360 or 390?
Post Fiend
Joined: Sep 2004
Posts: 5,231
Likes: 1
From: Near Fresno, California
That question has been asked a millon times - the only way is to measure the stroke. Remove a spark plug and insert a drinking straw into the cylinder. Rotate the engine until it bottoms and mark the straw against some convenient point. Then rotate the engine until the piston is at the top, mark the straw again. The differance between the marks is your stroke.
If it is 3.5" it is a 360, 3.75" is a 390.
If the engine is stock you can also tell by decoding the VIN number.
If it is 3.5" it is a 360, 3.75" is a 390.
If the engine is stock you can also tell by decoding the VIN number.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
dehamma Put your money on the 360. There were a few 390s installed but about 99% of them are 360s.
***** has a good suggestion, I think it would be adding time to the job trying to get the bottom and top. I usually recommend have a friend rotate the engine while you hold a Sharpie against what you are using in the hole and just mark the whole time on a couple revs. Either way you measure the length of the mark. Whatever you use the smaller it is the better. I perfer a piece of white 14-2 house wire with a slight curve in it. It is both small and ridig.
You may have to do it more than once, but you will find a method that works for you.
John
***** has a good suggestion, I think it would be adding time to the job trying to get the bottom and top. I usually recommend have a friend rotate the engine while you hold a Sharpie against what you are using in the hole and just mark the whole time on a couple revs. Either way you measure the length of the mark. Whatever you use the smaller it is the better. I perfer a piece of white 14-2 house wire with a slight curve in it. It is both small and ridig.
You may have to do it more than once, but you will find a method that works for you.
John
Post Fiend
Joined: Sep 2004
Posts: 5,231
Likes: 1
From: Near Fresno, California
Same here - thanks Jowilker, I learned something today. The next time I have my spark plugs out and help available I will give your methoid a try, sort of pound it into my head. If I've actually done it I am more apt to remember the idea.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Thanks gents, I tried the 1/4" wooden dowel and it was a pita. The wire is smaller, fairly easy to come by and simply stated works the best, at least for me. 
I have seen some heated arguments settled with it. A lot of guys are in denial about the 360.
John
I have seen some heated arguments settled with it. A lot of guys are in denial about the 360.
John
Laughing Gas
Joined: Mar 2000
Posts: 1,191
Likes: 0
From: USA
I loved the 360 in my '70 F100 when I owned it... great engine... however if I had to rebuild the engine I would go with the 390... no replacement for displacement
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Fleet Owner
Joined: Jan 2004
Posts: 26,007
Likes: 575
From: Brentwood,CA
also by now the enigne is proably not stock. but the 360 was common but a 390 crank and rod swap was easy.
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