what are the mechanics of cam break in?

[bikedude189]

so i'm interested to know exactly what goes on when the cam is being broken in. i know i need to keep the engine at 2k for the first 20 minutes, but i don't know why. could someone please explain, in as much detail as possible, what goes on in that first 20 minutes?

on another note, what is the purpose of torque to yield bolts like in the head?

[SilverXJ]

The whole cam break in is bs. They just want you to but a special oil, use their special lube, etc. All you really need to do is start it up and drive.

Just kidding....

What is happening when the cam is being broken in is that the lifter is being married to the cam lobe. The cam lobe will form a pattern on the lifter. Basically, wearing in. The reason to use high ZDDP oil is to protect them during this process. The reason for the 2000-2500 RPM is so the crank shaft throws around plenty of oil to get to the cam shaft.

I'm still a little foggy on the whole stock head bolt issue. They aren't true TTY bolts, but they say to only re use them twice. I don't know.

[woodjeep]

Like Silver XJ says..the high rpm is so the crank can throw enough oil on the cam to keep it well lubricated during the mating of the lifters and the lobes. I did not fully know the answer to your 2nd part so I did a search and got this fairly non complex answer. There is apparently an entire science behind bolt mechanics

http://www.enginebuildermag.com/Article ... angle.aspx

Basically a bolt offers no force to the parts it is joining until it begins to stretch. Better bolts can stretch more..thus more force. However they measure it..and I don't know how..torquing to the yield point is more accurate than torquing to a ft lb on a wrench due to the friction involved in turning the bolt. I guess for most applications normal torquing is good enough and special applications need something better. They say you can NEVER use a torque to yield bolt over again .

[amcinstaller]

i wouldnt trust a torque to yield or torque to angle bolt more than once. if its already stretched, you might be just stretching it more, or it could be more inclined to over stretch the second or third time and cause head gasket failure early. besides head bolts are cheap enough, its like cheap insurance, no one wants to do something twice, or three times like silver has had to. but in all fairness, he has bad luck im thinking.

[IH 392]

On the cam I too believe that they just want LOTS! of oil flying around in there? use lots of the recommended additives, I use GOBS! of Molly HT #2.

The Jeep engines DONOT use "torque to yield" bolts any where, they are simply saying they don't recommend using the head bolt more than twice, if you buy into this you should NEVER use ANY! bolt more than once or twice!, a true "torque to yield" bolt will NEVER!!!! take a second stretch, it will pull apart long before it ever tightens!, if you get a gasket set for a TRUE torque to yield engine the gasket set WILL include NEW head bolts!

[SilverXJ]

The Jeep engines DONOT use "torque to yield" bolts any where, they are simply saying they don't recommend using the head bolt more than twice, if you buy into this you should NEVER use ANY!

Not disagreeing with you as I don't know if they can or can't be reused, but what are you basing this on?

[IH 392]

I don't know how many times I've read on here that the FSM says to only use the head bolts twice???, personally I've never replace the head bolts in any AMC, Chevy, Ford or Chrysler with iron cylinder heads, if you've ever done a Ford Escort/Focus engine, for example, you get new bolts with the gasket set, you can't even get half way thrue the torque sequence before you start snapping the bolts in half! I've seen it done, don't even know where they found a head set for that engine with out the bolts!??? and they have STUPID torque sequences too, the Escort is 44ftlbs, back them off, 44ftlbs, 90*, 90* each step in the recommended sequence, this a typical scenario for a torque to yield engine, never seen anything like this for an AMC/Jeep???
Here's some explanations.
http://www.acl.co.nz/Tech/Torque%20To%2 ... dbolts.pdf
http://www.enginebuildermag.com/Article ... angle.aspx

I guess a more direct basis of what I'm basing this on is the five years I worked at the machine shop, two as a cylinder head machinist and the remaining three as an assembler, putting together anything that burned Gasoline, LP/Propane or Diesel, three to five engines a week give or take a few that was about 600 engines!, working in a production setting you get them right or you loose the business and gain a bad reputation!

If you guys think you need new head bolts, go ahead and replace them.

[SilverXJ]

If you guys think you need new head bolts, go ahead and replace them.

I never thought I needed them but the FSM say only to reuse them twice, so I was erring on the side of caution. I have heard maybe two other people say they are not TTY bolts, which makes sense... especially given their size compared to other head bolts.. The few times that I have reused them I have had no problems.

[bikedude189]

COOL! those are really interseting articles/explanations.

btw, SHE LIVES!!! i drove my new stroker for the first time today... project report coming soon.