I would believe that the thread looks as though it is a NPT but it more than likely is SAE straight thread with an o-ring. Both NPT and SAE can have the same threads per inch except one is tapered and one is straight. If it is a straight thread, tapping it out requires a special tap.
Keep in mind they can both look VERY much alike but are very different. It would only make sense to have a high pressure o-ring type of fitting in this installation. Latham and MM both use straight threads not NPT
Hope this helps

Wouldn't be the first time I've seen an NPT fitting in a straight-thread hole. A telltale sign is if the area around the fitting has a machined flat spot in a circular shape. They spotface the surface for the o-ring's washer.

Steering operates on a much higher pressure than engine oil. The hose and fittings are 3,000 psi rated.

The fitting I am talking about is IMCO part # FA-14, and the filter is part # HO-18. The catalog describes the fitting as "1/2" M.P. to Metric O-Ring (Brazil) adapter" The leak is where the fitting screws into the filter. The filter says "1/2" F.M.P.T. 1500 PSI max pressure". That makes me wonder? 1500 PSI Max? I assume IMCO knows what they are doing.

Thanks for the replies, I'll let you know how it goes.

The steering probably doesn't operate at a pressure much over 200PSI when in static, but it may go as high as 1,000PSI under load- probably a bit over that under extreme conditions. I'd imagine they are using an off-the-shelf filter mount- something similar to a remote engine oil filter mount. A suggestion would be to convert this to a commercial-duty hydraulic system filter. You can get them at any Aeroquip distributor, Grainger or McMaster-Carr. The outlets on the filter adapter would then be o-ring fittings. You could then buy filters locally.

I had the same problem with my IMCO steering a few years back and used Gasoila to seal the threads.

Here's a link.

http://www.gsasupplyco.com/gasharvartyp.html

I removed the fitting, cleaned the threads, used teflon tape, re-assembled. I ran for about 1.5 hrs. today and no leaks.

Hopefully this is the end of the story.

Dave M.

Dave- thanks for letting us know you resolved the problem.