Lapping Scope Mounts

**Andrew Leigh** · 27-08-2011, 18:17

Hi,

How many of you have lapped, your scope mounts? Talking of a general hunting rifle here.

**wrm** · 27-08-2011, 18:26

Essential. Ask Richard Boothroyd about it.

**Andrew Leigh** · 27-08-2011, 18:29

Is Richard a forum member?

**wrm** · 27-08-2011, 18:45

He is. He also has a website somewhere. Also, his phone number has been posted to this forum. In other words, Google is your friend.

**Andrew Leigh** · 27-08-2011, 19:06

Cool, thanks.

**CasperC** · 27-08-2011, 20:41

You will be surprised to see how misaligned your rings are when you start the lapping prosses. If you want one contact me, about R 550.00 for the 1". I am not a dealer but have means of making them.

http://jaracal.com/index.php?topic=3966.0

**Andrew Leigh** · 27-08-2011, 21:59

Originally Posted by CasperC

You will be surprised to see how misaligned your rings are when you start the lapping prosses. If you want one contact me, about R 550.00 for the 1". I am not a dealer but have means of making them.

http://jaracal.com/index.php?topic=3966.0

Hi,

will remember that. Was looking at the post you referenced, those mounts were more misaligned than I would have expected. That is worrying because how do you know that the scope remains aligned with the barrel, being that far out there must be an apparent problem. The mounts may be lapped but that may be all.

How were the mounts when viewed from above, were they aligned?

Cheers
Andrew

**Dick** · 28-08-2011, 10:23

Rings are always, and I mean ALWAYS, except by the occasional freakish good luck, out of alignment until lapped. That's because of manufacturing tolerances in rifles, bases and rings. It can seldom be seen visually, but the slightest amount is all it takes to do the damage. The risk of a scope not being parallel with the barrel is the least of your worries - it is seldom bad enough to be factor. The real problem is grip. Scope movement in recoil is prevented only by the frictional grip of the rings on the tube. That requires full contact over the whole area of the contact surfaces. If the ring is misaligned even a few microns, it will have zero surface contact, it will have only point contact at one edge. With heavy recoiling rifles, that will not prevent scope movement no matter how much you tighten it. Worse, heavy tightening can dent the scope, and movement will scrape the finish off the tube. How'd you like that to happen to your new R15K S&B ? The rings are fitted and lined up as near as can be done with the cope, then lapped in place with a precision turned rod. That gets them in 100% perfect alignment, and the full contact when the scope is dropped in, can actually be felt. It is an essential part of scope fitting but is seldom done. I fitted lots of 'em. If we (Allied Arms, Somerset West) were not the first in SA to do it, we certainly were among the first, and it is still a rarely seen technique. For your info, fitting a scope took two to four hours.

**Andrew Leigh** · 28-08-2011, 12:37

Done a bit more reading and yeah I can see how the recoil could affect the scope if not tight.

I would not discount the torsional effect on the optics, this can be substantial depending on the degree of misalignment. For me I would be more concerned about the optics, the scope tube holds the optics in place and baffles light, any flex in this tube will throw the optics out of alignment.

I think it is cause and effect, lap the scope to provide a mount that does not apply any undue mechanical stress to the optics and you automatically get a mount that will secure the scope with the maximum surface area. I made a 8" Newtonian telescope some years ago and disproportionate effort goes into designing a mount for the mirror (the primary objective optic) that will not introduce any mechanical stress to the optical surface.

In the link that CasperC provided it actually showed a picture of how misaligned a mount was before lapping. Personally I think these mounts were poorly machined to start off with for them to be that far out. Lapping generally refers to a "bedding in" rather than a regrind / line boring of the mounts which appears to have happened in this instance.

It can't be the receiver as the upper surface is milled without moving the receiver so it has to be the mounts. Surely they can get mounts machined better, I am not suggesting that one does away with lapping, but based on the pics I referred to those particular mounts appeared very poor. I will certainly lap mine when it comes, simply because I believe in excluding as many variables as one can and optically it is the right thing to do.

Am I missing something here?

Regards
Andrew

**Dick** · 28-08-2011, 23:34

It might seem logical that the top of a receiver machined at one setting should have zero error. Machining can be done to quite fine tolerance at modest cost, but zero error only at prohibitive cost. So a zero tolerance receiver is unlikely. Nonetheless, it is quite possible that bigger errors exist in bases. But so what ? There will always be some tolerance in either or both, and the slightest error is problematic for the scope. Lapping is a simple technique that gets close to 100% perfection, so why worry about tolerances in rifles and scopes that will always be present ?

But there is another aspect worth looking at. The contact between receiver and base is unlikely ever to be perfect. We don't worry about it because it is not a problem as long as the screws hold the base firmly in place. But a little spotting and scraping for fit wouldn't be a bad idea. The real answer, of course, would be to solder the bases to the rifle. But, as far as I can tell, there is no need, as tolerances in that department seem not to be a problem as long as the bases are fixed firmly enough not to move.