welding boiler plate

If the plate you are welding touches water, it is likely under full boiler pressure. I don't know what the 1870's unit runs but the 1900 versions run 100psi with a 125psi safety valve. And all of these need a state pressure boiler inspection and license.
The plate in an 1870s unit will be much closer to wrought iron than steel plate. It is going to want a very soft (low tensile/yield) weld in order to be compatable with what is there.
If you haven't done much of this, I'd really suggest you chat with your state inspector. He will drive a lot of what you are allowed to do.
cj wrote:

Roy gave very good advice. And if you want to know why, here's a link: Pete Keillor

Ouch!
Pretty common in the old days, that's why they have the inspectors.
Peter T. Keillor III wrote:

Do you know the make or model of the tractor? Have any pictures of the tractor and area to be repaired or replaced? You need find all the information you can on the unit so that your repairs are to code and will pass boiler inspection.

Below is a link to picture of 1870 Tractor built by Howard Farmers Engine - 8nhp. Works No. 110 - Reg. No. AP 9197