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schrader7032
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VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
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I posted this on the /2 forum...I think everyone over there was out riding! I got a start on cleaning the piston crowns today from my R69S. I found the following on the top of the pistons - 7192, 1/68, the letter H, and a triangle with unreadable info in the middle of it. If I understand things, the 7192 means it's 71.92mm in diameter also indicating that the cylinder bore is most likely the standard size at 72mm (+/-) (only 30K miles on the bike). The 1/68 is the manufacture month and year. What about the letter H? Is the triangle like a Quality Assurance mark or something? I expected to see the word "zorn" to identify which is the front for installation. I got to reading my workshop manual and see that is says that the R69S pistons are not offset but they should be installed with the lettering readable. Sound correct?? Thanks...

Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

woodbury7089
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VBMWMO #7089
Belmont, New Hampshire
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Zorn Marking?

Sounds correct Kurt. Don"t know what the H stands for but there usually is a directional arrow or the word "vorn" {german for front} or both together. If theres no directional marks and your crown has valve recesses the larger recess is the inlet and the smaller recess is the exhaust. Hope that helps in some small way. Good Luck Jim

schrader7032
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San Antonio, TX
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Zorn Marking?

Thanks, Jim. I realize now that it's vorn and not zorn... Good idea about the pistion recesses but here's a picture of one of the pistons:

http://pages.prodigy.net/kschrader/teardown/2-11-06/LeftPistonCrown2x.jpg

It's hard to tell, but it looks like there's no difference between the two recesses. The R69S doesn't have an offset in the piston/rod, so I've been hearing that it doesn't make any difference. Vech has told someone else on his R27, which also doesn't have any offset, that it didn't make any difference.

I'm tending to lean that direction. I'll probably install with lettering readable just so I know how it was done for future reference.

Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

woodbury7089
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VBMWMO #7089
Belmont, New Hampshire
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ZORN MARKING?

Yeah Kurt, I see what you mean they are really close. From your Pic the right recess looks a tad bigger but that could be shadowing. The best way would be to grab the calipers and measure but I think I"d have to agree they are probably the same.
Just curious are those the stock pistons or are they oversize. I see there stamped but did you mike them and your bore to verify dimensions. I know curiosity killed the kitty but am interested in why you tore it down to begin with did you have a problem?
I'm just assuming that maybe it was in pieces when you got it or forgot to notice piston orientation when you took it apart [Thats where the digital camera has it"s place in the tool box}
Either way I think Vech is right on with the fact that it probaby doesn"t matter and if thats the case you could always scribe the crown with your own arrow for the next person who tears into it down the road.
Keep me posted I'm interested in how it turns out. You never know I might own a 69s someday. CUL JIM

schrader7032
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VBMWMO #7032
San Antonio, TX
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Zorn Marking?

Jim -

I'll try measuring as best as I can. The stamping "7192" means 71.92mm in diameter which is stock, not oversize. I have measured the diameter and it is pretty close to that. From this, I gather that I'm the first one into this engine. I haven't measured the bore yet or even the ring gaps but plan to. I wish I had taken pictures before I took the pistons off but they were covered in a layer of carbon, I wouldn't have been able to see the markings anyway. I could have tried to clean the pistons in place but that would have been a mess. I soaked the pistons in Hyrdoseal for a week and was able to fairly easily get them this clean. I haven't decided if they need to be much cleaner. I still have to work on the ring grooves.

I took quite a few other pictures to document my progress. In some cases, I want to see what it looked like before I messed things up!!

The engine had 30K miles on it and I'm tearing into it to get the slingers cleaned. I'm doing as much work as I can and I'll be taking the bottom end to a mechanic in St. Louis. Once that's done, I'll be reversing the process and probably putting new rings in, at least. The compression was about 125 each side but I had a 10% leakage rate past the rings. No discernable leakage past the valves or guides.

Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

schrader7032
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VBMWMO #7032
San Antonio, TX
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Zorn Marking?

"woodbury7089" wrote:

If theres no directional marks and your crown has valve recesses the larger recess is the inlet and the smaller recess is the exhaust. Hope that helps in some small way.

Jim -

Just a follow-up...I measured the recesses as best as I could with my calipers...they measure pretty close to the same.

Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

woodbury7089
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VBMWMO #7089
Belmont, New Hampshire
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Zorn Marking?

Hi Kurt. I would imagine that if they both measure the same and the r69s is not an offset piston than it wouldn"t matter which way it goes . It would be nice to put them back in the way they came out for wear purposes { are you going to hone cylinders before reinstalling? }. I Will continue to research piston marks for pistons made by Nural or any aftermarket pistons just for my own curiosity. But I got to tell you I don"t find alot of specifics on such things as that. The good thing is the next time you will take a razor or wire wheel and clean the crown first before taking it out. Just a little tidbit I learned from workin with small engines is gumout carb cleaner will dissolve carbon on piston crowns very efficiently, you can just about wipe it clean depending on thickness of buildup. But I can"t imagine yours was that thick other wise it would have boosted your compression ratio and you said you got 125psi both sides.
125 - 147 is normal compression and you had no appreciable difference between cylinders. [Did you take your readings at operating temp? Was your throttle open so it could draw air into cyl? How"d your plugs look ?they will tell you more than anything whats going on in there.
Good luck on your project i"ll keep lookin for info CUL JIM

schrader7032
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VBMWMO #7032
San Antonio, TX
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Zorn Marking?

Jim -

I guess you and I have our little tech meeting going on!! I hope to learn something...

I plan on replacing the rings since I was getting a fair amount of leakage during my leak down test. Probably a light honing is best in order to break the glaze. I'll have someone look at the cylinders and give me some suggestions. I don't have a hone so I'll need to look into that. I'm a member of VMCA also and they have a tool loan program...maybe I can get the tool there.

I didn't make any marks on the pistons because I fully expected to find the arrow and vorn on it. That's all I'd every read regarding BMW pistons. Everything else I've removed, I've tried to mark left-right, up-down, etc., just be sure. I didn't know about the pistons. But if you can find some info out that would be great. The only thing that I haven't figured out is the letter "H" on the crown. But your suggestion to make a small mark somewhere is a good one.

For the compression test and leakage tests, they were all done after a warm up ride. I held the throttle open by hand as I stepped through on the kick start. Same on the leak down tests. My plugs look OK, but IIRC the right plug is a little darker than the left. I've been experimenting with carb settings and jets. I've ranged from 40mpg to over 50mpg. I got pinging at the higher (ie, leaner) gas mileage. I've settled on about 43-45 mpg and I get reasonable power with no pinging. I'll probably have to redo my tests once I get the bike back together.

Thanks for checking in...Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

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