How can I level out the bases on a M98-based sporter?

[TheCoachZed]

TL/DR: I need to figure out how to get scope bases properly aligned on an M98 sporter that had the front over-polished, resulting in a low scope mount.

The full story:

Got an Alpine Special .270 off the EE here, came with Parker Hale bases. Went to put Weaver bases on, and lo and behold--when they sporterized this rifle back in the '60s or whenever, they were pretty keen to polish off the old insignia, and really ground down the front of the receiver. It's somewhere between a small ring Mauser and large ring Mauser; the front doesn't have quite enough elevation. Not uncommon with the sporterized M98s, the gunsmith tells me, but he's too busy to do anything about it now. So, I want to see if I can remedy this myself.

One or two of the screw holes are a tad rough, but aligned correctly; running a tap through them might help here, as well as a one-piece scope base.

But how do I overcome the issue of the scope mount bases not being in proper vertical alignment? A front shim? Would a one-piece base make the problem worse? Should I just slap a scope on there and see how far the adjustment is off?

If I wanted to shim the front scope mount base, how can I measure it to see if it's more or less on? I presume I need to somehow ensure the bore is level, and then install a base and shim the front until front and rear are more or less level with the bore?

 

[Potashminer]

A Mauser 98 has a flat bottom that is more or less parallel with the bore. Remove your trigger. Check / verify that your receiver bottom is straight - not twisted, curved or "humped". It should be a plane. Use calliper jaws to check the bases - one jaw on bottom surface of receiver, other jaw on top of front base - get a measurement. Now switch to rear base - get a measurement. Should give you an idea of thickness of shim required. That also tells you whether your bases are square left to right. Snug up the mounting screws and use a straight edge along top surfaces to verify that the tops of both front and rear base are straight, not tipped forward or backward, so that top surface of the bases also make a plane that is parallel with the flat on the bottom of the receiver. Weaver bases can also be sanded on bottom to correct for tilt. Often use a bit of epoxy as a "shim". Use a lapping bar to verify that the bore of your rings are in line and concentric with each other.

 

[TheCoachZed]

A Mauser 98 has a flat bottom that is more or less parallel with the bore. Remove your trigger. Check / verify that your receiver bottom is straight - not twisted, curved or "humped". It should be a plane. Use calliper jaws to check the bases - one jaw on bottom surface of receiver, other jaw on top of front base - get a measurement. Now switch to rear base - get a measurement. Should give you an idea of thickness of shim required. That also tells you whether your bases are square left to right. Snug up the mounting screws and use a straight edge along top surfaces to verify that the tops of both front and rear base are straight, not tipped forward or backward, so that top surface of the bases also make a plane that is parallel with the flat on the bottom of the receiver. Weaver bases can also be sanded on bottom to correct for tilt. Often use a bit of epoxy as a "shim". Use a lapping bar to verify that the bore of your rings are in line and concentric with each other.

OK. This more or less makes sense for using two-piece Weaver bases.

What if I put a one-piece rail on there? I presume I could:

1. Make sure the bottom of the action was level.
2. Mount the one-piece base.
3. Take measurements off the bottom of the receiver with calipers, OR put a torpedo level on the one-piece rail to see if it, too, was level.
4. Shim as necessary.

EDITED to say I am guessing loading the rifle will be an absolute pain with a one-piece base, as it doesn't have a DBM, or even a spring floorplate. It's old-school fixed mag.

 

[cody c]

OK. This more or less makes sense for using two-piece Weaver bases.

What if I put a one-piece rail on there? I presume I could:

1. Make sure the bottom of the action was level.
2. Mount the one-piece base.
3. Take measurements off the bottom of the receiver with calipers, OR put a torpedo level on the one-piece rail to see if it, too, was level.
4. Shim as necessary.

EDITED to say I am guessing loading the rifle will be an absolute pain with a one-piece base, as it doesn't have a DBM, or even a spring floorplate. It's old-school fixed mag.

