Plug sizes/ engine differences

[Lucoousesox]

Hi, hopefully someone can help me out. What is the measurement from TDC to the cylinder head, the reason I ask is I have a prototype frame number 47x-000098. I have taken out NGK plugs BR9ES but the book says BR9HS, there is no damage to the pistons and it runs fine. I’m a little confused to say the least.
Any help would be deeply appreciated

Andy

[Jeff B]

HS is correct. ES has longer threads and would stick down into the combustion chamber farther. I highly doubt it would hit the piston. I suspect it would be more likely to burn a hole in the piston though.

[tacky1]

Grab a little house electric wire. Strip it to bare copper. Put a bend on the end. Slide it inside the plug hole and twist it and pull up till the copper hits the inside of the head. Mark the top where it touches the head. Pull it out. That’ll give you the dept of the thread.
But all the 500s I have owned over the years used HS plugs

[rz500guytexas]

I have always ran Champion plugs. L87 or something. Never an issue on my old stocker I sold or track bike