For the past six months , I've been requesting advice on switching to a 3.25 gear set for my '25 coupe. The car has a strong running engine with a Z head, Stromberg OF carb, aluminum pistons, a recent valve job, and Lockwood outside brakes. Before the change, top speed was 53 MPH ( electronic speedo verified with my GPS), but the engine RPM and vibration was uncomfortable to say the least. For the record,I really don't want, or need to drive that fast, but it served as a test point. I assembled another diff using a new 12 tooth pinion and a fairly good looking 39 tooth recessed crown gear as well as new bronze thrust washers and excellent original Hyatt bearings with two excellent original, and two repop sleeves. The C/P clearance is 0.018" and I had to install two shims behind the crown gear to get that. I also used Locktite and double-wired the crown bolts. With the 3.25 the car now tops out at 51 MPH, but is way more pleasant to drive as it does not seem to be trying to grenade at higher speeds. Acceleration is quite acceptable ( low pedal till around 15-18 MPH), and overall the car is much more enjoyable. However, I have one big problem and that is a rear end howl that is very irritating. I'm guessing that it is a mis-match with the new pinion and used crown and wondering if in time, it will diminish or should I tear it apart and find a better (new) 39 tooth crown. I really don't want to go back to the 3.63 gear, keeping in mind that my goal was to reduce the engine RPM, not gain any speed. Thanks : Bruce
Bruce, I have gone the same route with the Ruckstell to go in my roadster. I would give your gears some time to get used to one another before deciding they don't really get along. Get as many miles under your belt as your eardrums can stand and then re-assess.
My ring gear was as new from a 3:1 set, and was not recessed, so I had no real difficulty setting the gears up.
Allan from down under.
Thanks Allan. My gut feel is that somehow I have too little C/P clearance. Following that hunch, I added three paper shims (gaskets) between the spool and the center housing and the howl is much less.
Although I should have stated it in my original post, I want to thank all the guys that offered their opinions on changing from the 3.63 to the 3.25. I had everything from "don't even think about it to "go for it,you'll love it", and everyone was offering their genuine best advice based on their experiences. Needless to say, I was somewhat hesitant, but I knew that my 3.63 had babbit thrust bushings and had to come out anyway. Hopefully I'll get the howl gone and enjoy the 3.25. I think I'll try some Lucas or STP in it tomorrow. Thanks again : Bruce
STP is NOT a lubricant! It is a viscosity enhancer, and probably NOT what you need in there.
I have the same ratio in my speedster, and had the same howl. Since the 12 tooth won't come out the hole I fabricated a brass shim to sit behind the spool, split it, and was able to install it with not to much work, Steve
I have the 12 tooth, 3.25:1 Ruckstell setup in my '16 touring as well.
I have a howl as well.
It has diminished over time but is still noticeable.
I am running synthetic gear oil at this time.
This is a case where the classic 600W might be more appropriate.
Bruce, two shims behind the crown wheel / ring gear might be a problem. I've seen experienced restorers advise against the use of shims behind that gear - might be better to shim the steel thrust bearing on one side and thin the bronze thrust washer on the other side if you have to move the gear closer to the pinion.
But if you have accessory outside brakes, shimming the ring gear wouldn't be such a safety issue.
Here's one of the threads dealing with the ring gear shim risk issue: http://www.mtfca.com/discus/messages/411944/490794.html?1415109179
Thanks Roger: I had read the warning threads about shimming behind the crown gear and couldn't understand the reasoning, but just in case I have the bolts as tight as I dare take them and applied "Locktite" to each bolt as well as double-wired them, once in pairs and again in a single loop. I can't imagine them coming loose. Bruce
Concentrate more on a smooth meshing gear set, with good, even contact area, and less on backlash/clearance. Too many people will get distracted by trying achieve some magical clearance number, while ignoring gear mesh, and getting noise. When a gear set is rolling smoothly, backlash will take care of itself. I never even bother to measure it.
I totally agree
Steven, what thickness of brass shim did you fabricate? Right now I have four paper shims that add up to around 0.024" but they will compress a little. I'm just guessing but considering the angle of the pinion, pulling it forward 0.024" might be equivalent to moving the crown sideways around 0.008". The howl is still there but much less that it was. Bruce
Bruce, noone thinks your bolts will loosen up, but they may be sheared off due to the reduced friction. I'm quoting Tom Carnegie from the linked thread: "the ring gear should not be driving on the bolts. It should be driving from friction between the carrier and the ring gear. Each shim you use reduces that friction."
You have a situation where you have added shims between the carrier and the ring gear to reduce backlash - and now, when you want to add backlash again you add more shims between the axle and the spool - it might be better to reduce the total number of shims
(of course I understand the feeling of having to pull something apart you just had assembled - though it's much more pleasant doing it in your garage than having to do it some other year on a motel parking lot on a tour..)
Moving the pinion back is not equivalent to moving the crown gear sideways. Each move accomplishes different things with regard to gear mesh.
Thanks Roger and Jerry for your input. When I assembled the rear end initially there was over 0.045" C/P clearance, but the mesh looked fine and the tip of the pinion teeth were clearing the crown gear bolts. I didn't want to put a shim under the pinion which would have reduced the C/P clearance,but also reduced the distance between the pinion teeth and the crown gear bolts so I added 0.025" shims under the crown to get it closer to the pinion. Now without a complete disassembly, the only way I know of to increase the C/P clearance is to pull the pinion away from the crown by adding shims under the spool. What I was trying to say was that due to the contact angles of the pinion and the crown, pulling the pinion back a specific amount will increase the clearance less that moving the crown sideways the same amount. Cheers : Bruce
Yes, I know you were taking the angles into account. But, even so, backing out the pinion has a different effect an how the gears mesh with one another than moving the crown sideways.
Thanks again Jerry, and now you've really peaked my curiosity. I can see how the clearance would change, but given that the angles don't change and the fact that they are straight cut gears, I can't see how the mesh will change other than the contact points will be at a different place along each tooth, and eventually there would be less overall total tooth contact in length and depth. Cheers : Bruce
Contact point between gear teeth is complex--the surfaces are not flat, but curved, and what you want to achieve is the greatest amount of constant contact between the teeth--contact as they touch, and as they "roll" with each other and then separate. That's why it's not easy to explain the setting procedure.
Thanks David : That also explains what Jerry was getting at too. Bottom line is that I really am shooting in the dark guessing at the C/P clearances while knowing nothing about the mesh without tearing down the whole thing and starting over. Makes me wonder what 20 H.P. can do in the way of damage if the mesh is not exact versus the thought of another several days of miserable work to try to get it perfect. Oh well,that's why I have a Model T anyway. Thanks again : Bruce
Bruce, I think I added .005 and it made a world of difference. Steve