Steel main cap key way.....

[Exminiman]

Looking at Swifty and MED steel main caps, one significant difference is the thrust locating keyway.

Swifty one is very similar to manufacturers OE cap, with the key way not going all way down and also machined at a deliberate angle.

MED cap, and all other after market caps I have seen have a straight keyway from top to bottom.

Obviously the MED Type key way is easier and cheaper to machine, but the key way in the OE cap is so deliberate, its difficult not to think it is like this for a reason...

I wondered if the key way in the OE Cap was machined to reduce or stop hydraulic oil pressure getting behind the thrust and reducing oil film between bearing and bearing surface on crank journal....

Any one got any ideas ?

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[imack]

Interestingly standard A series and A+ ones are machined as a slot.

[Exminiman]

The plot thickens ......

Maybe it was seen as a desirable early on (for what ever reasons) and then later, changed to save money on machining operations....?

It was obliviously a change, I just wonder why....

[Hipwell]

Wouldn’t be too concerned, can’t imagine there would be a pressure build up there with the grooves in the thrust faces. It’s debatable if much pressure exists there anyway.

[andy1071]

The keyway slot is below the level of the thrust face because of cost..
It guarantees the slot is not higher than the thrust-face... in production it is machined as a separate operation, so it would be time-consuming/difficult to machine it at exactly the same height as the thrust-face, and if it is higher it will hold the thrust-bearing off its face
And everyone else just copies the original...

The Swiftune one doesn’t go all the way down because he understands that the corners at the bearing journal will be stress-raisers (where a crack can start), and weaken the part...
-BMC machined all the way through, as it was cheaper and they weren’t making race engines....

[Exminiman]

The keyway slot is below the level of the thrust face because of cost..
It guarantees the slot is not higher than the thrust-face... in production it is machined as a separate operation, so it would be time-consuming/difficult to machine it at exactly the same height as the thrust-face, and if it is higher it will hold the thrust-bearing off its face
And everyone else just copies the original...

The Swiftune one doesn’t go all the way down because he understands that the corners at the bearing journal will be stress-raisers (where a crack can start), and weaken the part...
-BMC machined all the way through, as it was cheaper and they weren’t making race engines....

Thanks for comprehensive reply, still wondering what the specific reason Is that the keyway was not machined all way through originally.

Are you implying the reason is its stronger if not machined all way through......and thats why?

[andy1071]

That's correct. It is stronger (more fatigue resistant) when not machined all the way through.
As I said, the sharp corner of the keyway where it meets the crank bearing is where cracks will start.

BMC probably found/calculated that for 'normal' cars it's not a problem.

But when you start to raise the engine power, and max. revs, the stresses go up rapidly...