I LOVE my GRP(Ninja) 21, ran about a gallon through it practicing today, did a bit of performance tuning...and WHOA, this thing puts out some SERIOUS power.
I need to reglue all my tires, the GRP power has torn them off of teh rims

Well, I whipped out the digital calipers and a bunch of plugs....

O.S. Turbo, GRP Turbo (C6), Mugen Turbo, and O'Donnell Turbo

Now rather than post my actual numbers, I'll post my findings. Reason being, calibration of digital calipers will vary so numbers may vary. The key is to use the same digital calipers for all the measurements, apples to apples comparison...

I measured total length from where the plugs sits on the button head to the end of the cone.

Length of the cone itself, from the shoulder to the tip or end.
Diameter at the end of the plug (filament end)
Diameter at the shoulder (Where the threads end, and just before cone starts)
Length of plug from where it sits to where the threads end....

In summary; I found the Mugen and GRP to be basically identical (no real surprise).

The O'Donnell was very close to the GRP that I'd have no fear of running it in my GRP. The differences I found were in the .001 range which I can attribute to my error....

The biggest difference was between the O.S. Turbo and the GRP C6...

The two big differences were in the (outside) diameter of the tip/end where the filament is. The O.S. measured .197, the GRP .202. The next biggest difference was in the diameter of the "shoulder". The O.S. measured .256 and the GRP .271

So, the O.S. is smaller by some. Other than that observation, I don't know what that really means....

I'd sure like to hear what others mic'ed out...

My suggestion is, it you ran an OS plug in the head button, use OS plugs for that button. If you want to use a GRP/Ninja plug, get a button for that glowplug. I have seen them leak, not just this brand, but all others when you switch up glowplug brands. I have heard people tried the OS plug and went back to the GRP glowplugs and said they didn't feel as good as the OS, but this could be because the taper is now not sealing properly.

I have been hearing the GRP is closer to the Picco taper than any other plug, but I would still say stick to GRP plugs unless you are certain you have no leakage issues with another brand.

Come on man,... you can see the freakin difference... geesh! My point here is Novarossi plugs wont work no matter what the caliper say's! LOOK! The angle and distance is the same as they are laying right next to each other...! The taper does not match, the pictures are not lying....!

The reason to actually figure this out is:

10 GRP plugs @ $11.50 x 10 = $115.00
10 Odonnel plugs $6.99 x 10 = $69.90

What plug would you rather run if they both fit and perform well?

Oh... I see you said no offense... sorry! Anyways, good job on measuring. No need for me to take close ups now. Cool.

Ninja, GRP, or Odo plugs got it!..

Thanks.

My understanding is there are 3 options
Ninja plugs
GRP plugs
O'Donnell 97 plugs< exact same taper, I run these

got to race my GRP mill this weekend and it's a winner. Plenty of power throughout the powerband and it was delivered really linear. I'm happy with it I was running a P3 plug as well

I asked Mike Truhe if I could run an O.S P3 with no problem in the GRP engine this is what he told me.justindyer: yeah that will fit just fine

One thing, does the stock GRP plug suck? I'm trying to figure out why everyone runs a P3 plug instead of what it comes with.

The one it comes with is a cold plug I think a GRP 6 mine died after breakin.

I ran the P3 with very good results. P3's are cheaper and easier to find. Plus they idle for 1/2 with P3s.

Yes, the grp6 is a medium plug. the grp5 is the hotter of their selection. I emailed Mr. Drake and he told me to run the 5. I'm a tool and die mold maker. I have the precision tools to get some hard facts about the plug dimensions. I will measure a grp and an os. I work second shift so, I will post my findings in the early hours Tuesday morning when I get home from my Monday. The difference of the end diameter from .197 to 200 isn't as important as the angle. If the angle is different, that's when damage will occur. I will get the bottom of this.

Truhe told me to run the 4.But told me the OS would work fine no problems.

Ive been running boss & ninja # 6 plugs with 25% with great results and i can get them alot cheaper here in Australia than the grp plugs. #6 plug seems to be ideal for Australian daytime conditions.

The problem is if you are looking for the "hot" plug (GRP), just about every place is out of it. I can't find it anywhere in stock. So I believe people are looking for alternatives until those are available OR a less expensive alternative...