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.030
An Autolite 258 is the recommended spark plug for a 6. 5 horsepower Tecumseh engine. A Champion RJ19LM also is recommended by the manufacturer.
My book for my John Deere snowblower , which has a 5 horsepower Tecumseh engine , shows to use the Champion RJ17LM spark plug
The Answer to your question RJ19LM spark plug set at 20 thousand of an inch.
Champion J8C
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Set the spark plug gap to .030"(.762mm).
Autolite 258
.030 inch ( on my 24 year old 5.0 horsepower Tecumseh engine )
J19lm
If that is a 5 horsepower Tecumseh engine - ( .030 inch )
Champion RN4C - Gap .030"