Lower Unit Guru's please read (1971 Merc 800)

In the process of putting back together a lower unit for a 1971 Merc 800. The disassembly thread is here

http://forums.iboats.com/showthread.php?t=401704

In summary, I'm replacing a driveshaft and using the existing bearing carrier, pinion, fwd and reverse gears in the same housing. Only other thing new is the tapered upper driveshaft bearing and race.

When checking the pinion/fwd gear engagement depth, I discovered something the just doesn't look right. The pinion gear appears to be not nearly deep enough to fully engage the fwd gear properly. The manual says to remove shim material from under the bearing race (tapered upper bearing), BUT it looks like I need to lower the pinion gear 1/8", not just a few thou.

As a comparison, I checked the "old" lower unit that was causing me troubles. The pinion depth appears to be about the same, 1/8" too shallow.

I've attached two photos showing the pinion position relative to the fwd gear in both lower units (one I'm putting together and one that was damaged).

From what I've seen, I would say the lower unit castings are wrong, as far fetched as that may seem. How can the engagement be out by so much?

Any insight is much appreciated as I hope to get this thing finished up this weekend.

Re: Lower Unit Guru's please read (1971 Merc 800)

does the d/s have a preload pin in the end of it?

Re: Lower Unit Guru's please read (1971 Merc 800)

The housing you are installing the new driveshaft in, did the old D.S have the tapered bearing (preload style) or a solid bearing? If it was trapped roller bearing housing will not work...

Re: Lower Unit Guru's please read (1971 Merc 800)

Replacing pre-load pin shaft with pre-load pin shaft. The unit I'm repairing is the one with the hypoid gear set in the photos. It has the tapered roller upper bearing. Dirveshafts are identical. Only new part is the upper bearing set.

The other unit in the photos with the straight cut gear set was a defective unit that I tore apart to find out what happened to it.

From the photos you can see that both units seem to have the same apparent issue.

Re: Lower Unit Guru's please read (1971 Merc 800)

I took the parts out of the LU housing and set them up on the bench for a better look. I "think"everything is OK from what I see now. I've attached a few photos of how the gear set engages.

Even thought the pinion depth looks odd when looking at the fwd gear (as would be seen looking into the LU housing), the engagement does in fact appear to be right when viewed from a different angle.

If I'm interpreting what I'm seeing right, then the pinion/fwd gear mesh looks perfect because the full face of the driving edge of the pinion gear is highly polished.

I'm going to go ahead now and check the contact pattern with the new driveshaft and bearing in the housing. Hope I won't have to mess around with new shims.

Re: Lower Unit Guru's please read (1971 Merc 800)

Well I put things together and checked the contact between the pinion and fwd gear an was happy to see the results. As can be seen in the photo the contact pattern looks pretty good. Contact is centered on the teeth and is oblong in shape indicating contact over a wide portion of the teeth.

Backlash for the fwd gear as best I could measure was 0.015" at 1" from the center of the driveshaft. Overall it turned out pretty good using the original shims.

Tightening the carrier nut was a bit if a challenge. The spec was 210 ft/Ib. I used a bit of physics and my 190 Ib to get it "close enough" for me.