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Old 10-04-2011, 09:26 PM   #76
kbasa
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Quote:
Originally Posted by Beemerguru View Post
I know it's always the right thing to do,,,measure for head interference when assembling a new engine...but in reality,,,all stock BMW parts,,rotates with no interference on assembly, has the proper valve clearance, etc, I would think everyone on this list would think the engine was good to fire up,

Of course when that first loud WHACK, WHACK comes out...I don't think I'd see if it would go away in a 100 miles. That head would be off in 15 minutes.

Measure the piston and measure the cylinder...this crowd has been in suspense for too long. Post the findings..everyone will grab their favorite manual and tell you if the problem lies in either part.
Valve seats are cut to correct depth and valve face is correct diameter at that point?
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Old 10-04-2011, 11:57 PM   #77
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Here's more pics of the heads, cylinders, and pistons. Measurements will happen this weekend.

https://skydrive.live.com/redir.aspx...FB71801A7E!766
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Old 10-05-2011, 12:35 AM   #78
tokyoklahoma
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Question Machined Case

I would bet that, at some point the mating surface of the case (where the jugs bolt up) has been flycut to clean-up the surface. Thus making the assembled height too short without a thicker gasket, or double gasket, or gasket and shim.
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Old 10-05-2011, 01:15 AM   #79
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What should the normal distance be from the piston crown to the head at TDC?
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Old 10-05-2011, 01:56 AM   #80
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Originally Posted by Padmei View Post
What should the normal distance be from the piston crown to the head at TDC?
I don't know the official figures, but on a race engine 35-40 thou minimum. On a road bike I'd guess about 50 thou .
You need about the same piston to valve clearance too.
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Old 10-06-2011, 07:12 PM   #81
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Old 10-09-2011, 01:19 PM   #82
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Talked with the local Motorcycle shop (Bill at Quality Motorcycles). Going to have him inspect the heads and pistons, and see what needs replaced.

We were discussing the cause, and most logically, it seems like somewhere in the last 40 years, something was machined down, either heads, cylinders, or cylinder pads. I'm going to repair the damage, and then put the low test gasket (1.2 mm) on, button it up, and clay the motor. Should fix the problem.
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Old 10-09-2011, 04:29 PM   #83
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You might want to measure some volumes and calculate the compression ratio of this particular collection of engine components. It won't run happily on available fuel with a 15:1 CR...
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Old 10-09-2011, 04:43 PM   #84
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You might want to measure some volumes and calculate the compression ratio of this particular collection of engine components. It won't run happily on available fuel with a 15:1 CR...
Probably would be the wise thing to do. These are all stock, so as long as there is clearance, it should be fine. Can't hurt to check though.
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Old 10-09-2011, 05:50 PM   #85
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this is one of the best threads I've seen in awhile.
Every time I get cocky about what (I think) I know, I run into threads like this and regain humility.

Some of the questions you guys (and girls?) are asking demonstrate a tremendous depth of understanding of how these bits of ironmongery work.

What an amazing braintrust.




... now don't get a swelled head.
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Old 10-09-2011, 06:41 PM   #86
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Originally Posted by datchew View Post
this is one of the best threads I've seen in awhile.
Every time I get cocky about what (I think) I know, I run into threads like this and regain humility.

Some of the questions you guys (and girls?) are asking demonstrate a tremendous depth of understanding of how these bits of ironmongery work.

What an amazing braintrust.




... now don't get a swelled head.


Also a good example of the combined knowledge of the internet. You combine all the gurus', as well as the tinkerers' knowledge, and you could fill volumes. Each person adds their little (or large) contribution, and you get something amazing. I'd like to see an airheads wiki.
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Old 10-09-2011, 08:58 PM   #87
DoktorT
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Quote:
Originally Posted by wmax351 View Post
I'd like to see an airheads wiki.
Now I know I have seen something like that somewheres. Maybe IBMWR or something like that.

Here is one down for service.
http://wiki.airheads.org/index.php?title=Main_Page

The there is wikipedia itself

http://en.wikipedia.org/w/index.php?...ofile=advanced
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Old 10-20-2011, 04:48 PM   #88
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Well, here's an update. Took the heads to the local motorcycle repair guy, who does a lot of work on airheads. He checked them out, and says they seem fine. We're going to have him look at the pistons too, but he thinks they should be run-able. We're assuming that at some point, something has been machined down. Rather than figure out what it is, we are just going to fix the problem. We'll start with the low test, 1.2 mm base gaskets, check for piston crown clearance with clay, then measure tdc volume with mineral spirits, and adjust each side to be equal with .5mm high test base gaskets.
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Old 10-20-2011, 07:28 PM   #89
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Sounds like a plan... :)
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Old 10-20-2011, 09:35 PM   #90
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Early (70-75) jugs I'm measuring this: see picture
76 and later are running around 118mm even or thereabouts.
I compared several original jugs. I have many several.
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