Cylinder stud length question

[hools100RS]

Hi all! I'm currently rebuilding a 90/6 engine from bare bones! On opening a pack of cylinder studs aquired on Ebay, I notice that two of the eight studs are longer than the others. Which studs go where? I'm in unknown territory here. Any help or suggestions appreciated. Thanks, hools

[Kurt in S.A.]

I saw a couple of posts by Tom Cutter on the Airhead list. Someone asked about long studs on an R100 and he said they both went into the two rear holes on the right side. Not sure if that is the same for all Airhead blocks.

In another post, he mentions the length should be 9-7/8 (9.875") to 9-15/16 inches (9.9375") or (250.8-252.4 mm) from the case gasket surface to the end of the stud. Use a simple carpenter's combination square, adjusted to 9-7/8", then lay the square head across two adjacent studs and adjust the studs until the ruler touches the gasket surface.

You may be able to figure out where the longer studs go by looking inside the opening in the block to see which of the spots has more casting material to hold a longer stud.

Kurt in S.A.

[chasbmw]

i just roughly measured the exposed length of all the cylinder studs on my R90/6 at 25.5cm, its the same for all the studs. Looking at my parts book dated 6/75, there is a single part no for all 8 studs and its the same part for all bikes from 500cc/5 to the R90/S.
I dont know if BMW changed the stud at a later date, you could use Realoem to check if part Nos changed?

[hools100RS]

Thanks Chas and Kurt for you responses. Guys I have discovered that the block I am using for my 90/6 build is a 1976 R100S block. Chas, I was worried when I read your post re: all the studs being the same size for the R90. The barrels and heads I have are R90 but the studs are definately for some other application and yes Kurt, there appears to be more crankcase meat under two of the stud holes, the right hand cylinder top and bottom rear studs. I don't know what change would have necessitated such a design change? Just a question: given that he studs screw into the block how in the hell does the oil get up the stud to oil the rocker gear? Is it just high pressure pushing oil past those treads or are the threads in the block cut slightly larger? I LOVE A MYSTERY! Thanks again guys, hools.

[SamH]

There's an oil way at the top of the top two stud holes, you can just see it when the studs are installed. It just peeks out at the end of the threads in the block. I was putting the engine together on my new R90 and had to get a torch to convince myself they were actually there. They're the reason you only use a smear of liquid gasket when putting the jugs on. A big bead is almost guaranteed to block them.

[chasbmw]

The way to check would be to go into realoem for your bike and check the part nos for the studs, the longer studs will have a different part no.

[Kurt in S.A.]

Seems correct. RealOEM for my R100/7 shows 6 studs 275mm long and 2 studs 297mm long.

Kurt in S.A.

[driggs]

I saw a couple of posts by Tom Cutter on the Airhead list. Someone asked about long studs on an R100 and he said they both went into the two rear holes on the right side. Not sure if that is the same for all Airhead blocks.

In another post, he mentions the length should be 9-7/8 (9.875") to 9-15/16 inches (9.9375") or (250.8-252.4 mm) from the case gasket surface to the end of the stud. Use a simple carpenter's combination square, adjusted to 9-7/8", then lay the square head across two adjacent studs and adjust the studs until the ruler touches the gasket surface.

You may be able to figure out where the longer studs go by looking inside the opening in the block to see which of the spots has more casting material to hold a longer stud.

Kurt in S.A.

Yep it's true....the right side rear studs are longer, because the treaded portion in the block that accepts those two studs are recessed into the block about 1/2" to 3/4" or so, such that the first part of the stud mounting hole is smooth, not threaded...this is on a '78 /7....not sure why, I understand that the oil delivery gallery is built into the top two studs...

[SteveD]

Boxerman Phil Hawksley concurs...http://www.bmbikes.org.uk/Forum/viewtop ... 70&t=16815