Capscrew Under Head Radius

[Bill Hermsted]

I decided to read the posts backwards for the first time. I have no idea why, but it's a good thing for me.
What was missed was a March 21, 2008 entry calling for grinding a chamfer around the con rod cap's holes to clear the under head radius on the screw, for the Macs.
I grabbed a spare rod, praying for even a slight factory chamfer. It wasn't there. The edge wasn't even broken. How could Mac, a designer and builder of genius, overlook this?
It breaks my heart to think of how many Macs out there are relying on the radius alone to clamp the cap home. It's not like they built millions of racing motors.
Now my Mc 45 is facing another partial teardown. Thank God the motor hasn't been run yet.
To my relief, upon checking the Power Products cap holes, it was found that the alignment sleeve lies down a bit, leaving a pocket for the radius to occupy. You see, they HAVE been running.
I understand the concept, but still failed to check while assembling these Two Snappers.
Shame on me! There's always so much more to learn. I guess too much faith was put in Mac.
THANK YOU, REAR.
bill

[ted johnson]

Bill, check the I.D. on the rod cap. It may be large enough to clear the tangent diameter on the stock Mac bolts. The Bonbright 310KSI bolts have a large under-head fillet radius, and I know that Dave B. recommends chamfering the holes when you use his bolts on a Mac. TJ

[Bill Hermsted]

Ted- sorry for being such a spaz. A cooler head prevaled (yours).
I couldn't see where the tangent diameter blended with the screw head to measure accurately, so a # drill gauge was found. Now screws could be tried in ever increasing diameter holes, holding the gauge edge on over a flashlight, until all the radius was in the hole.
Here's what was discovered: the standard Mac screw head banked flush to the plate in the # 11 ( .191 ) hole.
I use the spline drive Mc 101 screws, which bank flush in the # 6 ( .204 ) hole. The holes in the cap measured .210, so we're good. The cap holes are concentric to the rod holes when the parting line is in mesh, right?
I owe a big apology to Mac. I only have a few motors, and that's why my approach to messing with them is one of overcaution.
Thanks, Ted !
bill

[ted johnson]

Bill, I'm pretty sure the machining operations are completed prior to cracturing the cap, so I imagine the holes are concentric. Good you checked the tangency diameter with the drill gage. Good idea! When we're dealing with equipment this old, a little extra caution does not come amiss! TJ

[Bill Johnson]

I have been building Mac's for 40 years and I dont have a clue what you are talking about here.

[steveohara]

Bill,

It is an issue that turns up in design and fabrication projects on a regular basis but it is not and never has been an issue on OEM Mac rods for the reasons discussed above.
Here is what Bill H. is talking about.... most cap screws of any type will have a small radius where the transition takes place from the seating surface of the head to the shank or threaded portion of the screw.
If the hole that the screw is going into is sized very close to the OD of the shank and the hole has no break, chamfer or radius at the corner then the screw will not sit all the way down into the hole as the radius on the screw will interfere with the sharp corner on the hole.
Carroll Smith covers the subject nicely in on of his books, I beleive it is Prepar to Win.
Steve O'Hara

[ted johnson]

The only reason this came up in the first place was that I installed the Bonbright 310 KSI screws in my 101, just because I had a set, and I wanted to be able to reuse them. The Bonbright bolts have a large underhead fillet, and Dave wanted me to accomodate this on my rod. I used to work at Sandia National Labs on satellite design, and we had had quite an issue with previous designers and engineers using close diameter clearance holes on a critical assembly. We started checking things, and realized that we were dealing with a critical issue for fatigue and crack propagation. Several of we designers and engineers wrote up a guideline to cover the various potential problems. Anyhow, it's something to keep in mind for critical assemblies. I found a cone shaped fine grinding stone to chamfer the holes. The whole thing is a moot point when using Mac bolts, even the spline head ones.