I have an old Tikka T3 and want to change the scope mount holes to 6-48. the current ones are metric 3.5x.5

First, if the problem is you can't find screws, call Evolution Gun Works. Only place I know that has M3.5X0.5 screws.

For a retap, you're looking at the same diameter and 48 tpi verse 50.8 tpi. It may work, may not. Depends on the tolerance of the original, I would think. I've not tried.

Sizing up to 8-40 is the sure thing.

Jeremy

Yep, EGW has them for their rails. I thought about going that route, but want to minimize weight. I'm mountung Talley aluminum rings. I looked high and low at every fastener supplier and manufacturer for the M3.5-0.5 screws. I did find some, but in 1,000 quantity.

I had planned on calling them tomorrow. The ones they have are shorter, but they may have longer ones. Need 8.5mm thread length,

THanks!

What happens is this; you have a 2.8 thread mismatch, Per Inch. If your thread engagement is say, .150 inch, then you will have a mismatch of .42 threads. Less than half a thread.
Go for it. The 6-48 might thread in as is.

I suspect there are a lot of these running around that were chased with a 6-48 tap.

Jeremy

For mounting rings directly to the receiver, I need to move the front ring back due to my long LOP and the scope I'm using. So, I can tap the front hole when my tap arrives and let you know.

Yes it will re-tap

Are you SURE the existing thread is not M3.5X0.6? That is a Std ISO size, though not a prefered size, and is what Sako and Tikka use in their scope rings, etc these days. Running in a 6X48 tap into a hole tapped M3-5X0.6 would make a BIG mess. 6X40 would be closer, but still not ideal.