After 4 tanks of gas piston starts hitting spark plug

[digital_life]

Took the head off trying to figure out why the motor will not start I found that the spark plug gap was non existent then noticed a little ding in the piston.

I have run this motor a few times using about 4 tanks of gas and travelled about 150~200 Kilomters ran great always started up right away, Last week I took the plug out to see how it looked and put it back in today I tried to go for a boot and it would not start.

About the motor: It's a Brand new GRUBEE SkyHawk GT5B 69cc I created a thread about it with pictures a while ago

Brand new GRUBEE SkyHawk GT5B 69cc compared to GRUBEE SkyHawk GT5A 66

I got my new GRUBEE SkyHawk GT5B 69cc Angle Fire Slant Head and decided to compare what differences my older 66 Grubee made in 2017 were to the 2021 model. In most of the pictures below the new GT5B is on the right hand side.

motoredbikes.com

Anyone ever seen this happen before, The only maintenance I have done is torque the heads and tighten the spark plug after almost every ride.
The plug I was using was a ngk b6hs, after it happened I tried a champion l86c an the same thing happened, Both of those plugs worked really good on my older Grubee's.

Any thoughts -- I got another head for It I will try that but kinda worried about it happening again after another 200 kilometers, This motor has 2 gaskets on the bottom end should I add another and make it three or use two on the head or could there have been some kind of defect on how the threads the spark plug hole were made.

 

[GW's Motorized]

What plug are you using? It could be too long of a plug

 

[digital_life]

First one was a ngk b6hs, after it happened I tried a champion l86c an the same thing happened As far as I know both are a direct replacement for what is recommended on this forum

 

[GW's Motorized]

Did you have a head gasket in there or was it just the rtv silicone?

 

[digital_life]

Did you have a head gasket in there or was it just the rtv silicone?

I used both,The gasket was made of copper and the rtv was ultra copper gasket maker

 

[Wrench]

Use a indexing S/P washer to shim up the S/P a bit
BTW don't use RTV on the head gasket ,

 

[digital_life]

Use a indexing S/P washer to shim up the S/P a bit
BTW don't use RTV on the head gasket ,

I have never seen or heard of a indexing spark plug washer until now I googled it and it seems like a great idea. Just to confirm from what little I have read should the gap point towards the intake.
One more question about indexers: Could I use a normal washer that I have in my nuts and bolt bin

This is my third motor, My first one I got over 3000 kilometers until the clutch broke on the inside of the motor, The second one that I have since fixed got over 3000 maybe closer to 4000 kilometrs untill I leaned the carb a bit to much and the piston ring blew. All of them I did the same thing the rtv I used said it was good to a temp 550 F (or was it 600F ?) no leaks no problems what so ever

 

[Chainlube]

Screw the plug into the head tight and test how far it extends into the combustion chamber with a spare piston. If the plug leaves a mark, then you need a new head. I don't think the plug is the issue.
Pull the cylinder off and check the piston pins and rings.

 

[digital_life]

Looking in my parts bin I found this head, The pistion will have absolutely no chance of hitting the spark plug

66cc / 80cc High Performance Head

Compatible with GT5, GT5AWoody WoodPecker's High Performance / High Compression run cool head.Brushed aluminum finish. 6cc combustion chamber; Means high compression. It Fits 66cc and 69cc engines using M8 head bolts and 47mm dia. pistons. Extra cooling fins and extra mass means better engine...

motorizedbicycle.ca

Woody WoodPecker's High Performance / High Compression run cool head.​

Brushed aluminum finish. ​

6cc combustion chamber; Means high compression. ​

It Fits 66cc and 69cc engines using M8​

 

[Chainlube]

Looking in my parts bin I found this head, The pistion will have absolutely no chance of hitting the spark plug

66cc / 80cc High Performance Head

Compatible with GT5, GT5AWoody WoodPecker's High Performance / High Compression run cool head.Brushed aluminum finish. 6cc combustion chamber; Means high compression. It Fits 66cc and 69cc engines using M8 head bolts and 47mm dia. pistons. Extra cooling fins and extra mass means better engine...

motorizedbicycle.ca

View attachment 194368
View attachment 194369

That's a nice head. Did you check the piston?