Jeep Strokers

yeah, pin height is definitely where the difference came from. so, assuming i could get to that 9.09:1 compression ratio and using the stock cam specs. what does everyone think about the fuel i'll need to be running? oh, and it's also worth noting that i'm in denver and the one mile altitude lowers dynamic compression as i understand.

I like it flash, what kind of royalties do I get?

riversidefort wrote:yeah, pin height is definitely where the difference came from. so, assuming i could get to that 9.09:1 compression ratio and using the stock cam specs. what does everyone think about the fuel i'll need to be running? oh, and it's also worth noting that i'm in denver and the one mile altitude lowers dynamic compression as i understand.

The 9.09 is staic compression and doesn't take the cam into account. This is why I said it is kind of a useless #. Depending on when the cam closes the intake valve on the compression stroke determines your actual compression or dynamic compression ratio. Static compression ratio is the compression ratio from bottom dead center to top dead center with a closed cylinder (i.e. both valves closed). As far as I know there isn't a cam that actually closes the intake vavle at BDC. So as your piston starts its upstroke, the intake valve is open and your not compressing anything. Once that valve closes you begin to compress and that is your actual compression ratio (dynamic compression), simplified for ease of understanding.

I used KB pistons calculator, but I've been told it is not accurate.

oletshot wrote:I like it flash, what kind of royalties do I get?

Hmmmm.........lets say 10% of the profits i make off of it

riversidefort wrote:yeah, pin height is definitely where the difference came from. so, assuming i could get to that 9.09:1 compression ratio and using the stock cam specs. what does everyone think about the fuel i'll need to be running? oh, and it's also worth noting that i'm in denver and the one mile altitude lowers dynamic compression as i understand.

Sence the 4.0L SCR is 8.8:1 and your using the stock cam.............If the quench hight was the same or less, and the compression ratio was only raised buy 0.29:1......If the 8.8 didn't ping. chances are the 9.09 won't either.

............Ok changing my tune a little..............althow the same cam is being use and similar compression ratio............The DCR will be higher, because of the longer stroke of the 258 crank.

Look like we need to do a DCR for the two.

riversidefort,
if You don't know the difference between SCR a DCR non of this will make any sense.

Flash

yeah, i'm still kinda wrapping my head around some of the more technical aspects of the project, dynamic compression was a new one for me. so, i have the dynamic compression as calculated by the kb-silvolite dynamic compression calculator. the stock 4L comes to 8.055:1 and my plan comes to 8.340:1. not terribly sure i did it correctly though.

I guess this is probably correct thread for my question. I left my 258 crank shaft to machine shop to be ground to dimension for undersize 0.02 bearings (a must) but where can I find information what is recommended journal diameter for those bearings?? Both rod and main.

I figured to take average of manual original dia and minus 0.02" but it leaves too much tolerance for rod bearings. In manual OEM dia 2.4996"-2.5001" (0.0005") and preferred oil clearance 0.001-0.0025" for main bearings this is quite OK, but for rod bearings 2.0934"-2.0955" (0.0021") leaves too much to guess.

Any advice?

Killman wrote:I guess this is probably correct thread for my question. I left my 258 crank shaft to machine shop to be ground to dimension for undersize 0.02 bearings (a must) but where can I find information what is recommended journal diameter for those bearings?? Both rod and main.

I figured to take average of manual original dia and minus 0.02" but it leaves too much tolerance for rod bearings. In manual OEM dia 2.4996"-2.5001" (0.0005") and preferred oil clearance 0.001-0.0025" for main bearings this is quite OK, but for rod bearings 2.0934"-2.0955" (0.0021") leaves too much to guess.

Any advice?

Maybe I'm completely missing you Question but, bearings will come matched to the the crank turn......if they turn the crank mains .020 and the rods .010 (for example) then you would order the bearing with these number........Plastic gauge can be use to check the machine shops accuracy.


Flash.

For clarity, the crank I have in my hands has worn out of OEM specs. By measuring it was found that even 0.01 under size bearing is not enough, but 0.02 is needed. Now, what is the size that journal must be ground to meet 0.02 under + required oil clearance?

Killman wrote:For clarity, the crank I have in my hands has worn out of OEM specs. By measuring it was found that even 0.01 under size bearing is not enough, but 0.02 is needed. Now, what is the size that journal must be ground to meet 0.02 under + required oil clearance?

The bearing come in .020,.030 and .040 ( forty thousands of an inch)

The bearing are all made precise the same........the crank has to be ground to match the bearing(.020" .030" exc)

For and example, if they turn the crank under size for .020" bearing, and you find that there is only .001" of oil clearance,( With plastic gauge) and you want .0025 oil clearance, the crank would have to be polish down .0015 more or the crank would have to become smaller by 1 and 1/2 Thousands of an inch.

The spec that the crank should be ground down to would be .020" smaller then the standard journal diameter.

With in .0005 or 1/2 of one thousandth

Unless your building a High rpm, full on race eng.........Oil clearance of .001 to .003 will work grate.

Just plastic gauge it when there done, to make sure that it is done right.

Flash

Killman wrote:I guess this is probably correct thread for my question. I left my 258 crank shaft to machine shop to be ground to dimension for undersize 0.02 bearings (a must) but where can I find information what is recommended journal diameter for those bearings?? Both rod and main.

I figured to take average of manual original dia and minus 0.02" but it leaves too much tolerance for rod bearings. In manual OEM dia 2.4996"-2.5001" (0.0005") and preferred oil clearance 0.001-0.0025" for main bearings this is quite OK, but for rod bearings 2.0934"-2.0955" (0.0021") leaves too much to guess.

Any advice?

Factory Service Manual specs for the crankshaft

Main Bearing Journal
Bearings #1 through #76 63.489 - 63.502 mm or 2.4996 - 2.501inch diameter
Bearing #7 63.449 - 63.487 mm or 2.4980 - 2.4995 inch diameter
Bearing clearance #1 - #7 .03-.06mm or .001-.0025 inch
Preferred clearance 51 mm or .002 inch

Rod Journal
53.17 - 53.23 mm or 2.0934 - 2.0955 inch diameter
Bearing clearance .025 - .076 mm or .001 - .003 inch
Preferred clearance .044 - .050 mm or .015 - .0020 inch

Rod bearings and main bearings are two different boxed set of bearings, You might have a crank with the mains .020 under and rod journals .010 under etc. A machine grinder is looking at factory size or turning smaller by .010 increments until the fit falls within the clearance range.
John

Thanks for comments, actually I know now right question:

What is standard dimension of journal from which to deduct 0.02 in + oil clearance?
It would be perfect if bearing manufacturer includes in a bearing package for which journal dia they have been manufactured.

Correct, the clearance must be checked with plastigauge, but it is time consuming and expensive to grind it, test and grind again.