R75/5 Repair of Stripped Cylinder Head Stud Threads in the Engine Block

The first sign of the problem was this bike would not idle well. I thought the carburetors got dirty and all I needed was to clean them. But, before I started on that task, I decided to check the valves and I found the left intake valve lash was 0.012 inches instead of 0.004-0.006 inches. Hmmm .... how did that tappet get so loose?

Before I reset the valve clearance, as I always do, I torqued the cylinder stud and head bolt nuts. But, one stud just spun. Bummer. That means the threads in the engine block are stripped.

It turns out a friend of mine, Dick, had the same problem on his 1972 R75/5 a couple of weeks earlier. I contacted him and learned he made arrangements to rent a jig to repair his stripped threads from Northwoods Airheads. So, we jointly repaired our stripped threads in "Brook's Airhead Garage" when the jig arrived.

This write-up is based on the work we did on both bikes.

• 11 BMW R75/5 Repair Stripped Cylinder Stud Threads

Here is a link to a video of how to use the Northwoods jig to drill out the damaged threads, tap new threads for a 20 mm long M10x1.5 Heli-Coil insert, and install the insert.

Brook

Your documentation of repairs is always first class and has been helpful to me several times. Thx much for the effort.

Very nice on a number of levels Brook
I was wondering if you are a fan of making the parts “squeaky clean” before using the Locktite?
Gary

thanks for taking the time to do that video... I sure it will be referenced a lot... unfortunately.

beemerphile said:

Brook

Your documentation of repairs is always first class and has been helpful to me several times. Thx much for the effort.

Click to expand...

You're quite welcome Lee.

Omega Man said:

Very nice on a number of levels Brook
I was wondering if you are a fan of making the parts “squeaky clean” before using the Locktite?
Gary

Click to expand...

Gary,

Indeed, I mentioned that I cleaned the threads with acetone until my Q-tip shows no trace of discoloration indicating all the oils and grease have been cleaned off. I want to give the Loctite every advantage to reach its full bonding strength, particularly in this situation.

When I use "blue" Loctite I always use alcohol pads to clean the male and female threads on fasteners. I get a box of them at my Walgreens pharmacy.

Best.
Brook.

grahamm said:

thanks for taking the time to do that video... I sure it will be referenced a lot... unfortunately.

Click to expand...

Hi Grahamm

I think you are correct about this topic being referenced a lot as time goes on. Unfortunately the Haynes manual lists much too high torque settings (28-32 Ft-Lbs) for the cylinder stud and head stud nuts. The Clymer values are safer (22-28 Ft-Lbs). I'm going to settle on 23 Ft-Lbs as my standard going forward which is at the low end of the Clymer range and a bit lower than Bob Fleischer's (aka Snowbum) 25 Ft-Lb maximum recommendation.

An interesting property of aluminum is that unlike steel, under cyclic stress, there is no stress for which an infinite number of stress cycles can occur without failure. With steel, you can keep the stress below a specific value and it will never fail under cyclic stress.

I have wondered if this is a contributing factor to cylinder stud thread failure in the aluminum block of these bikes. Combined with using the upper range of Haynes and Clymer torque values, I suspect those higher torque values reduce the number of cyclic stress cycles the aluminum threads can take before they fracture.

Best.
Brook.