Saturday, April 4, 2020

Fixing an R80G/S Fork Leak

Fixing an R80G/S Fork Leak:

A Journey into the World of O-rings and Loctite

I had been looking for a R80G/S for three or four years and found nothing that grabbed my attention. About a year ago I managed to find a 1982 R80G/S in pristine condition – or – so I thought. The bike was located about 100 kilometers West of Brussels in Zwevegem Belgium. I live in Nürnberg Germany, about 700 kilometers away. The bike appeared to be extreamly nice, and after several telephone discussions with the owner, I thought it was “The One”, so… Road Trip.

The bike looked as good in person as it did in the pictures. I took it for a ride and it ran great. The deal was made, loaded the bike into the van and headed back to Nürnberg, with a small side trip to Stuttgart to see a friend.

Upon unloading the G/S at my shop I noticed a small bit of oil coming from the left fork - from the bottom, not from the top Slider seal. My first thought was the drain plug was leaking or the Slider retaining bolt that threads into the damper rod was leaking. Wrong!

When I drained the oil in the left and right fork legs I found some pretty dirty oil. I replaced the crush rings on both drain plugs and checked the tightness of the Slider retaining bolts (damper rod bolts). Here’s where it gets ugly. When I checked the tightness of the left Slider retaining bolt the Slider Base rotated. This should not be and I concluded this was the source of the leak.

OK, well this is disappointing. I need to rebuild the forks. But, no big deal. I’ve rebuilt plenty of forks – BMW and others.

I pulled the left fork leg off, disassemble it and clean everything. I did the same for the right side as well with the assumption if one is leaking, the other one will start as well.

The Marzocchi forks on the early G/S have an unfortunate design in the fork Slider. There is a Slider Base (sometimes referred to as a “plug”) that is inserted into the bottom of the Slider. Between the top of this Slider Base and a lip within the Slider there is an O-ring. It is not in a groove, it just sits on the lip. The top of the Slider Base has a slight taper and presses against the O-ring, pushing it into the lip, and the Slider Base is held in place by a circlip. The pictures below show this configuration clearer. I am not certain if the first cutaway picture is supposed to be for a R80G/S, but it is the only cutaway picture I could find and it is pretty darn close. The only difference is that the height of the Slider Base shown in the cutaway is much lower than on the G/S forks. This discrepancy in this rendition could be due to the artist’s misunderstanding.

Note: the height of the Slider Base shown in this picture is much narrower than the actual height on the G/S fork
Picture was sent to me by an unknown source, but I believe it came from (Robert Fleischer, AKA

Left to Right: Circlip, Slider and Slider Base
Picture from the Author

Closer view of Slider with the O-ring in place
Picture from the Author

Slider Base inserted into the Slider before the Circlip is inserted
Picture from the Author

In addition to the circlip, the factory applied an adhesive to the Slider Base to “permanently bond” it to the Slider. The permanent bond part is a relative term. Permanent does not necessarily mean for thirty-five or so years. It seems that when the adhesive gives up the Slider Base can rotate when the Slider retaining bolt is tightened and the O-ring may or may not maintain a seal. In this case it did not.

When I disassembled the forks the left Slider Base fell out of the Slider. This was a pretty good hint that the permanent bond was no longer permanent. The right Slider Base came out with practically no resistance.

OK, simple to fix, right? Get the proper O-rings and some adhesive and rebuild the Slider. And so, my Journey Into the World of O-Rings and Loctite began.

The Hunt for the O-ring and Adhesive Specifications
Upon looking at the parts finch for the G/S forks I did not see any indication of the Slider Base or the O-ring – only a complete Slider. I looked at forks for other BMW models as well and found nothing. As far as I could tell these forks are unique to the G/S model.

After much consultation with people who know much more than me, I was enlightened about the “permanent bond” theory. Since this brilliant design idea was considered to be permanent, BMW apparently decided there was no need to provide repair parts and therefore no specs for the O-ring size or what type of adhesive to use.

