Hello all,

I'm currently building a Benson stock bore block with a k24 99mm crank, Wiseco 87mm 9.5comp pistons and matching K1 rods (for k24 conversion) and have a few questions you experienced engine builders might know the answers to. In the process of obtaining the correct rod oil clearances I've run into an issue. While using the +0.001 ACL bearings (4B1972HX) I get oil clearances that are too large:

1 0.0026
2 0.0028
3 0.0026
4 0.0027

I say they are too large because in using the formula Big.end.dia - Journal.dia - 2xBearing.Wall.Crown = Oil Clearance, I should theoretically obtain a clearance of 0.0017". Big.end.dia = 2.008", Journal.dia = 1.8893, Bearing.wall.crown = 0.0585", or 2.008 - 1.8893 - 2*0.0585 = 0.0017. I can live with 0.0017 on a Turbo engine, but 0.0026-0.0028 is too much from what most engine builders recommend.

Have any of you run into this discrepancy on any of your engine builds? Are ACL bearings notorious on bad quality control? Am I using the incorrect method to calculate the oil clearance?
Thanks in advance for your insight.

 

Nobody has had clearance issues with ACL bearings?

 

On the bench, install the rod bearing halves on the rod big end and on the rod cap. Assemble cap to rod with bearings installed, then torque the rod bolts to spec. Just as you normally would assemble it, only w/o installing the rod on the crank. Next, use an inside micrometer to measure the installed bearing ID at several "clock" positions to get an average. They are usually slightly oval, wider at cap parting line than they are tall across it. Then us an outside micrometer to measure the OD of that rod's corresponding crank journal at several positions along the journal's axis, to get an average. The journal should not have much taper along its axis, so the readings should be pretty consistent.

Subtract the average journal OD from the average bearing ID to get diametral clearance (since its the difference between two diameters). Radial clearance is half of diametral clearance. Finally, you must find out if that 0.0017 clearance spec that "they" suggest is referring to diametral or radial clearance; and whether or not it is the clearance number you really want to run. I would get the bearing manufacturer's recommendation on clearance for your application. If for no other reason than to check it against your 0.0017 recommendation. They should at least be in the same ballpark (just remember the distiction between radial and diametral).

If you don't have calibrated inner and outer micrometers of the proper size to get accurate readings, you can use "Plastigauge" to get a pretty accurate clearance measurement. It's much cheaper than good micrometers, but not as accurate. You can Google "Plastigauge" to find manuals and info online, and probably even vidoes on Youtube of how to use it.

 

I use a dial bore gauge which I zero at the crankshaft journal bearing diameter using an outside micrometer. The rod bolts are stretched to 0.006" as per manufacturers instructions. The measurements (vertical oil clearance) are put in the formula I described above, which is consistent with those found in blueprinting books and Laskey's website. The oil clearances are diametral.
Unfortunately the ACL rod bearings are in back order and I can't test a set in the standard size.

 

What is wrong with oem? Get the color that gives you the correct clearance.

 

I have had clearance problems with ACL bearings. Depending on your crank journal size it may not be possible to get correct clearances with the ACL's. Your crank, rods and block should have bearing bore and journal codes marked somewhere. Depending on the bearing codes, the manual recomends the bearings you should use. Honda stock bearings come in I think 24 different combinations to get clearances correct. I ended up using them instead of the ACL's to get proper clearances. Also the Honda manual recommends using plastigage for both rod and main bearing clearance. I used plastigage, it works fine.

 

I'm looking at doing a stroker mind to give me a run down on set up

Sent from my DROID RAZR using Tapatalk 2

 

Based on the math alone, the Honda bearings will not give enough clearance. I'm looking for 0.001" per 1" of journal diameter, which will be at around 0.0019" - 0.002". The largest Honda bearing, the Red bearing, will only give anywhere from 0.00122" to 0.0015" according to the math.

 

Thanks for the suggestion. Keep in mind I'm not using the manual's suggested bearing clearance since my engine will be turbocharged and developing 3X more power than stock. I used Honda bearings for the mains and was able to obtain acceptable clearances following the method I described in the opening post. I had to guess the bearing thickness by inferring the Green bearing is equal to the ACL standard bearing thickness as no one has posted a bearing thickness chart for F2X engines.

 

The thing that keeps bothering me is that the formula you're using to calculate clearance has no way to account for the slight interference fit of the bearing halves once they are installed in their bore and the bolts are torqued down. The OD of two bearing halves together is greater by a thou or so than the ID of the bore they fit in. This slight interference fit causes the bearings to crush when the bolts are torqued down. This exerts enough outward radial pressure (and therefore friction) to prevent the bearings from spinning in their bores. This interference MUST have some affect on the actual bearing ID, and I can only imagine it making the clearance smaller.

I personally would not live with that uncertainty. Especially when it is so easy to verify the clearance by either of the two time-proven methods I mentioned above. You are already going to the trouble of torquing the bolts in order to measure the ID of the bores. Just install the bearings before you do that step, and avoid any uncertainty. You don't have to replace the measured set of bearings once they've been crushed. As soon as you loosen the bolts, they spring back. Be careful not to scratch the bearing babbit surface when you're measuring them. But that goes for the thickness measurement you had to do using your method as well. If you are just using the published bearing thickness from the mfr website, then that is another potential uncertainty as well.