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Old 07-27-2013, 02:42 PM   #1
Humboldt Airhead OP
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Thick base gasket

Hi!
I'm putting together a 74 R90/6 motor and was wondering if I should use the thick base gaskets. When I took it apart it had 2 thin gaskets.
The motor does not have dual plugs and is stock. I did a valve job on the heads and the pistons are nice and clean.

Thanks!
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Old 07-27-2013, 02:48 PM   #2
disston
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My 1975 R90/6 runs fine on one stock gasket. 2nd oversized pistons. Stock ignition points. Mechanical advance. Regular gas.
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Old 07-28-2013, 02:03 AM   #3
Bill Harris
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Quote:
When I took it apart it had 2 thin gaskets.
Does that mean two thin (normal) base gaskets, sandwiched, per cylinder or two all together (one per cylinder)?

If the former, put it back with a thicker (low-test) base gasket. Unless you are honing the cylinders, there may be a slight ridge at the top of the cylinder and the thinner base gasket may allow the piston ring to bump this ridge.

And the previous owner probably had thicker base gaskets for some reason...

--Bill
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Old 07-28-2013, 09:24 AM   #4
Humboldt Airhead OP
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Yes, it had 2 thin gaskets per side. The pistons were soda blasted and cylinders were honed and new rings are going in.
I do not know the history of the bike but from what I have found it has been the R50 & R60 that had most of the trouble back then. My old R75/5 ran great with stock base gaskets.

Quote:
Originally Posted by Bill Harris View Post
Does that mean two thin (normal) base gaskets, sandwiched, per cylinder or two all together (one per cylinder)?

If the former, put it back with a thicker (low-test) base gasket. Unless you are honing the cylinders, there may be a slight ridge at the top of the cylinder and the thinner base gasket may allow the piston ring to bump this ridge.

And the previous owner probably had thicker base gaskets for some reason...

--Bill
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Old 07-28-2013, 01:58 PM   #5
Bill Harris
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Then give the normal base gaskets a try. If you get pinging it's (relatively) not a big deal to change out the base gaskets. The R60 has an odd camshaft figure that increases the effective compression and makes pinging a real problem.

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Old 07-28-2013, 02:25 PM   #6
Plaka
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Quote:
Originally Posted by Humboldt Airhead View Post
Hi!
I'm putting together a 74 R90/6 motor and was wondering if I should use the thick base gaskets. When I took it apart it had 2 thin gaskets.
The motor does not have dual plugs and is stock. I did a valve job on the heads and the pistons are nice and clean.

Thanks!
Standard gaskets should be fine. If you are planning a lot of trips to Mexico, etc. then lowering the compression might be a good idea.
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Old 07-28-2013, 04:49 PM   #7
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Thanks for the help. The bike belongs to my nephew that lives near Seattle WA and I'm in N. CA. so I can't be of much help if it starts to ping after he gets home.

I'm thinking of finding the Step 1 gasket that is .028 thick to be on the safe side.
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