8.5” BOP rebuild

Discussion in 'Got gears?' started by Darron72Skylark, Oct 13, 2019.

  1. Darron72Skylark

    Darron72Skylark Well-Known Member

    Took several sessions of cleaning and 6 cans of brake cleaner but the axle is pretty well cleaned up and applied the sealer to the tube joints. Hopefully this stops the axle from weeping gear oil.

    Attached Files:

  2. Bens99gtp

    Bens99gtp Well-Known Member

    Yep spray her your favorite color now and should be good
    Darron72Skylark likes this.
  3. Darron72Skylark

    Darron72Skylark Well-Known Member

  4. Darron72Skylark

    Darron72Skylark Well-Known Member

    One thing discovered today, the gears are NOT 3.42 as previously thought. 3.08 instead. Ring gear 40 and pinion 13.
    Still trying to figure out the mistake in earlier calculations. But counting teeth is pretty definitive. Kind of changes the thinking for future projects like the 5 speed transmission. Going to need a gear change, so all of this will get opened up again at some point.
    Would have been better to get it all done at once, but that’s the way it goes sometimes.
  5. BrianTrick

    BrianTrick Brian Trick

    You can get 5-speeds with different gear clusters.
  6. Darron72Skylark

    Darron72Skylark Well-Known Member

    So here is the strange thing.
    I was convinced that the gears in the back were 3.42 ratio because the car cruises in top gear at engine rpms that make sense for a 3.42 ratio.
    Today, for instance, I drove at 60 mph with the engine speed right around 2600 rpm. Verified that the speedo was accurate by using a stopwatch between mileposts. I have 235/55R17 tires; they are 27.2 inches tall.

    Here is my math: 3.42 x 60 mph x 336/27.2 inch tall tire = 2535 rpm Not exact, but at least in the ballpark.

    On another occasion I made the following observations: about 3000 rpm while travelling 70 mph down the interstate. Also verified speedometer calibration using the stopwatch method and mile markers.

    3.42 x 70 mph x 336/27.2 inch tire = 2957 rpm

    Yet the ring gear in the axle is clearly marked 40-13, which makes it a 3.08 gear set.
    I can't find any mistakes in my math; when I use online calculators to run these numbers, I get similar results. When I use 3.08 for the rear ratio, all calculations show that my engine speed should be much lower. The 3.08 gears should have the engine turning at just under 2300 rpm at 60 mph and 2660 rpm at 70 mph.
    I don't think my speedo calibration is off and I have no reason to think the tachometer is wrong (the sound of the engine correlates with what the tach is reading). This is a manual car with no torque converter.

    What the heck can be going on? Is it possible to have that much clutch slippage when it is engaged and be super consistent about it? It is not like the engine is surging while I'm cruising down the road. I can't think of anything else that might explain what is going on.

    I hope my fellow Buick guys and gals can enlighten me. I'm kind of stumped.
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    If it is a stock tach, it could be that far off.
  8. Bens99gtp

    Bens99gtp Well-Known Member

    I would verify tach, most digital timing lights have a tach feature built in, just use it and ide . Rev to 2500 on your tach and have someone else read the timing light
  9. Darron72Skylark

    Darron72Skylark Well-Known Member

    That’ll be the next troubleshooting step: verify tach.
    It’s not a stock tach, it is a reproduction from Original Parts Group. About 2 years old if I remember right.

    I’ll update when I know more.

    Last edited: Dec 5, 2019
  10. Darron72Skylark

    Darron72Skylark Well-Known Member

    Had a chance to compare the tach readings with the rpm reading pulled off of a Mac digital timing light.
    At 850 rpm on the tach, the Mac showed about 830-840 rpm.
    At 2500 rpm on the car's tach, the Mac showed 2360 rpm.
    At 3100 rpm on the car's tach, the Mac showed 2930 rpm.

    So as expected, there is a small error from the analog tachometer in the dash, but it is not a huge difference. A little over 5% error.

    This 5% difference does not explain the large differences in my rpm calculations. Is my theory about the clutch slipping possibly right? It's state of wear has been greatly highlighted by the upgrade in rear axle traction, so I need to replace it anyway.

    Also got out the tape measure and verified that the 27.2 inch diameter looks right.

    I could borrow a radar gun and try verifying vehicle speed that way. Or maybe take the Garmin GPS off of my bicycle and try that as a way of verifying the speedometer.

    Any other ideas out there?
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I would verify speedometer next.
  12. Darron72Skylark

    Darron72Skylark Well-Known Member

    Radar gun, here I come!
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    There are 3 things you need to know to figure out what DRIVEN gear you need. Two of them you know, tire height (27.2") and gear ratio (3.08). All you are missing is the number of teeth on the DRIVE gear which is on the out put shaft. Take a peek at it by removing the speedometer gear sleeve and rotating the drive shaft in neutral with the rear wheels off the ground. See what DRIVEN gear is in the sleeve now. Then put your numbers into this calculator and see if it agrees,

  14. monzaz

    monzaz Jim

    LOL... will look for the MUG shot...LOL. :D JIm
  15. Bigpig455

    Bigpig455 Fastest of the slow....

    There are apps that will give you MPH based on GPS. The on I use is call simply " speedometer" . whenever I do a gear change, I do use a calculator to get the right speedo gear, then verify with the app.
  16. Darron72Skylark

    Darron72Skylark Well-Known Member

    Borrowed a radar gun this past weekend. The speedometer has a small error when checked with radar gun and a Garmin GPS.
    The speedometer reads a little under actual speed by 2 mph: 30 mph on the speedo registered 32 by radar and GPS, 45 on the speedo was 47 by radar and GPS, and 60 on the speedo was 62 by radar and GPS.
    The 2 mph discrepancy does not change with vehicle speed, its pretty much constant.

    So all I'm left with to explain the symptoms is a worn clutch. Hope to get that changed over the holidays and see what happens after that.

    Thanks everyone for all the help and suggestions.
  17. Bens99gtp

    Bens99gtp Well-Known Member

    I would think if you clutch was slipping that much it wouldn't hold in you going slow. Say 25mph in 4th gear and floored......the power should blow through the clutch if its slipped several hundred rpm at cruise and light throttle
    DasRottweiler likes this.
  18. Gary Bohannon

    Gary Bohannon Well-Known Member

    2 mph at all speeds, would just be the speedo needle on the pin/shaft that holds it. Pull it off and push back on until the you get the exact spot.
    There is no ratio of gears to fix it.
  19. Darron72Skylark

    Darron72Skylark Well-Known Member

    Now that I have a rebuilt rear axle with great traction, that is exactly what happens. If I'm cruising along at light throttle and suddenly punch it, the engine revs up but vehicle speed stays the same. Kind of surprising that I didn't really notice it with the open differential. It is really obvious now. Actually, even a sudden partial throttle will cause it to slip - don't have to hit it very hard.
    Since the clutch is clearly shot, it has to be replaced anyway. We'll see if cruising rpms go down after replacing the clutch.

    Agreed that a consistent 2 mph error is not related to the gear ratios. Disassembling the dash and instrument cluster is one of my least favorite things to do, so with such a small error, I'm probably just going to leave it alone.

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