Picture from BMW Parts Fiche

After numerous search attempts on the Web, numerous correspondences on the Forums and searches on You Tube, I was eventually advised to use Loctite 573 for the adhesive. More on the Loctite 573 later. But, I found no mention of the O-ring size. At least no one was giving up the answer to the size of the mysterious O-ring.

The Journey into O-ring World
Fortunately, there is a shop in Nürnberg that deals in high pressure commercial hosing and fittings. They happen to sell all manner of O-rings known to mankind. I trotted over to the shop with a Slider, Slider Base and the old O-ring in a cloth sack (a very common method of carrying stuff around in Deutschland). My venture into the Über O-ring shop was a bit of a challenge, as this is not the type of shop that gets walk-in traffic. It is a shop for trades people, and I suspect most people going in are regulars. I got a few odd stares, but then a few people sort of gathered around, interested to know what kind of “high pressure valve” I had brought into the Über O-ring shop.

With assistance from one of the guys in the Über O-ring shop we started the Journey of the O-rings. The old O-ring no longer had a round cross section due to being compressed for thirty-five years. It was sort of triangular in shape. This made measuring the cross section diameter a guess. The only thing for certain was the inside diameter of the Slider, 42mm. That became the baseline for choosing an O-ring.  Since the cross section diameter was the mystery, a SWAG (Scientist Wild Ass Guess) was required. After much discussion, and trial fitting some different size O-rings for fit with the Slider Base inserted, we decided on a 39mm x 1.5mm O-ring.

A small side note from my Journey into O-ring World
If you are not familiar with how to measure, describe and discuss the size of O-rings in a commercial Über O-ring shop, it goes like this (at least in Germany):

First look at this picture.

Picture from the Web

You will see an inside diameter (ID), a cross section (CS) and an outside diameter (OD).

ID + CS + CS = OD

So, a 39mm x 1.5mm O-ring has a 39mm ID and a 42mm OD

39mm + 1.5mm + 1.5mm = 42mm

OK, back to the O-ring selection
The inside diameter of the Slider where the Slider Base is inserted is 42mm, so the OD of the O-ring needs to be 42mm. The outside diameter of the O-ring, as described above, is 42mm. At the top of the Slider Base is a small taper that allows the Slider Base to slightly fit into the inside diameter of the O-ring (39mm). Then, when some pressure is applied to the Slider Base, it pushes the taper farther into the O-ring. Once the Slider Base is in far enough, it is on the against the Slider lip and the taper section squishes the O-ring up and out against the lip and side of the Slider, hopefully forming a seal.

The 39mm x 1.5mm O-ring seemed to fit correctly, offering some resistance when the Slider Base was pushed into the Slider, and thus in theory creating the seal between the Slider Base and the Slider (more on this theory in a moment).

Slider Base showing the O-ring and taper relationship
Picture from the Author

Now, back in my shop, with all the parts clean and dry, I did a trial fit for the O-ring size. With O-ring in place, I inserted the Slider Base into the Slider. In order to insert the circlip some pressure was needed to push the Slider Base in far enough and compress the O-ring enough to get the circlip into the groove. Actually, it took a lot of pressure. I jury rigged a large wood clamp to apply enough pressure to get the circlip in place. I had assumed that some amount of interference and pressure would be necessary in order for the O-ring to seal. The question was how much interference was needed. Too much interference and there is too much pressure required. At this point the fit seemed OK, but I was still walking in the dark.

The Journey into Loctite World
The Slider Base OD is 41.085mm, so it’s a snug fit into the Slider’s 42mm opening. The gap is .015mm. The Loctite 573 specs say it will fill gaps up to a .025. Good to go.

