Wrist Pin Materials

[Tom Kelly]

After having some problems with wrist pins, I would like to make my own. I am considering using M-2, but could use S-7 as well. Any input on this topic?

 

[Martin Hamilton]

Speak with Jim Allen on this forum, im sure he will advise you.

 

[moparbarn]

what martin said .

 

[Jim Allen]

After having some problems with wrist pins, I would like to make my own. I am considering using M-2, but could use S-7 as well. Any input on this topic?

M-2 or M-42 hardened & ground blanks, -.0000" to +.0002" or +.0000" to -.0002", can easily be obtained in wire, fractional & metric sizes from William T. Hutchinson Co. M-2 wil be 62 to 64 Rc & M-42 will be 66 to 68 Rc. The blanks can be obtained in different lengths & the harder material is more expensive. Wrist pins should be solid in the middle & drilled from both ends. Both materials can easily be drilled with a sharp four faceted C-2 grade carbide drill.

Wrist pins should be machined with a chamfer on ends that make contact with wire circle clips. Total end play should be held to .005"

Jim Allen

Note: AISI-S-7 is an excellent tool steel that I use for very high performance connecting rods in my .90 cuin & 26 cc engines. (RPM's to 42, 000 in the .90 cuin & 28,000 in the 26cc gas) When S-7 is used for the connecting rod, there is no bushing used. The M-2 wrist pin material runs directly in the ground S-7 connecting rod's upper hole. Typical clearance of .0002"- between the rod & wrist pin with 8% oil.

 

[Tom Kelly]

Thanks for the information. The stock wrist pin is hollow on one end and solid on the other. I normally put the solid end toward the exhaust port to block off exhaust from entering the wrist pin. If I drill in from both ends, the exhaust could enter halfway. Would that cause any poblems?

I make the carbide drills from end mill shanks. The Niagra brand carbide seems to work the best.

 

[Jim Allen]

Thanks for the information. The stock wrist pin is hollow on one end and solid on the other. I normally put the solid end toward the exhaust port to block off exhaust from entering the wrist pin. If I drill in from both ends, the exhaust could enter halfway. Would that cause any poblems?

I make the carbide drills from end mill shanks. The Niagra brand carbide seems to work the best.

Tom,

The reason that I drill the wrist pin from both ends is so that the solid part in the middle sits at the edges of the wrist pin bosses. This places the most rigid part of the pin in the middle & prevents any flexing of the pin. Wrist pin flexing has been found in larger size two strokes to cause the piston boss area to bulge outward. I am not positive this means anything in smaller size, under 50 cc, engines.

Pluging or blocking the wrist pin hole showed no HP increase or decrease providing there is no communication between the exhaust and rear ports.

Jim Allen

 

[Tom Kelly]

I will let you know how it turns out.