Gear case lapping question

[raglover]

What is the proper grit sandpaper to use when lapping omc gear case halves before reassembly?

Is it possible to get the halves too smooth to not give the 847 good adhesion? I have used 400 grit on glass and am having a time getting a good seal.

[chris-p]

I don’t think I have ever done that. Just clean well with acetone.

[collectorinspector]

You should not be using "Sand Paper".

You use the water proof Hermes or similar brand wet rubbing carbon type….like on paintwork before buffing.

Wet and Dry it is called down here.

Use a sharpie and color the surfaces. Give a brief figure of 8 to each. Any sharpie color left indicates low spots. Compare the relevent positions of same both halves.

Post a pic or two for better answers.

B

[mr-asa]

As Collector said, lapping doesn’t involve sandpaper.
When I do it, I get a piece of 1/4" thick glass (at a minimum) that’s about a foot square, then I just use valve-lapping compound like you’d find at any auto parts store.

With a flat enough surface you shouldn’t need the sharpie to see the high and low spots after a few passes.

[Casey Lynn]

How about dropping the need for lapping altogether and install a new piece of spagetti seal and 3M 847 or Adhesive M and be done with it.

[chris-p]

quote Pappy:

How about dropping the need for lapping altogether and install a new piece of spagetti seal and 3M 847 or Adhesive M and be done with it.

That’s all Ive ever done, who knows how many gearcases Ive sealed that way.

[collectorinspector]

And here I am thinking that the case halves were so rooted that they need such treatment.

I thought that there was a reason to do that because the Spag leaked?

Looks like LOTS! of questions are required Aye. Also why not on insisting on pics before a Q is answered for correctness from now on?

Pics are easy and "Wee all Like Pics Aye"

B

[raglover]

Thank you for the replies. Sorry for the wrong terminology.

I used a piece of high quality wet/dry paper glued to the glass. I used it wet on glass to do exactly what is being suggested….looking for low spots.

I had a casing that had some noticeable low spots and knicks I was trying to remove. Got it polished down and removed all low spots and knicks, it is very smooth and flat. I too usually just do the 847 and seal, have resealed many. This one is just being more troublesome that usual so I suspect my "new" process is at fault. We sand the bearing casing halves of large turbines before using anaerobic sealer in between but cannot remember roughness spec.

I would say that it is at least as smooth as with lapping compound, but is much smoother than the original machined surfaces. I can go back and try the lapping compound to compare the difference

Pictures I do not have, but try to add them when it is needed. Did not mean to cause frustration over people posting/not posting pictures or the question. Just curious if anyone an idea if it was possible to get too smooth and if some roughness was needed for good adhesion.

As always than you for your help I will try to update if the lapping compound works.

Allen

[collectorinspector]

😎

B

[Casey Lynn]

I think you are missing the point here.
You can have several imperfections (common sense prevails) in the mating halves. The spagetti seal is what creates the seal along the mating halves plus the large O-ring.
Seems we are once again placing the cart before the horse….. I see that no one has asked if you are having an issue or repeat issue getting it sealed and how you are checking to see where the leak is. Care to fill in the blanks?

[Author]

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