I decided to go ahead and assemble the pieces with the Loctite 573 that I had ordered on Amazon. More to come on the Loctite selection. Everything was thoroughly cleaned, and cleaned again with Loctite 7063. The Loctite was applied in a thin, even layer to the outside diameter of the Slider Base and it was inserted while carefully rotating to spread the adhesive evenly. Using the jury rig wood clamp, I compressed the Slider Base enough to insert the circlip (this took some grunt) and then sat the Slider to the side. The Loctite 573 takes about 96 hours to fully cure. Next step was to repeat the procedure on the other Slider.
After I allowed time for the Loctite to cured I reassembled the forks, reinstalled them on the bike and put in the proper amount of oil. I pumped the forks up and down for five minutes (my work out for the month) and no leak from either side. YEAH SUCCESS!... Wrong again.

I took the bike for a twenty kilometer ride and when I returned both forks were seeping oil. I went home and had a beer.

After having given great deal of thought to my predicament I was back in the shop the following day. I pulled the forks off the bike and carefully disassembled them. What I found confirmed that the leak was coming from between the Slider and Slider Base.

It turns out that the Loctite 573 does not cure hard, but stays rather slightly tacky. I went on the Loctite Web Page and read a bit about 573. It is designed to be a gasket application where there may or may not be gasket material, such as paper, etc., and two parts will be disassembled at some time in the future. Therefore, it is not intended to cure hard and solid.

I got in contact with the Loctite folks at Henkel and asked a few questions. The answer came back that the right choice would be Loctite 638. It is intended for slip fit applications where the inserted piece is intended to stay solid. If you look at the picture on the Loctite containers there is actually a pictogram that gives you a pretty good hint.

Note the pictogram on the 638 and 573
Picture from the Author

So, off to Amazon again to order Loctite 638. By the way, this stuff is expensive. The two bottles of 638 and 573, 50 ml each, cost nearly €75. It took a couple of weeks to get the 638 (I suspect that Loctite was making an order just for me). However, that gave me time to think about the O-ring thing again.

Return to the Journey into O-ring World
It did not make sense to me that, even though the Loctite 573 may not be the best solution, the oil would leak between the Slider and Slider Base with an O-ring properly in place. At least, not immediately. This gave rise to the idea that the O-ring was the wrong size – more specifically, the cross section was too large. If this was the case, the inside diameter of the O-ring would be too small for the taper of the Slider Base to slip into correctly and the excessive pressure require against the Slider Base to get it in far enough to insert the circlip might be distorting or pinching the O-ring in a way that does not provide a seal.

I was pretty convinced of my new theory. The O-ring still needed to be 42mm OD, so this means I need a 40mm x 1mm O-ring. Back I went to the Über O-ring shop to buy a few 40mm x 1mm O-rings.

Round Two
The Loctite 638 arrived and I had the new O-rings. Time to test my new theory.

First. I did a trial fit with the 40mm x 1mm O-rings. I spread a very thin coat of white lithium grease on the taper section of the Slider Base to use as a telltale. I placed an O-ring into the Slider and then inserted the Slider base. With a minimal amount of pressure, the Slider Base went far enough in to insert the circlip.

I removed the circlip and Slider Base and it appeared that the white lithium grease telltale transferred to the O-ring was distributed in a way that would work to provide a seal.

Next was to clean everything up properly and reassemble with the new O-rings and Loctite 638.

I got a good feeling about the Loctite 638. After spreading a thin even coat on the Slider Base, I partially inserting it into the Slider and carefully rotated it slowly to spread the adhesive. I continued doing this as I pushed the Slider Base farther into the Slider, but not too far. I did not want to disturb the O-ring with the twisting motion. I could feel the adhesive starting to cure and drag a little – this good I thought. Once the Slider Base was in, it only took a little finger pressure to allow the circlip to be inserted.

I repeated this assembly on the other Slider and then once again let the Loctite cure for 96 hours.

Once the cure time was finished I reassembled the forks and installed them on the bike, added oil and went for a ride.

YIPEE… NO LEAKS… And still no leaks after a couple more rides.

The jury is out on the Loctite 573 virus Loctite 638. The 573 may have been just fine and the real problem was the incorrect O-ring. However, because the original factory assembly had the Slider Base bonded to the Slider I feel better with the 638.

And finally, my Journey into the World O-rings and Loctite came to an end.