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The upper shroud chainplates (the larger two in the picture) were installed with 1/4" tound head machine screws. The chainplates themselves had been countersunk to accept flathead screws, leaving the contact area between the plates and screws rather small and allowing some (although slight) elongation of the holes. The chainplates do not have any stress or fatigue cracks and appear to be in good condition. Should I:
a) drill out the countersink holes to 3/8" or 1/2" and replace the machine screws with the appropriate size?
b) replace the 1/4" roundhead machine screws with flathead ones?
c) re-install then as they were. (this is not an option I would normally choose)
d) do something else
I know that stainless is hard to drill and requires specific techniques.
Thanks
While you think you're green, you're growing,
but when you think you're ripe, you're rotten.
How slight is the elongation? If it is very slight, I would simply replace the screws. If the elongation is more significant, then you'll cause point loading which will only make the problem worse. In that case, then you will need to drill new screwholes to accomodate the larger screw.
I think that option B is probably sufficient.
I'd rather drill out for 3/8" bolts though. I don't know that I trust my ability to put sufficient torque on a screwdriver, I'd take the opportunity to switch to proper bolts.
Drilling stainless isn't so bad. cutting oil is your friend.
1. Cutting stainless steel works best with cobalt bits, cutting oil, and very slow cutting speeds -- my drill press goes down to 360 RPM and even that is probably a bit too high. You'll know that you've got it right when long spirals of bright metal are coming off the bit. Slow and lots of pressure.
2. I normally don't like 1/4" bolts because the heads are too easy to torque off and the bolts themselves can bend. I try to use 5/16" or larger for any high load applications on any of my boats. I've never had a 5/16" head shear off.
3. However, after looking at the pictures, I would think that drilling the holes out to the outer limit of the countersinks wouldn't leave enough metal on either side of the chainplate. I would be afraid that a sudden and slightly off center load would crack the more heavly loaded side of the chainplate at the first bolt.
4. In your application, the countersunk 1/4" bolt heads should spread the load out over a large enough area of chainplate so that the holes won't elongate too much.
Enlarging existing holes in stainless isn't that hard to do, since most of the hole is already cut. Slow speed and cutting oil help.
However, I agree with those that said that enlarging the holes to the outer limit of the current countersink would be excessive. It just doesn't leave much meat in the chainplates. 3/8" I don't see any reason why you couldn't slightly enlarge the holes to 5/16" and use regular hex head bolts. Just apply fender washers on each side, including under the head.
I also think, as said in an earlier post, that slotted/phillips machine screws--whether flathead or roundhead--don't have a place in a chainplate because of the difficult of torquing them properly with only a screwdriver to hold the head. Bolts are far superior.
From the photo, the chainplates look to be in generally good shape. Slight elongation of the holes is normal, and you'd probably be fine sticking with the original-sized bolts. But the slight upgrade in size wouldn't hurt, and you'll have better luck with 5/16" bolts than 1/4.
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