Thin Base Gasket??

[bprigge]

Hello fellow Airheads,

Tonight I started the joyous task of reinstalling the heads on my 1976 R75/6
after having hardened exhaust valves installed by Memphis Motor Werks.
The heads are beautiful by the way owing I think to the shops ultra sound cleaning process.

Anyway, the cylinder base gaskets that I purchased at the local BMW dealer
are about 1/2 the thickness of the gaskets that were on the motor. The originals (maybe not originals?) look to be about 1 mm thick and the replacement gaskets look to be about .5 mm thick.

I know that this will raise the compression ratio a little and I'm not really concerned about that end of it as I always run high test gas. What I am concerned about is creating a piston to valve clearance problem. Does anyone out there have any experience or knowledge about thin verses thick
base gaskets on these motors? Any advice will be appreciated. Bruce

[boofer]

What is the history of this bike? Did it have extra gaskets installed to lower compression when unleaded gas was first introduced? Is there a "ridge" in cylinder bore at top of stroke?

When I changed to no lead, at 210k, had the cylinders honed to remove any top ridge (still could see crosshatching in cylinder walls) and used a thin gasket but never miked it's thickness.

Tim

[bprigge]

Tim,

Thanks for the reply.

I don't know the history of the bike before August of 2004 other then its
original owner lived in Pennsylvania and it spent a couple of years in Los
Angeles prior to me buying it last year and riding it home to Minnesota.

Very good point about the cylinder ridge, i hadn't considered the possibility
of breaking a ring or piston ring land because of a ridge. The odometer shows
about 48,000 and still works.

I'll check out the ridge issue tonight. Can you reuse the base gaskets if they
appear to be undamaged?

thanks, Bruce

[boofer]

I can't see a problem with using the base gaskets over. Many replace pushrod seals without removing the barrels. Maybe a little never sieze on both sides, thats what I have done in the past.

Tim

[bprigge]

Tim,

I think I'm going to use the old ones again. They are over a MM thick.

Did some research on the web and found some related information on this
website: http://pweb.jps.net/~snowbum/engineinternals.htm.

The fellow, snowbum, describes 3 different base gaskets that can be used to vary compression from the standard 9.2:1 down to 8.6:1. I'm pretty sure
I've been running the thickest gasket. Last summer in Montana I stopped for
gas with just a pint or so left in the tank and the station had nothing but 87
octane, so thats what I bought. I couldn't make the bike ping on the 87
octane but I thought the lack of pinging was related to the high altitude.
Now I'm thinking it was more do to with running those chubby base gaskets.

I sure would like to have that extra compression though. If I hone it then i'll
have to change the rings and the pistons have some markings on them that
indicate that they are not the originals, raising the hassle factor considerably
when finding rings for it. Bruce

[kbasa]

Back in the late 70s, lots of folks installed low compression base gaskets so they could run cheaper gas on their bikes. My dad did this with his 75/7.

[kbasa]

Tim,

I think I'm going to use the old ones again. They are over a MM thick.

Did some research on the web and found some related information on this
website: http://pweb.jps.net/~snowbum/engineinternals.htm.

The fellow, snowbum, describes 3 different base gaskets that can be used to vary compression from the standard 9.2:1 down to 8.6:1. I'm pretty sure
I've been running the thickest gasket. Last summer in Montana I stopped for
gas with just a pint or so left in the tank and the station had nothing but 87
octane, so thats what I bought. I couldn't make the bike ping on the 87
octane but I thought the lack of pinging was related to the high altitude.
Now I'm thinking it was more do to with running those chubby base gaskets.

I sure would like to have that extra compression though. If I hone it then i'll
have to change the rings and the pistons have some markings on them that
indicate that they are not the originals, raising the hassle factor considerably
when finding rings for it. Bruce

Are they marked +1 or something? If so, they're probably BMW one over pistons and you can get rings for them from BMW.

[bprigge]

I pulled a piston today and buffed out the piston crown. The piston is marked with '81.975'. According to my Clymer book the 81.975 mm piston is one of the three grades within 'standard'. The piston also is marked with
R + 03. Thats the part that makes me wonder if they are oversized pistons?

Bruce

[bprigge]

For anyone that might be interested, my R75/6 is back together with the
THIN base gaskets, freshly honed cylinder bores and new standard BMW rings.
The motor also has new Black Diamond exhaust valves and new valve guides,
along with a new set of aftermarket OEM looking mufflers. The motor runs
well, in the garage anyway. The roads are still snow covered in my part of Minnesota. Bruce

[kbasa]

Bruce,

Who did your heads? I'm going to need to do the ones on my CS and am looking for sources. I know Bob's will do them and my local shop will do them, but I'm looking for a little porting and bigger valves on the exhaust side. I had it done on my RS and it really helped.