Interesting. Id kinda wanna put the rings on a lapping bar, then wrap the action with sand paper, and work the bottom of the bases down till they both sit flat. Then lightly grease the action and epoxy bed them still on the lapping bar. Know what I mean?

 

[TheCoachZed]

Interesting. Id kinda wanna put the rings on a lapping bar, then wrap the action with sand paper, and work the bottom of the bases down till they both sit flat. Then lightly grease the action and epoxy bed them still on the lapping bar. Know what I mean?

Yeah. But I don't have a lapping kit and am not likely to buy one for this job. I have tons of tools for fixing motorcycles, but no real amount of tools for fixing guns. I've fixed plenty because I had to, but never enough to buy the specialty tools

 

[trevj]

Nothing magic about a lapping kit, it's simply a straight aluminum bar that is accurately measured to be at the diameter of the rings, with a handle added on to it.
Grab a jar of fine Clover valve lapping compound at the auto parts place. And then you have a lapping kit.

Personally, I would go with a taller front mount and fit it that way.

You are pretty much stuck modifying the rifle to make it work, or modifying the mounts to make it work, and there is less work in modifying a taller front mount down to the height needed, which is like to have been the route taken by those that originally modded the rifle.

 

[Potashminer]

OK. This more or less makes sense for using two-piece Weaver bases.

What if I put a one-piece rail on there? I presume I could:

1. Make sure the bottom of the action was level.
2. Mount the one-piece base.
3. Take measurements off the bottom of the receiver with calipers, OR put a torpedo level on the one-piece rail to see if it, too, was level.
4. Shim as necessary.

EDITED to say I am guessing loading the rifle will be an absolute pain with a one-piece base, as it doesn't have a DBM, or even a spring floorplate. It's old-school fixed mag.

If the receiver surfaces that the one piece base tightens up to are not appropriate, the one piece base just flexes or twists as it needs to, under the pressure of the mounting screws. Shows up as soon as you test lap the rings. There was a website that is now defunct that showed to snug a front screw - check whether that lifted the rear end off the receiver. Then snug second front screw - again check rear end for clearance. Then remove front screws and do the same for rear screws - tells you how much shim might be needed and where. If you chose a one piece base, the top flat on it still needs to end up as a flat plane, more or less parallel with the underside of the mauser receiver. Your steps 1 through 4 will get you there as long as the receiver is straight and the one piece base stays straight. You can use a couple pieces of straight key stock - say 3" long - laying cross ways to eyeball whether the surface is twisted or not. Straight edge to check for straight.

 

[Potashminer]

A lapping bar can be steel - just has to be 1.000" - ish. 10" to 12" long. And straight. Drilled and tapped for a handle (a 3/8" bolt at right angles) is handy. The Weaver scope mounting kit has a steel lapping bar. I also had one turned at a machine shop. I messed up way too many dollars of scope tubes before I caught on to always at least check the ring alignment. Works very well for aligning the dovetail style rings.

 

[TheCoachZed]

If the receiver surfaces that the one piece base tightens up to are not appropriate, the one piece base just flexes or twists as it needs to, under the pressure of the mounting screws. Shows up as soon as you test lap the rings. There was a website that is now defunct that showed to snug a front screw - check whether that lifted the rear end off the receiver. Then snug second front screw - again check rear end for clearance. Then remove front screws and do the same for rear screws - tells you how much shim might be needed and where. If you chose a one piece base, the top flat on it still needs to end up as a flat plane, more or less parallel with the underside of the mauser receiver. Your steps 1 through 4 will get you there as long as the receiver is straight and the one piece base stays straight. You can use a couple pieces of straight key stock - say 3" long - laying cross ways to eyeball whether the surface is twisted or not. Straight edge to check for straight.

Yep, this all makes sense.

I'm not going to do this this weekend or anything, I guess, so I'll keep an eye out for a scope mounting kit on the EE an if I find one, or don't, I'll likely tackle this either way in January at this point.

