Piston Head Thickness

Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic

   Here is a quote from my CJ2A Service Manual, page 33:
"NOTE: Effective with engine S/N 50705, the piston head thickness was increased from 3/16" to 1/4" to provide greater strength and heat conductivity.  When replacing pistons singly, or in sets, procure the correct parts or engine vibration will result from unbalanced pistons."

When I took my motor apart, I found three thick head pistons, and one thin head piston.  The thick head pistons are 1/16" taller than the thin head ones if you place them side by side on a flat surface.  The difference is noticeable in the width of the top piston groove, which is 1/16" wider.  This must result in a difference in compression.

My question is why can't I find any new late style thick head, high compression pistons?  All the pistons I see for sale everywhere are the early style, thin head pistons.

Edited by Willy Nilly - 06 Oct. 2022 at 12:25am

Author	Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic
Willy Nilly Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 23 Aug. 2022 Location: San Diego Status: Offline Points: 106	Post Options Post Reply Quote Willy Nilly Report Post Thanks(0) Quote Reply Topic: Piston Head Thickness Posted: 06 Oct. 2022 at 12:24am
	Here is a quote from my CJ2A Service Manual, page 33: "NOTE: Effective with engine S/N 50705, the piston head thickness was increased from 3/16" to 1/4" to provide greater strength and heat conductivity. When replacing pistons singly, or in sets, procure the correct parts or engine vibration will result from unbalanced pistons." When I took my motor apart, I found three thick head pistons, and one thin head piston. The thick head pistons are 1/16" taller than the thin head ones if you place them side by side on a flat surface. The difference is noticeable in the width of the top piston groove, which is 1/16" wider. This must result in a difference in compression. My question is why can't I find any new late style thick head, high compression pistons? All the pistons I see for sale everywhere are the early style, thin head pistons. Edited by Willy Nilly - 06 Oct. 2022 at 12:25am
	1946 Willys CJ2A military clone

cpt logger Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 23 Sep. 2012 Location: Western Colorad Status: Offline Points: 3040	Post Options Post Reply Quote cpt logger Report Post Thanks(0) Quote Reply Posted: 06 Oct. 2022 at 2:25am
	Are the taller pistons actually the thick head pistons? IIRC, they are the thinner head pistons. It has been decades since I had one of each to compare the piston head thickness, so I may be wrong about that. You have them both so figure out how to measure the piston head thickness. I seem to remember using a drill press, a brass shaft & a machinists ruler. IIRC the difference is about 1/8".

Joe Friday Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Sponsor Member x 2 Joined: 26 Dec. 2010 Location: Jeep Central Status: Offline Points: 3654	Post Options Post Reply Quote Joe Friday Report Post Thanks(1) Quote Reply Posted: 06 Oct. 2022 at 7:47am
	The head is thicker, the distance from the center of the pin is the same. They have the same compression ratio.

Oldpappy Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 09 Apr. 2018 Location: Tennessee Status: Offline Points: 4903	Post Options Post Reply Quote Oldpappy Report Post Thanks(1) Quote Reply Posted: 06 Oct. 2022 at 8:47am
	Having a mis-matched set as you describe is exactly what the service manual quote in your post warns us about. As Joe points out, both the early and later pistons are the same compression ratio. I always use new pistons when I rebuild an engine, and in your case if you plan to rebuild that engine you will need a new set of quality pistons. Avoid the imported crap a lot of suppliers sell. Classic Military Vehicles has Silvolite pistons for $100 a set, and has Hastings Rings for $26 a set. These are IMHO as good of quality as you can get. http://jeep1942.com/?page_id=7504 Edited by Oldpappy - 06 Oct. 2022 at 8:48am
	If you can't get there in a Jeep you don't need to be there!

oldtime Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 12 Sep. 2009 Location: Missouri Status: Offline Points: 4183	Post Options Post Reply Quote oldtime Report Post Thanks(0) Quote Reply Posted: 06 Oct. 2022 at 9:39am
	I certainly agree with Oldpappy. Go with Silvolite and Hastings every time. AFAIK Willys started using these heavier pistons because of the F134 engines. Likely since the the F134 was put into production in 1950. Since 1952 M38-A1 and 1953 3B production the skirt thickness was also increased. On my final 134 engine build I used a set of Extra Heavy OEM NOS pistons from W/O.
	Currently building my final F-134 powered 3B . T98-A Rock Crawler using exclusive factory parts and Approved Special Equipment from the Willys Motors era (1953-1963) Zero aftermarket parts

Willy Nilly Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 23 Aug. 2022 Location: San Diego Status: Offline Points: 106	Post Options Post Reply Quote Willy Nilly Report Post Thanks(0) Quote Reply Posted: 06 Oct. 2022 at 1:01pm
	I measured the distance from the piston pin to the top of the pistons, and indeed both piston variations are the same, and therefore would be the same compression ratio. So that's good to know. I don't know why I didn't think of that. I am doing a complete rebuild, and of course a new piston set.
	1946 Willys CJ2A military clone