Valve seat conversion -- unleaded

[DiabloADV]

I have this new-to-me R90/6. Not now, but some time this summer or fall I'm going to pull the top end off and replace pushrods and seals. Also give things the general lookover.

When I'm in there, how would I be able to tell if the heads have ever received new valve seats suitable for unleaded fuel?

[Airhead Wrangler]

If they used OEM BMW valve seats the good ones have 2 scribed lines around the inside circumference, the straight section. The first (too hard) version of the BMW unleaded seats has one scribed line. You might have to scrape away some carbon to see the lines. See below for example of the first unleaded seats with one line:

[BeemerShop]

Originally Posted by Airhead Wrangler

If they used OEM BMW valve seats the good ones have 2 scribed lines around the inside circumference, the straight section. The first (too hard) version of the BMW unleaded seats has one scribed line. You might have to scrape away some carbon to see the lines. See below for example of the first unleaded seats with one line:

I thought I would chime in here since I overhaul these heads for a living. BMW did not have "scribed" lines in the valve seats during the /5/6/7 years, that began with the 1981 models. The bike in question is a 90/6.

To answer the OP question, it can be difficult to tell. If the valve seats were replaced with Martin and Wells seats, you will see machine marks across the top of the seats in a lateral fashion, if they are Tucker seats, it would be hard to tell. If they are BMW factory replacement seats which few machinists use (overpriced and only available in 2 sizes per application), there will be double scribe marks on the interior. If the heads have never been overhauled there will be no evidence of seat grinding, in other words each angle on the seat will be undressed. This is hard to explain, but during a valve job, the seat angles are ground and there will be evidence of that. If there has been no grinding on the seats, you can be sure the seats are original. If you could shoot me a picture with the seats cleaned up I could tell you what you've got there.

HTH

Ted Porter
BeemerShop
www.BeemerShop.com

[DiabloADV]

Many thanks, Ted.

My bike is said to have been built by "Brian at SJ BMW..." and then owned for a few years by Ezra in Santa Cruz. It has the King/Bosse swingarm treatment. Any chance you've ever seen it...? I'm trying to get a feel for its history before starting on it.

[Airhead Wrangler]

Originally Posted by BeemerShop

BMW did not have "scribed" lines in the valve seats during the /5/6/7 years, that began with the 1981 models. The bike in question is a 90/6.

Right, but he's asking how to identify the updated unleaded seats vs. the originals.

[BeemerShop]

Originally Posted by Airhead Wrangler

Right, but he's asking how to identify the updated unleaded seats vs. the originals.

Right, and that would depend on the valve seats used. After 30 years of doing this work, I can tell you most of us are NOT using BMW seats when we do the seat conversions so it's not likely that they would be in his 90/6 heads, could happen, but not likely.

In many cases after removing the valve seats the seat pockets are not round which requires milling the seat pockets and fitting an oversized seat. BMW seats aside from being overpriced are often not available in the sizes needed for the O.D. of the seat pocket after machining. Furthermore we lost the lead in our fuel first here in the USA and figured out the unleaded seat material while BMW fumbled with it and produced bad valve seats (while they still had lead in their gas).

We have top notch valve seat manufacturers here in the USA with time tested proven products available in whatever size you want at a lower price (my personal fav is the Martin and Wells Welltite seat). There is really no reason for a pro shop to use a BMW valve seat. Therefore it is rare in my experience to see a BMW seat used in an Airhead unleaded conversion.

Ted Porter
BeemerShop
www.BeemerShop.com

[BeemerShop]

Originally Posted by DiabloADV

Many thanks, Ted.

My bike is said to have been built by "Brian at SJ BMW..." and then owned for a few years by Ezra in Santa Cruz. It has the King/Bosse swingarm treatment. Any chance you've ever seen it...? I'm trying to get a feel for its history before starting on it.

Sorry that one doesn't look familiar. I know many of the folks who have come and gone over at SJBMW but "Brian" doesn't ring a bell, sorry. I've only been in CA since 2000, so maybe it was before I moved here. Cool bike!

Ted Porter
BeemerShop
www.BeemerShop.com

[Bill Harris]

Best thing to do in thsi case is to take the top end to a skilled and competent shop that knows air heads, explain your concerns and let them evaluate the situation. This is not something you'll do on your own, and no need second guessing the machinist.

Good posts, Ted.

[DoktorT]

Quite a good looking trials /6 there. With a 37/11 rear drive, it would rip trenches in the trails.

[wirewrkr]

I guess I'll Chime in here. I believe the "Brian" you are referring to is Brian B.... from the now defunct BMW of Santa Cruz.
I worked with him for years and he often talked about the /6 he built into a dirtbike.
The4 dealership closed it's door a year or two ago and we all went our separate ways.
I hear that he is working in a lawnmower shop somewhere.
Ted, you know who I'm referring to. Any idea where Brian B.... ended up?
You could also try checking with Bill at Wunderlich America, he may know.
Robert

[DiabloADV]

Originally Posted by DoktorT

Quite a good looking trials /6 there. With a 37/11 rear drive, it would rip trenches in the trails.

That's the rear it has on it. Very nice.

[DiabloADV]

Originally Posted by wirewrkr

I guess I'll Chime in here. I believe the "Brian" you are referring to is Brian B.... from the now defunct BMW of Santa Cruz.
...

Very cool to know that! The bike apparently went through 3-4 years of not running and just sitting. I bought it from a retired longtime airhead owner who got it running.

I have got to finish my CL175 scoot and clear room in my garage to start refreshing this overgrown dirtbike. It's a really nice build.

[batoutoflahonda]

Maybe this will help when you get them off and start looking them over.

Here is a picture of my '81 R80 r/h head. You can see the valve recession on the exhaust. The bike had 76,000 miles on it.