I Need some help?

[mrbeerbaitnammo]

ok, i have a predicament, I am Stationed in Germany, i want to bore my engine 30 over, there is a machine shop here that will do it, i just have to give them a good measurement, in mm's, if i can get the diameter in inches i can convert, now i want to see if i am atleast in the ballpark with what i came up with. for a Stock 242 4.0L HO the standard bore is 3.875 inches now to go 30 over i would add .030 inches, which would be 3.905 inches, in the Jeep Engines book 3rd edition it says to have .0012-.0013 piston to bore clearance, would i add that to 3.905? i was told you can not bore this engine more than 40 over, how are people boring 60 over with their stroker? and the other confusing part is places i look for new pistons will say bore 3.875 for a 30 over piston, can anyone explain please. This Soldier need some serious help...badly

[Gary4]

Standard cyl. bore= 98.45 to 98.48mm or 3.8759 to 3.8775in.
Piston-to-bore clearance= .018 to.038mm
If your boring .030" over the cylinder will have to be finished honed for correct piston-to-bore clearance. Mic the new pistons (if not coated pistons),and mic the cylinder bore with a cyl. bore guage to get the correct piston clearance. The manufacture of the piston should be able to give you the correct clearance for their piston.

[mrbeerbaitnammo]

so i should hae the new pistons to take to the machine shop as well and the cylinders will be bored to those pistons?

[OhioYJ]

so i should hae the new pistons to take to the machine shop as well and the cylinders will be bored to those pistonsYes they should match the cylinders to the pistons.

[mrbeerbaitnammo]

one more question, are pistons for all 4.0L 242's compatible or do i have to get my year for pistons?

[StealthTJ]

Originally posted by mrbeerbaitnammo:
ok, i have a predicament, I am Stationed in Germany, i want to bore my engine 30 over, there is a machine shop here that will do it, i just have to give them a good measurement, in mm's, if i can get the diameter in inches i can convert, now i want to see if i am atleast in the ballpark with what i came up with. for a Stock 242 4.0L HO the standard bore is 3.875 inches now to go 30 over i would add .030 inches, which would be 3.905 inches, in the Jeep Engines book 3rd edition it says to have .0012-.0013 piston to bore clearance, would i add that to 3.905? i was told you can not bore this engine more than 40 over, how are people boring 60 over with their stroker? and the other confusing part is places i look for new pistons will say bore 3.875 for a 30 over piston, can anyone explain please. This Soldier need some serious help...badly The problem with boring over 40 (actually over maybe even 30) is the fact that you loose too much metal mass and it's metal mass that's required as a heat sink to cool the engine.
People who bore 60 over (and they're several that I know {not Jeep engines}) usually have a hell of a time keeping the engine cooled.


Fred

[OhioYJ]

People who bore 60 over (and they're several that I know {not Jeep engines}) usually have a hell of a time keeping the engine cooled.I think this depends greatly on the block, some blocks are thicker than others, the FE block in my dads Cougar is bored .110" over, with no problems, but its super thick everywhere.

[mrbeerbaitnammo]

i was just wondering because i have heard a million different things, but i think i am gonna go with 30 over, seems to be a happy medium, i was looking at my block, its pretty darn thick, i guess i lucked out, its from a 1991 yj, anyone reccomend a good piston thar aint too bank breaking?

[Slowride]

What kind of machine shop is this? Any good machine shop that knows what they are doing should know all of this and thus not have you do the measurement legwork for them. I'd find another place to do the work. I know you may be limited shopwise over there, but machine work isn't something you want to do twice if you know what I mean. As for boring the block, I'd stay with standard sizing like .030 rather than a "odd ball" .040. I'd also be real weary of going .060 over right away because that leaves you pretty much maxed out on cylinder bore and thus no more "room" to bore out the cylinders for a future rebuild.

[mrbeerbaitnammo]

very true about future rebuild, its a machine shop with a good reputation with the soldiers over here, well for those that know about it, i was just told to tell them exactally what i wanted, i do have 2 last questions where i am confused, do i get pistons that say 3.875 bore 30 over or do i get pistons that say 3.905 ? and last question whats the difference between metric and inch piston rings? i really appreciate all the help from everyone, thanks for the patience this is my first jeep and first engine rebuild by myself

[Gary4]

Take the measurements I posted above and tell them these are standard bore measurements. Now tell them you want a .030" oversize piston.