HELP!! 28hp cross threaded spark plug hole

Mthirsch14

Seaman Apprentice
Joined
Jan 26, 2019
Messages
46
I used the wrong size compression tester fitting and cross threaded the top cylinder hole. Now the spark plug will go in halfway and then it stops. I’m too afraid to take the powerhead off and take it to a machine shop because most of the bolts are rusted and I don’t want to break them. Would a rethreader be my best option? Thanks in advance for any help.
 

Ibl0wstuffup

Petty Officer 3rd Class
Joined
Mar 10, 2011
Messages
85
Try a tap, and if that doesn't work you should be able to install a helicoil or timesert without removing the head.
 

Mthirsch14

Seaman Apprentice
Joined
Jan 26, 2019
Messages
46
Correct me if I’m wrong, but if i install a helicoil or timesert wouldn’t that make the hole too small for the spark plug?
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,036
No it will not !----These are well engineered thread repair methods.
 

harringtondav

Commander
Joined
May 26, 2018
Messages
2,438
Bar the motor over until the piston is at TDC. Try a spark plug thread tap first. Re-tap using compressed air to blow the chips out along the tap flutes. No lube. Tap dry and slow. 1/2 turn, and then back up to break the chip. Repeat. When the tap gets 4-6 full threads below the top of the head, switch to a bottoming tap, or grind the lead in threads off the end of the tap and run the tap in until it contacts the piston. Then blow it out thoroughly.

If you can torque up the spark plug -18 ft lb, you're good. If not, you're into the helicoil repair. Buy the full kit with correct drill, tap and insert tool. You'll have more chips to deal with. Use the compressed air and avoid lube.
 

Mthirsch14

Seaman Apprentice
Joined
Jan 26, 2019
Messages
46
Will this work? Are they one size fits all? I don’t see a specific size
 

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Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,300
they are not one-size fits all.

there is a whole chart of sizes per SAE J548d
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,036
Hurry run to a machine shop for proper help.----This may save you a lot of grief in my opinion.
 
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