Originally Posted by Geo Y-Rock

I did some measuring today at work. I have the Grp .21 and I'm running the OSp3. The engine comes with the grp6 turbo plug. So, I measured the two and compared them. The angle is 30 degrees on both of them. To me, that's the critical thing. If the lock up angle is correct, no damage will occur. However, I did find a slight difference in the end diameter of the plugs. The OS plug is slightly smaller on the tip. What this means is, the OS plug protrudes slightly further into the cylinder than the stock plug. But only by .003 on an inch. I measured this with my Mitutoyo digital drop indicator. I then used Dykem blue dye to test the fit of each plug into the button. This gives evidence of the contact surfaces of the plug and button. The dye showed that there is a perfect seal with both plugs.
Conclusion: I've been measuring steel as a tool and die maker for twenty years now. I believe that these two manufactures of plugs are interchangable. But an engine expert, I am not. With the tests I've done, I believe you cannot damage your button by using OS plugs in your Grp button because the angle is correct. But the question is the .003 of depth between the two plugs. Will this make a difference? Personally, I don't think it will matter, because the surface area of the glow plug that is inside the cylinder is VERY small. I've had no problems using the p3 in mine and will continue to do so.
Notice the blue dye in the angle which shows the contact between the os plug and grp button is perfect.

The grp plug depth from the bottom of the button to the face of the plug is .116

The os depth is .113