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95hp "K" Blowby around Spark Plug

Posted: Sun May 05, 2019 2:18 pm
by Ravenswood
Just got the engine in my '41 Chris Craft Deluxe Utility rebuilt. After rebuild it was run on the block and then water tested by the company that rebuilt the engine.
When I went to run the engine the first time I noticed some staining in the new paint around the #6 spark plug. When I cranked over the engine, every time #6 came up on the compression stroke it puffed out air around the spark plug. I'm going to be calling the rebuilder on Monday but would like to have some advice as to what the problem might be prior to having a discussion with him.

Re: 95hp "K" Blowby around Spark Plug

Posted: Sun May 05, 2019 6:22 pm
by Captain Nemo
Did you pull check if the plug is loose? They don’t have to be torqued too much though. Pull the plug to see if the washer is intact.

Re: 95hp "K" Blowby around Spark Plug

Posted: Mon May 06, 2019 7:47 am
by Bilge Rat
Might be worth a try to change the plug. Even though its new, might have a manufacturing defect in the threads or even the porcelain insulator. Possible a Chinese product, there have been some documented counterfeit knock offs.

Re: 95hp "K" Blowby around Spark Plug

Posted: Mon May 06, 2019 7:53 am
by drrot
Bilge Rat wrote:
Mon May 06, 2019 7:47 am
Might be worth a try to change the plug. Even though its new, might have a manufacturing defect in the threads or even the porcelain insulator. Possible a Chinese product, there have been some documented counterfeit knock offs.
Great place to start. Exchange it with another plug and see if the problem moves with the plug or stays in #6

Re: 95hp "K" Blowby around Spark Plug

Posted: Tue May 07, 2019 7:16 am
by homeiste
Another possibility is a bad heli-coil repair job.

Re: 95hp "K" Blowby around Spark Plug

Posted: Tue May 07, 2019 2:04 pm
by Ravenswood
Thanks to all for their input.
Found the problem.
A close inspection of the #6 spark plug seat on the head revealed a fairly deep divot in the head surface where the metal gasket for the spark plug should seal against the head. Should have been caught by the machine shop when they checked the head, should have been caught by the boat repair when they installed the spark plugs or when they ran the engine.

Re: 95hp "K" Blowby around Spark Plug

Posted: Sat May 18, 2019 6:26 am
by jim g
Thats a simple fix. Get a package of spark plug index washers. Put shipping tape on one side and cut the center out. Put some JB weld in the pit and put the washer on tape side down. Then screw the spark plug in. Wait a few minutes then unscrew the spark plug by hand while holding the washer down with a small screwdriver.

Remove move the washer after a few hours. Then install the spark plug with the thinest index washer.