71 r75/5
One collar on each cylinder pushrod tube is a slip fit and one is tight. What is correct and can they be tightened/loosened? Thanks in advance.
In the old days, when this bike was new, tapping the collars down to cinch-up the PRT oil seal was a common mechanic's trick. It's highly conceivable that yours got tapped once too often.
The collars should be fitted so tightly (or brazed into place) on the PRT that a hammer and collar drift could to be used to move the PRT inside the cyl head. So if you have some collars that are so loose as to slide up and down, then part replacement is going to be your only answer. Re-brazing the collar back on while the PRT is in-place will not work.
Those collars not only allow positioning of the PRT, but they also apply pressure to the PRT seal at the base of the cylinder. Without that constant pressure pushing the seals against the motor block, you proabably also have a good sized oil leak.
Repair
The only course of action would be to start planning the next replacement of the PRT oil seals. That would include buying 4 new PRT seals, new top-end gaskets, new o-rings, and 4 different (new or good used) tubes. It will take proabably 2 to 2.5 hours per side, if you take your time and do a good job of cleaning gasket surfaces. Getting the old "collar-less" tubes out is going to be your most time consuming part of the task. It will require heat and a special drift.
If "tapping the collars" indeed caused the collars to part from the tube, then the projection length of the tube has also been disturbed. So you'll want to have that dimension before you remove the old tubes, so that you can install the new tubes to the correct depth.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly Why won't brazing work?
The tube is pressed into the cyl head, making very good thermal contact. So first off, there's the issue of applying enough heat.
Secondly, when you dump all the extra heat into the tube, with the top end assembled you'll simply melt the PRT seals. So with dead seals, you'll end up pulling the top end off anyway.
Once the heads are off the engine, brazing the rings back into place won't be such a challenge. But, you'll still need to inspect the length of exposed PRT to get the "crush" set correctly for the new PRT seals.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
According to Snowbum (bmwmotorcycletech.info), until around 81, the collars were not braised and there was a tool to drive them tighter against the seals. A tool can be made from conduit. There is also a link to Brooks Reams's rebuild of a 77 which uses those non-braised collars.
Thank you for inputs..
I like the idea of being able to lovetap the collars to find the anti leak sweet spot for the PRT seals like the original design. The notion of moving the whole pushrod tube in the jug to tweak a rubber bellow type seal seems more likely for repercussions. Instead of brazing would it be feezible to peen the collars, reream for size then reinstall or if they are available (to anyone's knowledge)? new collars? The tubes are in good shape and I don't want to change them if a practical solution exists... I like to keep it simple.
The best way isn't always the easiest looking way. That's all I know.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly Thanks for your insight. After cleaning up the tubes, I have discovered they are slightly smaller in diameter at the ends where the seal fits and the collars are a sliding fit up to a .004" step, then a press fit. I agree they were probably knocked in too far through repeated sealing attempts.I have new seals. The cylinders are on the bench and I will set the collars to the correct location. On this /5, which has the .020" base shim, what is the dimension from the cylinder base plane to the collar please? I have found 3 different numbers,ranging from flush to 5mm. and not sure which to go by. Thank you.
On this /5, which has the .020" base shim, what is the dimension from the cylinder base plane to the collar please? I have found 3 different numbers, ranging from flush to 5mm. and not sure which to go by. Thank you.
Sorry, but I can't help you with that.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly thank you. I will take careful measurements of the matching tapers and guestimate the distance to set the collars trying to factor the rubber pliability. If others can contribute from experience it would be helpful to prevent laundry arguments and will be appreciated by the wife.
On every seal that came out, the fins were mashed down at the top of the seal and each had about the same amount of squish so maybe the bottom fin started to seep. After decades of a combination of stiff rubber and adjustment, these seals are pooched. Time to put on the thinking cap and open the best ear and try get it the best I can. Thanks
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