Replacing Sash Window Glass

Hi, I have Victorian sash windows which I found has 3 mm glass in them which I would like to replace. I was wondering if anyone had any recommendations for doing this particularly on making the groove bigger to take the newer thicker glass.
Thank you

3mm glass is still used for greenhouses.

It is not as flat as modern glass so it will look more authentic.

A combination plane could be used on the rebates or you could just use 1mm less putty.

The windows will ba heavier afterwards with 4mm glass (if you went to 6mm they woulkd not be strong enough for the weight)

Yard - it'll only be the sashes top rails which will be grooved and these are likely to be bigger than 3mm anyway - they were commonly ¼" to allow for a putty bed. 3mm glass (the modern equivalent of your 1/8" or the old 26 oz) can still be used in smallish sashes, say 400mm x 600mm. Bigger than that consider 4mm but even with that size you may not have to enlarge the groove ('cos ¼" = 6.4mm). If you do have to enlarge the width of the groove just use a sharp chisel to pare-away some of the material - a perfectly satisfactory solution.

The additional weight of the thicker glass will need to be considered (as per JohnD's post) as the sash weights are meant to counter-balance the sliding sash. Weigh the newly glalzed sash and check against the weight of the 2 weights (some weights have their value cast into them or just stick-em on the scales). You can increase the weight of the weights ( by adding lead 'caps' to the weights).

you can do as symptoms suggests about the lead caps,but you can get different sizes of weights from your local reclamation yard or scrappy.