After 4 tanks of gas piston starts hitting spark plug

[Chainlube]

It went in normally with my fingers but seemed a bit weird when tighning it with a socket, now that I think about it it did seem to go bit to far

The compression washer on the plug must be flat now. Go finger tight, and then just snug it up with a wrench/socket, not even an1/8th turn.

 

[digital_life]

The compression washer on the plug must be flat now. Go finger tight, and then just snug it up with a wrench/socket, not even an1/8th turn.

Ya it defiantly went more than 1/8, I remember when I first started riding it had a small leak from the plug so I tightened it a bit to much.
I don't think the tolerance should be so tight, On my last 2 grubee's I reused the plug s much I'm sure the washer got squished a lot and had no problem.

Anyway I got it apart today and it looks good no damage or missing bits and I am going to use the larger head, Problem solved I guess

 

[Chainlube]

Ya it defiantly went more than 1/8, I remember when I first started riding it had a small leak from the plug so I tightened it a bit to much.
I don't think the tolerance should be so tight, On my last 2 grubee's I reused the plug s much I'm sure the washer got squished a lot and had no problem.

Anyway I got it apart today and it looks good no damage or missing bits and I am going to use the larger head, Problem solved I guess

It'll run cooler.

 

[Whitewill1412]

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

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I have one of those heads. I didn't know it was a woody woodpecker

 

[Daytripper24699]

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.

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I use 2 head gaskets on my Grubee and no problems in almost 4 years so far.