, etc. I can get those to you if you do not have the Assembly Manual.
1971 Cutlass/442 Assembly Manual
I do not see any differentiation between a 455 and a 350 stated anywhere; therefore, I assume the diagram is correct for both. If you need diagrams of the radiator supports, etc. I can get those to you if you do not have the Assembly Manual.
Except for that note "Shroud (for part number and usage see page 52)". Of course, the fan is in exactly the same location relative to the radiator for both BBO and SBO, so there shouldn't be any substantial differences, other than radiator thickness.
I did review the Note before posting. I didn't find any "applicable" differentiation. Honestly, I haven't gone back to re-read (above^) to determine if the fan shroud is the OEM fan shroud and/or the radiator is the correct radiator with accompanying support brackets for an OEM fan shroud.
Thank you for the diagrams, these are very helpful. With everything here I should be able to post some positive results very soon. I do not want to continue to run the flex fan if I can help it, it definitely doesn't move enough air through like the clutched fan does.
Joe has worked these engine frame mounts & motor mounts until he's blue in the face. If you have any pretty pictures of both the engine frame mounts (perches) and the motor mounts, it would probably assist significantly in determining whatchagotgoin.
I did review the Note before posting. I didn't find any "applicable" differentiation. Honestly, I haven't gone back to re-read (above^) to determine if the fan shroud is the OEM fan shroud and/or the radiator is the correct radiator with accompanying support brackets for an OEM fan shroud.
I would guess that any difference in the shroud would have to be minor changes due to radiator thickness or fan diameter, if there is any difference at all.
Would a "quick check" of correct motor mounts/frame stands be to measure the gap between oil pan and crossmember? Too large or too small of a gap would tell you if the engine is mis-positioned.
Another thing to consider is how much the fan is contacting the shroud. Note in the Assembly Manual diagram there are shims that go between the shroud and the upper radiator support. If those shims are missing, tightening the bolts can draw the shroud up enough so the fan contacts the bottom center of the shroud. I had that happen on my car and the fan would graze the bottom of the shroud and make a lot of noise. I finally realized the shims were missing and adding the shims corrected the issue.
June 6th, 2020, 06:19 PM
I didn't find any "applicable" differentiation. Honestly, I haven't gone back to re-read (above^) to determine if the fan shroud is the OEM fan shroud and/or the radiator is the correct radiator with accompanying support brackets for an OEM fan shroud.
