Boiler washout plugs.

Hi all,
I have an early 4" Burrell (with the copper tubes) and have just had my boiler inspection done for this year, it passed no problem and all good. However after the steam test I noticed one of the washout plugs was dripping water along with wisps of steam escaping so I thought I'd take it out again and clean the threads in the boiler boss. I take all 4 plugs out at the end of every season anyway and clean up the threads on the plug every year so no issue getting the plug out (I used PTFE tape on the taper thread), but haven't up till now cleaned the thread in the boiler boss other than a rub down with a miniature wire brush. I ran a 3/8" BSP plug tap through the boss which worked a treat and cleaned up the thread through to the water space, but having had the old style iron washout plugs with a taper thread on them, the thread in the boss obviously isn't like new.

Thinking the threads on the old original plugs were getting worn, I rang up STW to obtain replacements and found out these have since been replaced with brass plugs with a hex head so I ordered 4. These arrived today and I noticed these now have a parallel thread. So my questions are,

1. Has anyone out there had any experience going from the old iron tapered plugs to the new brass parallel ones and had any issues with the new plug sealing on a thread that's had a taper plug fitted?
2. Do you guys use Foliac on the threads to seal them?
3. Is there any washer used under the head of the plug? or just the plug tightened up to the boiler boss? The only assembly instructions/drawings I have are for the old style iron taper plugs.

Any advice appreciated...

I've not had any issues using parallel threads, I swap my front plugs with a set of taps to allow blow down of either side of the boiler. These I fit with copper washers and some PTF just on the very flange area rather than the whole thread as I am not a fan of PTFE tape on threads as it tends for damage the thread in the end.
I also swap between these and the tapered plugs with no issue, with the tapered plugs all I use is a smear of copper slip on them and thats all.

Others may have different ideas but mine is like yours an early MW boiler with copper tubes, in fact mines the first control model for the batch ones