Thread: QR25 Bearing info

I wrote this up a long time ago when I did the 07 rod swap. posted by request. Someone lookup and post the charts from the b16 FSM for b16 bearing selection.

I've been getting a lot of questions on this, and I wanted to archive the info:

Engine bearings are not typical ball, or pin type bearing. They are sleeve bearings, and the bearing itself is just a precision way to control the thickness of the layer of oil between the the various components. There is also a slight arch in the bearing itself to control the way in which the rod floats. The angle and height of this incline along the circle can be tuned for lower resistance (and less life), but selection too extreme of a bearing can cause knock especially at low-mid rpm as the crank slaps against the bearing surface. Some race cars idle at 4k rpms to avoid this engine damage.

If you are using OEM bearings which are bi-metal copper/aluminum bearings (high life bearings for low-mid power) then you're job is much easier as you only need to know the size. Unfortunately, if your crank is remanufactured (resurfaced journals) then you won't be able to use OEM bearings. Common practice for a remanufactured engine supplier is to document how much size was taken off (either by restamping the back of the counterweight or by keeping records at thier office or both..) A quick call to them can help you determine bearing sizes.

If the rod journals on the crank are "-10" or "-20" (ie, -.01" or .02" diameter then your job is more difficult. You will need to order the rods, order the right size bearings (you should only have to ask for "-20" bearings, or whatever amount was taken off.) Then you have to use plastigauges to verify the bearings have .015" to .025" gap. If the gap is too small (unlikely) then have them shaved. If the gap is too large (highly unlikely) then order new bigger bearings.

It's best to just have the journals measured with a micrometer anyway to be sure they are perfectly round (not oval) to allow for longer bearing life.

b16 bearings are the same design as b15 bearings except that they come in more sizes, allowing a more perfect fit.

Rod bearing selection is not complicated.

There are codes stamped on the rods, block, and crank. These can be used to reference a chart in the FSM to determine the sizes you have (ID on rods and block, and OD on the crank.)

If the engine is out of the car, simply remove the cradle to examine the crank counterweight and reference the b15 FSM to decode the bearing sizes. To do main bearings, you need the code from the block and the code from the crank (because those are what the bearing is between.) To decode rod bearings, obviously you will need the code from the NEW rods, and the crank to figure out what size to get.

If the engine isn't out of the car you must use a micrometer because you will not be able to read the stamped-in crank codes for the crank journal sizes.

If the engine is remanufactured (they usually turn the crank rather than buy a new one) then you will need to measure the journal sizes with a high end micrometer. I didn't replace the main bearings, only the crank bearings. (at first. later I did all the bearings when I built the whole block.)

-JNY

Originally Posted by pimpsmurf

Then you have to use plastigauges to verify the bearings have .015" to .025" gap. If the gap is too small (unlikely) then have them shaved. If the gap is too large (highly unlikely) then order new bigger bearings.

-JNY

??



did you mean .0015" to .0025" ?