Repair 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 to 25 Ft/Lbs. 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.

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.

 

2 thoughts on “Repair Stripped Cylinder Head Stud Threads in the Engine Block

  1. Thanks again for hosting this event, I learned a lot about this particular service/repair task….you ever hear of owners tapping on the base of the pushrod tube with a home made pipe with cutout, to drive the pushrod down a bit and cause the leaking seal to seat better….thereby postponing the inevitable removal of the head/cylinder to replace the seals?

    • Hi Dom,

      And thank you for posting a write-up about this work on your blog. Feel free to reference this one and the video in yours if you wish.

      To your question about tapping on the push rod tube collar to seat it against the push rod tube seal (but not on the seal itself) using a cut-off tube is, yes I’ve heard of that and seen such tools. I want to be clear that this is tool isn’t used to push on the seal, but the metal collar that the seal butts up against.

      But, at a deeper level, why is the seal not firmly seated in the engine block? If it is, then it will only leak when it is cracked and that’s when it should be replaced, or when you get tired of all the oil on your boots and gunk on the engine 🙂

      IIRC, the early /5 engines used a push rod tube that had a collar pressed onto the outside of the tube ant the collar is what the seal butted up against. Over time and vibration, that collar would walk up the push rod tube reducing compression on the seal so it started to leak. The cut off tube was used to tap on the metal collar on the outside of the push rod tube to push it down tight against the seal. This is a losing proposition over time as the collar gets looser over time and you keep chasing the leak.

      Later versions of the push rod tube came with a brazed collar so it wouldn’t walk up the push rod tube. Should you use the cut off tube to pound on that collar you can break the brazed seam and worsen the problem. So with these newer push rod tubes, why can you still get leaks even though the seal isn’t cracked?

      One cause that can contribute to the seals being loose, or splitting is the distance the push rod tube protrudes from the cylinder. The push rod tube is a shrink fit into the hole in the cylinder base. It can move over time and that can cause leaks if its too short, or it can cause splitting of the push rod tube seals if it is too long. I show how to install push rod tubes to the proper depth in this write-up from the 1977 RS rebuild.

      –> https://brook.reams.me/bmw-motorcyle-rebuilds/1977-bmw-r100rs-rebuild-project-index/11-bmw-1977-r100rs-install-connecting-rods-and-assemble-top-end/#Install_Push_Rod_Tubes_and_Cylinder_Head_Studs

      This work has to be done carefully as you do not want to damage the push rod tube hole in the cylinder base.

      More modern versions of the push rod tubes, those in stainless, are a good substitution for /5 push rod tubes for two reasons; they have a welded collar and they don’t rust.

      One other point to make is the part# for the push rod tube seal. There are two versions due to changes in the push rod tube diameter (starting in the 1976 model year I think when the 99 mm cylinder base was introduced). The earlier are 15 mm (part# 11 32 1 250 267) and the later are 17 mm-ish (part# 11 32 1 262 995)

      I hope this helps.

      Best.
      Brook

Leave a Reply to breams Cancel reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.