TheSamba.com :: View topic - Rebuilt 912 engine number three and four pistons hitting spark p

Fri Jan 31, 2025 11:37 am

Hi All, I had my father‘s Porsche 1966 913. The engine was rebuilt couple years ago. Haven’t been able to do anything due to fighting Cancer, now that I’m cancer free I’m starting to work on it having so much trouble getting this thing going the guy who built it came down eight months ago, and the Pistons were hitting the spark plugs, closing gaps. The builder said well You just have to shim the spark plugs. Now I’m not an engine builder, but I’ve had enough experience on engines that this is not right in my eyes. His excuse was you gave it to me so I rebuilt it and of course, if it were wrong, I never would’ve approved it if he had asked, I was under the assumption that he knew what he was doing. So I don’t want to damage anything. There are 0 miles on this engine. I know I’m not the first one that has experienced this, but shimming the spark plugs. Will it run without problems? or is it better to strip it down take off the heads and have another place machine them to match or replace the one head so that there is clearance. The other curious question I have is compression if the heads have been decked so low that they’re hitting the spark plugs when the piston comes up is there enough compression that matches the other side? Any on all comments would be great. I’m dying to get this car going, but I’ve waited so long that if I had to pull the motor out and redo that head Then I’m going to do that.

Thank you so much for your input Scott

Fri Jan 31, 2025 5:13 pm

https://forums.pelicanparts.com/porsche-912-technical-forum/115239-best-spark-plug.html

See 4mor 5 post down. Could be extended tip plugs were installed.

Sat Feb 01, 2025 6:40 am

Wolfgangdieter wrote: https://forums.pelicanparts.com/porsche-912-technical-forum/115239-best-spark-plug.html

See 4mor 5 post down. Could be extended tip plugs were installed.

Some (old only?) NGK spark plug catalogs seem to list wrong plugs for 356, 912.
With NGK B7 HS, I have or had no problems since about 20 years.
Engines and complete ignition system original.
The geometric "style" of plug is and must be the same, as for beetle.
Plugs with extended tip can burn holes in pistons. (I heard about some bikers)

Compression, ratio etc.:
it´s most important, the pistons do not touch the heads.
To check, if Is there is something completely wrong, put out all plugs, turn the crank several times and listen and feel, if it knocks.
If it sounds good, put in the right NGK B7HS, or a aequivalent and try to start.
When ideling, listen again for some knocking. The reason for knocking only on left side, can be "tolerance".
With luck, wrong spark plugs are the single reason for your problems.

Sat Feb 01, 2025 7:46 pm

Thankyou I will be checking into this. Thank you so much for your input.

Fri Feb 14, 2025 9:53 am

if the heads have been cut too far then you could have this problem, which means shimming the barrels. Compare your sparkplug length to the proper one a bosch wr6bc and see if it protrudes further