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Spark plugs will not go in....

64SF

64SF

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This is a weird one...in final assembly of engine and tried to install the plugs to keep things clean inside. None of them will go in... neither used ones or new ones. The heads are 452 castings. A plug thread chaser goes in fine, a compression tester fitting goes in fine but not the plugs. Tried the old champions and a new set of NGK's. Any thoughts, comments or suggestions would be appreciated.
 
Between the thread chaser, and the plugs, compare thread per inch, and outside diameters.
Only thing I can think of.
 
By "old champions" do you mean ones that were installed in the heads before?


Also, are they not threading or just getting tight as they go in?
 
Sure you have the same tap and spark threads? If you can get a tap in, you should be able to fit the spark in. Inspect the diameters of the holes and the sparks, and inspect the threads.
 
Champion RJ12yC and NGK XR5's. Both (as I read it) are for the application. Using a 14mm x 125 thread chaser and picked up a 14mm x 125 tap. The tread chaser and tap have cleaned the threads nicely....it's the darnedest thing I've seen. I bought the heads and installed new valves, springs, seats and guides. The heads are installed I may have to pull them off to hopefully fix the problem.

Can the head gaskets be reused after installation and torqued to specs?
 
The Champion RC12YC has a thread diameter of 14mm and 1.25mm pitch like you are using. There is a difference between a tap and a chaser. You need to use the chaser not necessarily the tap. I've never had the problem before, but researching this you might need a Back-Tap Thread Repair Tool to straighten them out.

https://www.amazon.com/Powerbuilt-6...r/dp/B0014FGVVW?tag=vs-auto-convert-amazon-20

I found this thread on the subject. Even noticed a famous member there!

http://www.lightningrodder.com/foru...spark-plug-holes.html#/topics/72671?_k=bcw9me
 
OK...found the problem.

There is a small nub/hump right below the bottom of the spark plug hole. The bottom shoulder of the plug is larger than the space allowed for the plug.

I took an old plug and ground it down small enough to clear the nub and it goes in without a problem.

As I mentioned in my earlier post this is a set of 452 castings...guess I'll grind off the nub/hump. Anyone else encountered this problem before?

Thanks all for your responses.



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It appears that you have the motorhome 452 heads that use the Chevy style taper seat spark plugs with the 5/8 nut on them and not the 13/16 nut gasket style plugs.
 
IQ52 said:
It appears that you have the motorhome 452 heads that use the Chevy style taper seat spark plugs with the 5/8 nut on them and not the 13/16 nut gasket style plugs.
Click to expand...
Could very well be...I'm going to pull the full casting number for a double check. You happen to know what the plug number is for this application. If not I can research.
Question: Does the motorhome head have water passage holes around the combustion chamber?
Thanks
 
This is my 452 head. Plug diameter on the end is .795 almost 13/16. Your plug hole has a taper, my plug hole has a flat shoulder.

20161113_200821.jpg
20161113_201900.jpg
 
yup, those are tapered seat heads, for the smaller 5/8" plug....also known as "motorhome heads"

the 13/16" plug will not work in them
 
Yep.....looks like they are Motorhome heads. Thanks for all the replys it's been very helpful!
 
The plugs you need are called peanut plugs same crap as a chevy small block.
 
Interesting...glad you found out what's going on.

For my own information, what's the advance/disadvantage of the motorhome head? Is everything else the same (chambers, ports, etc)? Thanks.
 
the motorhome head has extra cooling passages in the head, located between the cylinders....they are just small holes....everything else is teh same
 
What years were those 452 heads used? I have not had that problem with 76 heads.
 
I have 2 1976 440 engines both have 452 heads, one engine uses regular plugs and 1 uses the small plugs
 
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