A Better Crankcase Seal

[Jim Allen]

I have used the very low drag type seals shown in the photos for several seasons. The RCMK stroked crankshafts used in these seals have been "carefully trued". (.0002" total indicator run out end to end) The precision ground shaft shown (.4722") in the photo not only checks the alignment of the four bearings used & therefore the crankcase bearing bores, but it also aligns the final boring of the bored seat in the seal mount on both ends of the crankcase. This also tells me that the two locating pins are located correctly. The final boring of the seal mount is done in the assembled crankcase. The seals are standard steel backed lead teflon bushings that are readily available & easily replaced if worn.

Keep in mind that the nominal ID of any bearing's inner race is always .0001" or .0002" smaller than the listed size. For example, a 12 mm (.4724) precision ground shaft WILL NOT pass through a 12 mm X 28 mm bearing. Also the OD of the bearing's outer race is always .0001 or .0002" larger than the listed size.

Jim Allen

 

[Daniel's Racing]

Keep in mind that the nominal ID of any bearing's inner race is always .0001" or .0002" smaller than the listed size. For example, a 12 mm (.4724) precision ground shaft WILL NOT pass through a 12 mm X 28 mm bearing. Also the OD of the bearing's outer race is always .0001 or .0002" larger than the listed size.

Jim Allen

Jim,

12 X 28 bearings will usually fit and slide on a 12mm hardened shaft. If they don't one or the other is out of tolerance and tightening the fit.

The bearings would be pretty worthless if they were 12 mm but a 12mm shaft doesn't fit in them. What bearings would people run on a 12 mm shaft ?

Open the I.D.'s on all the bearings ? Cut some O.D. off the shafts ? I don't think so.

12mm I.D. bearings absolutely slide right on a 12mm hardened shaft.

If that was the case across the board it would be pretty difficult to build anything using shafts and bearings if none of the bearings fit on the same size as the shafts.

When a given size bearing doesn't fit a similarly sized shaft it is because either the bearing was made undersize or the shaft was over or little of both but they normally are a compatible slip fit.

6 brand new high quality phenolic retainer 12 X 28's bearings slid on a 12 mm hardened shaft.

 

[John Beardslee]

Hi Daniel,

I think Jim's point is that 2 objects cannot occupy the same place at the same time ie: 12mm shaft dia. will not go in a 12mm I.D. bearing. One or the other may be 12mm nominal diameter but when accurate precision measurement is taken, there must be a clearance created some how or way. Slip fit, shrink fit etc.

Thanks, John

 

[Daniel's Racing]

I think most realize that if something is larger than the hole it is going in something has to give.

I was just pointing out that his exact example he used fit's just fine. A bearing of a given advertised size normally does fit the shafts of the same advertised diameters. That is the idea I think so they are compatible and can be used together. Most shafts need to run in bearings so it makes it a bit easier if they fit each other as delivered.

I just happen to know that the 12 mm X 28 mm bearings fit shafts that are full 12 mm diameter as I make a bearing alignment/installation tool that uses a 12 mm hardened shaft to go through the bearings then a machined collar pushes on the outer races to install the bearings in the cases while the cases are assembled so the outer races are dead true to the other on the other side. Bearings installed with this tool allow the crank to spin true in the ball bearings so all the balls are contacting the same throughout the entire 360° of rotation.

If either bearing is merrily just installed in the case halves individually while they are apart there is no way of getting the bearing true and facing each other exactly so the bearing will fail prematurely as the bearings are in a partial bind and are not running true with all the balls true and square in their grooves. Just slamming the bearings home individually in heated cases is the best way to insure you have installed a pair of misaligned bearings once they are assembled they will almost never be exactly true to each other. The difference doing them that way and with the alignment tool is night and day.

Put four bearings in a set of cases without the tool and when you be done a 12mm alignment shaft won't even slide freely through the I.D. of the bearings. Install the bearings with the tool and the alignment shaft glides freely through the bearings. Imagine how out of true they must be for that to even be possible. Then think about that spinning at 18,000 rpm with the force of the prop thrust pushing on a single bearing just in front of the crank. If that bearing isn't true all the balls will not make contact evenly and it just burns it up.

Sold several hundred of these tools so they must be fitting every ones 12 mm X 28 mm bearings.

 

[Jim Allen]

You are correct John. What most individuals do not know about any standard radial bearing is the two things that I stated in my post. The precision ground harden M-2 steel shaft shown in the photo actually was ground with two different diameters. The shaft was ground, between centers, on our Kellenburger OD, ID grinder. The end that passes through any 12 mm X 28 mm bearing is .4721" & the end that will not pass through is .4724". I use a micrometer that reads in .00005", mounted in a micrometer stand, to make an accurate measurement!

If anyone is interested in how we check the radial play amount, inner race roundness or outer race roundness of special order high clearance radial bearings, I would be happy to show the special fixture required to do this.

Jim Allen

 

[Jim Allen]

Here are some photos of crankshafts ground with a .0001" step to ensure that they are pressed into the inner race of their bearings. They are then shrunk into the engine's crankcase. (.0003" to .0005" shrink fit) The press & the simple tooling shown makes this process repeatable! This can only be done with special order high clearance bearings. It should be noted that the inner & outer races of radial ball bearings were not designed to support themselves, therefore, the press fitting & shrink fitting of both is necessary for bearing performance.

Jim Allen