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Old 09-28-2013, 06:19 AM   #1
StephenB OP
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Modifying post-75 /6 barrels (w/ white o-ring) to fit /5 block

This is a bridge I haven't crossed before:

What I have is a /6 barrel with 99mm spigot and a white o-ring (a "C", a "6" and Mahle imprint with a "77" embossed on the foot of the barrel) and a /5 block with 97mm.

Is this "just" a case of machining the spigot (removing 2mm on the diameter) and everything fits the /5 block as it should? Could I would even go as far as re-cutting the groove for the o-ring and use o-ring and gasket?

Are these good dimensions to use for the machinist:

- turn spigot down to 96.9mm
- re-cut o-ring groove 2mm wide and 1.7mm deep at same location (I'll use the 93x2 o-ring)
- leave overall barrel length alone

Is there something else I should be aware of?

Thanks in advance.

Stephen
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'72 R75/5 The Blues

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Old 09-28-2013, 07:01 AM   #2
JonnyCash
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I don't have any advice for fitting them to you /5 bottom end, but it sounds to me like those are /7 jugs you have there, with 77 cast into them.
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Old 09-28-2013, 07:10 AM   #3
Plaka
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Quote:
Originally Posted by StephenB View Post
This is a bridge I haven't crossed before:

What I have is a /6 barrel with 99mm spigot and a white o-ring (a "C", a "6" and Mahle imprint with a "77" embossed on the foot of the barrel) and a /5 block with 97mm.

Is this "just" a case of machining the spigot (removing 2mm on the diameter) and everything fits the /5 block as it should? Could I would even go as far as re-cutting the groove for the o-ring and use o-ring and gasket?

Are these good dimensions to use for the machinist:

- turn spigot down to 96.9mm
- re-cut o-ring groove 2mm wide and 1.7mm deep at same location (I'll use the 93x2 o-ring)
- leave overall barrel length alone

Is there something else I should be aware of?

Thanks in advance.

Stephen
"C" is the tolerance grade. You must also use C pistons.

Some prefer to machine the block. Then it fits lots of things in the future.

What CC jugs?

What do you have left for thickness after you turn it down AND cut an o-ring groove?
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Old 09-28-2013, 08:44 AM   #4
StephenB OP
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Plaka,

These are 600cc jugs. Don't want to machine the block, like to keep that original.

With 97mm OD at the foot of the barrel and a 73mm piston bore, the wall thickness is about 1/2in. Taking 1mm off and cut a 1.7x2mm groove shouldn't be problem.

I should have about 10mm of meat left over at the groove.
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Stephen Bottcher
Ontario, Canada

'72 R75/5 The Blues

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Old 09-28-2013, 09:01 AM   #5
Plaka
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Originally Posted by StephenB View Post
Plaka,

These are 600cc jugs. Don't want to machine the block, like to keep that original.

With 97mm OD at the foot of the barrel and a 73mm piston bore, the wall thickness is about 1/2in. Taking 1mm off and cut a 1.7x2mm groove shouldn't be problem.

I should have about 10mm of meat left over at the groove.
I asked about the CCs because depending on cost, it might make sense to trade around for some 60/5 jugs. If you aren't in a hurry, you could possibly do it for almost no money (some shipping?). Also depends on what you have going on in pistons.

You might call around to some of the regular bone yards too. The /6 might be a more salable item for them than the /5. Dunno. or maybe they would take them on trade in.

Do you already have some pistons?

are you going down from 750 jugs?
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Old 09-28-2013, 09:22 AM   #6
StephenB OP
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Plaka,

I have the original matching "C" pistons and wrist pins and the barrel is already honed. Machining doesn't cost me much and my guy is local, so modifying the barrels is my "sensible" approach. Don't wanna mess around with trading/shipping as this is already a "deal gone bad".

The project is now in the "must complete before winter" mode.
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Stephen Bottcher
Ontario, Canada

'72 R75/5 The Blues

www.stephenbottcher.net
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Old 09-28-2013, 09:25 AM   #7
Plaka
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Quote:
Originally Posted by StephenB View Post
Plaka,

I have the original matching "C" pistons and wrist pins and the barrel is already honed. Machining doesn't cost me much and my guy is local, so modifying the barrels is my "sensible" approach. Don't wanna mess around with trading/shipping as this is already a "deal gone bad".

The project is now in the "must complete before winter" mode.
Yeah, I got a heated shop too, if I dress warm enough. it was winter here this morning. Frost on the squash.
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Old 09-28-2013, 09:36 AM   #8
StephenB OP
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Plaka, do you have by any chance a /7 barrel where you can measure the o-ring groove dimensions?
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Some of the above is fact, some is fiction, some is my personal imagination and some is just simple truth. [me]

... i'm not touring around the world, but neither are most of the guys i see running overloaded spam cans ... [bmwblake]

Stephen Bottcher
Ontario, Canada

'72 R75/5 The Blues

www.stephenbottcher.net
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Old 09-28-2013, 10:38 AM   #9
Plaka
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Originally Posted by StephenB View Post
Plaka, do you have by any chance a /7 barrel where you can measure the o-ring groove dimensions?
Circa 1984:

2.75 mm wide x hmmm..getting from .75mm to 1.00 mm deep. It's the ragged edge of where my caliper will fit. If you know the o-ring cross section you can look it up. it's standard. Anton mentions the ratio on his oil filter canister pages I believe but you can just Google it too and get a table. This is a static fit and figure the durometer is 65 shore A in BUNA-N.

The spigot has three different diameters. Starts small, about halfway it steps up, then the o-ring groove, then it steps up on the other side of that.

I can measure it maybe. Biggest mic. I have is for pistons. Never tried cylinder bases.

I can mail you the profile if you can wait a week or so to get it.

An X ring would be sweet if you can find one.

It is easy to pad out the groove if you aren't getting the compression you want on your ring. it is difficult to make the ring smaller however.

edit: The hole in the block is stepped as well. The /5 block simply has a chamfer leading into the bore, the /7 has a small chamfer, then a step, then a big chamfer leading into the bore. The o-ring is getting compressed 2 ways.

Clarification: The groove is deeper on one side than the other because the spigot diameter steps up immediately following the groove (just eyeballing it)

Plaka screwed with this post 09-28-2013 at 10:56 AM
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Old 09-28-2013, 07:09 PM   #10
bmwrench
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Long ago, in a universe far away, I built several motors with cast-off R80 Nikasil cylinders and small hole cases. We had the spigots on the cylinder turned to size by a local machine shop. I didn't bother trying to find an o-ring to fit the smaller spigot.
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