Got any pictures?

Dave

[bprigge]

Dave,

I had my heads done by Leo Goff at Memphis Motor Works.
I didn't talk to him about port work so I don't know if he gets involved in
'performance' head work. I did some port work and installed larger valves
into a set of Ford E7TE heads a couple of years ago and its VERY labor intensive as you may already know. Unless a guy had proven success doing
BMW airheads I wouldn't risk it myself. Porting is an art form and the special
talent it takes sure isn't spread evenly amongst gear heads. Do a search
on Leo Goff and you should pull up Memphis Motor Works. Bruce

[bikerfish1100]

I would suspect that San Jose BMW can do beneficial head porting on airheads. I also would think that the Harley masters- Branch- could set up the heads on their flow bench and make those puppies whoosh for ya!

[28796]

I don't believe a 76 would have had any metal base gasket originally, only the 2 small o'rings & 1 large one + sealer. Thats the way my R90/S was & is. I thought 74 was the last year for the stock metal gasket & they were only used after that to drop compression which should never be an issue on an R75.

[bprigge]

This one only had the thick base gasket, a little over a mm I think, no O rings
at all. I reassembled it using the thin metal gasket the local dealer said belonged there, with a very thin coat of high temp silicon seal on it, cured for about an hour before assembly. I did look for machined recesses in the bottom of the cylinders thinking it should have O rings but there was nothing
there that looked like it would take an O ring. My Clymer book shows a gasket in the exploded view of the cylinders also. Actually I wouldn't mind running with no gasket if it didn't leak. I think a 750 that is rated at 50 hp or whatever low number it is rated at could benefit by a little bump in compression. Not that I'm complaining, the bike is a joy to ride just like it is.

While I'm thinking about it, how does the top end get oil on these motors?
I didn't see any way for oil to move up to the rocker arms.

[flash412]

While I'm thinking about it, how does the top end get oil on these motors? I didn't see any way for oil to move up to the rocker arms.If you had looked CLOSELY at the block where the TOP cylinder studs exit, you would have seen a small hole in the stud threads in the block. Those holes connect to the oil gally. Oil runs up the hole in the cylinder around those two studs. When it gets to the rocker block, it continues up inside the rocker block to where the rocker shaft crosses. The oil runs down the rocker shaft to lubricate the bushings or bearings in the rocker arm.

When rebuilding a motor, it is not a bad idea to pour some oil all over the rocker assemblies a little while before you start it for the first time. When you start it for the first time, you might want to consider leaving the valve covers off. Once the oil light goes out, take a look at the rockers. Run the motor a couple minutes and the rockers should start FLOWING oil. Then you KNOW that the top end is getting proper lubrication and can shut it off and button it up.

BTW... after reinstalling a top end on an airhead, I like to get it GOOD and warmed up ONCE and then let it sit overnight. Retorquing the head and readjusting the valves COLD the next day, after ONE good thermal cycle, will make the adjustment STICK until the next service.

[bprigge]

Thanks for the info Flash412.
Now I think I better pull the rocker covers and make sure its oiling the top end. From what you describe I could easily have some silicone seal 'squeeze out' in the oil holes. I've only run it about 5 minutes in the garage and the rockers were well lubed when I put it together but I better check it anyway.

Those hole must be to small for the 'mature eye' to see. Bruce

[barryg]

My suggestion is ask Leo Goff at Memphis Moterwerks. He did your heads for you and I can assure you he knows more about putting that head assembly back together than any of us out here. He has over 30 years experience in engine building and assembly. Do a web search on Leo and you will be surprised at what it will show. Leo has built many show winning cars and motorcycles over the last 30 years. He has also built many performance cars and motorcyles that have won races in many venues. He is most wellknown for his Norton drag bike that held several class records in IDBA in the 80's. That bike was retired undefeated. Good luck with the reassembly Barry

[bprigge]

Thanks Barry,

I know what your saying. I researched Leo a little before I sent my heads down. He is somewhat of a legend. To tell you the truth I thought about calling him on a couple different assembly points but decided it would be a
little like asking God how to lick a stamp. I really didn't want to bother him
about what seems pretty trivial. The motor is together and running well
now (in the garage anyway) and its been a great learning experience.
I'm looking forward to the first ride of the year with my slightly higher compression! But first I'm going to tackle the clutch spline lube project. I
take this machine out on cross countries and reliability is a pretty big issue.
Bruce