 

[rgg_7]

Can you post a picture of the top of your receiver to show hole locations? Somebody on here may have the correct height bases and rings. Nothing wrong with a one piece base however 2 pcs make it a whole lot easier.. Ron

 

[TheCoachZed]

Can you post a picture of the top of your receiver to show hole locations? Somebody on here may have the correct height bases and rings. Nothing wrong with a one piece base however 2 pcs make it a whole lot easier.. Ron

 

[rgg_7]

Take a hair dryer or heat gun and some 0000 steel wool and clean off the locktite or epoxy that mounted the previous bases. Holes are set up for 2 pc bases. Do you have rings and bases? looks like somebody did a decent job laying out the holes and profiling the receiver. I'd start with standers 2 pcs bases and measure height of each from base of action. Ron

 

[guntech]

Put bases and rings on, mount a scope, shoot it... then decide if the heights are too far off. A scope should be pointing downwards... it may be just fine.

 

[TheCoachZed]

Take a hair dryer or heat gun and some 0000 steel wool and clean off the locktite or epoxy that mounted the previous bases. Holes are set up for 2 pc bases. Do you have rings and bases? looks like somebody did a decent job laying out the holes and profiling the receiver. I'd start with standers 2 pcs bases and measure height of each from base of action. Ron

It came with Parker Hale bases, but not rings, so I was going to switch over to Weaver mounts and rings.

 

[rgg_7]

I'm with guntech on this. Nothing wrong with Parker Hale (Front A22/Rear A28). Why not use them? If you want Weaver make a trip to Cabala's or Bass Pro and pick up the Weaver set up. Once you shoot it you can determine if anything further is required. Ron

Just noticed this......appears your front receiver ring has been milled down....I see a step either side just above the stock line. Is this the case? Large ring Mausers should be the same diameter.

 

[TheCoachZed]

I'm with guntech on this. Nothing wrong with Parker Hale (Front A22/Rear A28). Why not use them? If you want Weaver make a trip to Cabala's or Bass Pro and pick up the Weaver set up. Once you shoot it you can determine if anything further is required. Ron

Just noticed this......appears your front receiver ring has been milled down....I see a step either side just above the stock line. Is this the case? Large ring Mausers should be the same diameter.

I don't have the rings for the Parker Hale mounts, and I thought they were proprietary, and hard to find, and expensive. I was under the impression I could switch over to Weavers for practicality. Is this not the case?

And yes, front receiver ring seems to have been milled down. I presume this was to remove Nazi markings or something like that, as this certainly appears to me to be a surplus K98 action that the Brits rebuilt into a sporting rifle. That is the source of the woes to start with.

 

[rgg_7]

Can you post a picture of the underside of the Parker hale bases. Is the anodized finish present? Maybe they have been fitted? What are you trying to use for a scope, It may been better to find the PH rings - they are out there! Ron

 

[TheCoachZed]

Can you post a picture of the underside of the Parker hale bases. Is the anodized finish present? Maybe they have been fitted? What are you trying to use for a scope, It may been better to find the PH rings - they are out there! Ron

The front is an A5 mount and has indeed been machined down.

I was going to put an American Rifleman on it, for the generous eye relief, but I missed the Cabelas sale and am thinking of slapping on an old made-in-Japan Tasco 4x I recently picked up, for the same reason. Lots of eye relief, very clear, and I prefer fixed-power scopes. Less to go wrong.

 

[rgg_7]

Looks like somebody has made the necessary adjustments. I would reinstall your bases and get a set of PH rings...you might be able to get away with a set of low rings with the scope set up you are looking at. ron

 

[TheCoachZed]

Looks like somebody has made the necessary adjustments. I would reinstall your bases and get a set of PH rings...you might be able to get away with a set of low rings with the scope set up you are looking at. ron

I think we tried a ruler on them before taking them off and they were a bit skewed, but maybe not so much that they aren't usable.

Where might one find PH rings in Canada, and which ones am I looking for here?