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Old 10-03-2011, 05:18 PM   #46
Max Headroom
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If the damage is confined to one side only, it would suggest that either the base of the barrel has been machined or the head has been shaved.

I had a similar situation with an R75/5 earlier this year, and it was resolved simply by installing a 0.020" spacer under the barrel.
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Old 10-03-2011, 05:22 PM   #47
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I agree that it looks like both valves are hitting the piston. If true, this provides a clue. Also, they clearly can't be open very much when it's happening or things would almost certainly be much, much worse.

What kind of valve overlap exists on an airhead? I don't see how the piston could hit closed valves, so wouldn't the evidence indicate the valves might be hitting the piston at the end of the exhaust stroke (when the exhaust is nearly closed and the intake is just starting to open?)

So check the timing first.A couple of teeth off would seem to be the most likely culprit.

Alternatively, would very weak valve springs allow the piston to hit the valves and cause this kind of mark?
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Old 10-03-2011, 06:36 PM   #48
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Quote:
Originally Posted by D-Mac View Post
Alternatively, would very weak valve springs allow the piston to hit the valves and cause this kind of mark?
At high revs only, yes. The intake valve being larger and thereby heavier would be the first to float though. It'd be worth measuring the free and installed lengths of the valve springs because it's easy, but I doubt that's it.
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Old 10-03-2011, 08:47 PM   #49
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Originally Posted by outfit View Post
Dont be silly, why do you turn the crank 360 to do the valve lash on the opposeing side
Cam timing is off 180?

Is it even possible to do that?
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Old 10-03-2011, 09:41 PM   #50
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Cam timing is off 180?

Is it even possible to do that?
No. If you set the cam shaft 180* off the engine will run normal. 180* off is really normal cam timing.

Look. The cam rotates once for every two times the crank rotates. Got that. They don't care which side is at TDC. So if you set the cam mark 180* off the engine will still run normal.

In fact when you changed the chain or whatever and had to set the timing marks? Put them in alignment and then turn crank one rotation, now the cam is 180* off.
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Old 10-03-2011, 10:01 PM   #51
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My R100RS was built by S.J. BMW as a hotrod. It had a set of their factory rocker arms modified for a greater ratio and thus greater lift. It was my first BMW so I wondered why the adjuster end looked modified but didn't recognize it as not stock. I didn't have any interference issues but I was working for Ozzie in Chico at the time and when he saw them he insisted that I remove them.

You have a photo of the rocker arms? It's unlikely but who knows.
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Old 10-03-2011, 10:27 PM   #52
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They were pretty savage.
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Old 10-04-2011, 07:58 AM   #53
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Talked with Max last night.

Look at the pictures...both valves on both sides are hitting the heads.

When the engine was set up,..there was the proper end play for all the valves.

It has a set of SJ push rods installed and they measured out to the same length as the stock ones.

He installed the smaller base gasket,,,the .5mm and not the 1.5mm option.

If the heads were skimmed, I KNOW the machinist always includes a set of washers to put under the rocker arms to keep the correct height..but i don't think these were skimmed.

That doesn't leave much other than weak springs...but I know those are always replaced too.

The thicker base gasket will probably solve the problem but is that really the cause..don't think so since there was proper end play to begin with...the rocker arms would have locked up just trying to set the valve clearances.

Timing gears have been checked at least 3 times for alignment against TDC.

The mystery continues..heads are going to be examined as well as each part of the engine..bound to be something obvious,,I hope.
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Old 10-04-2011, 09:40 AM   #54
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One consideration from Ted Porter,
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Originally Posted by Ted Porter
I would have a lot of questions about this assembly. Those look like the special high compression 750 pistons, is that true? Is the cam stock? I would also want to look at the depth of the valves in the head. Obviously you need some clearance somewhere. ;)



Do those look like stock pistons? I'm not sure. They're .5 over, 82.465 mm. That puts them .5 over on a R75/7, but still not 100%.
They are first oversize, "A" bore group. The domes do look big, bigger than what I pulled out of my R75 last year IIRC. I might be able to post a better pic for comparison tonight. This is a first OS, "B" bore, 82.475mm:



Hard to see a difference sitting in the ring squeezer, but there might be one.
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Old 10-04-2011, 10:50 AM   #55
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Quote:
Originally Posted by Guru
Talked with Max last night.

Look at the pictures...both valves on both sides are hitting the heads.
If the cam, etc was set up right, could the crankshaft be a later on with a longer stroke? What size flywheel bolts and type flywheel does it have? Don't know for sure, just grabbing at straws.

I'm thinking it will be ultimately related to it being a Beemer In A Box and Frankenbeemering parts together.
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Old 10-04-2011, 11:20 AM   #56
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Clay the valves for clearance on assembly and these things won't surprise you. Too many unknowns unless you know all the parts history. Guessing leads to disaster.
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Old 10-04-2011, 11:40 AM   #57
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Clay? I rather use solder. On a beemer you can measure valve clearance with solder through the spark plug hole. You can put it together for keeps and only have to take it back apart if there is a clearance issue. Plus solder is much easier to get a caliper around and accurately measure down to the thousandths.

Most of us beemer guys would call this issue highly unusual. All original stock BMW parts? No need to measure.
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Old 10-04-2011, 12:01 PM   #58
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Quote:
Originally Posted by Beemerguru View Post

It has a set of SJ push rods installed and they measured out to the same length as the stock ones.

The mystery continues..heads are going to be examined as well as each part of the engine..bound to be something obvious,,I hope.
Try fitting a pair of standard BMW pushrods. I know of another recently rebuilt engine which had issues with rough idling and loss of power when hot, yet it ran fine when cold. It was also fitted with a set of SJ pushrods, and eventually it was discovered that they were growing when hot and the valve clearance would close up.It wasn't an obvious problem, and kept folk guessing for quite a while.

Between that and a thicker base gasket, most of the issues would be sorted.
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Old 10-04-2011, 12:02 PM   #59
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Quote:
Originally Posted by Bill Harris View Post
If the cam, etc was set up right, could the crankshaft be a later on with a longer stroke? What size flywheel bolts and type flywheel does it have? Don't know for sure, just grabbing at straws.

I'm thinking it will be ultimately related to it being a Beemer In A Box and Frankenbeemering parts together.
The only crank with a different throw is the R65 - and it's shorter. The flywheel bolt size only affected which flywheel it would accept.

Mixing parts doesn't necessarily cause problems, but in this case, some un-noted change was made previously that must be tracked down by careful observation, comparison to standard parts, and measurements.

At this time I don't see any other possibilities other than already mentioned. All the likely suspects have been rounded up and are currently getting their mug shots and finger prints.
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Old 10-04-2011, 12:09 PM   #60
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Try fitting a pair of standard BMW pushrods. I know of another recently rebuilt engine which had issues with rough idling and loss of power when hot, yet it ran fine when cold. It was also fitted with a set of SJ pushrods, and eventually it was discovered that they were growing when hot and the valve clearance would close up.It wasn't an obvious problem, and kept folk guessing for quite a while.

Between that and a thicker base gasket, most of the issues would be sorted.
My NOT SJ 4130 pushrods were doing just the opposite. They "shrunk".
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