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Author Topic: Rusted Piston cb750  (Read 1362 times)

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Bill

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Rusted Piston cb750
« on: February 09, 2008, 08:42:06 PM »
My name is Bill Tew. I am in Jackson, MS. My son brought me a 74 cb750 that he bought for 80 bucks. My Dad had the same bike and my son thought it would be cool to restore one. Anyhow we are finished with the tear down and are sending stuff off for painting etc...And now we are working on the motor. Totally locked up. I've read the other threads about rusted pistons and am several days into the soaking and tapping phase. Where my situation is different is that the cylinders are about half off and the other 3 pistons have dropped free. The remaining piston is at the bottom of the cylinder and about 1/16 inch is showing below the cylinder. Any additional ideas for this situation?
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Online Terry in Australia

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Re: Rusted Piston cb750
« Reply #1 on: February 09, 2008, 09:21:25 PM »
Is there movement in the crank Bill? If so, put some softwood blocks under the cylinder block and on top of the cases, and using another block of wood as a drift, tap that sucker free! You're probably gonna be up for a new set of pistons and a re-bore anyway, but I wouldn't use anything harder. Cheers, Terry. ;D
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Offline Aaron J Williams

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Re: Rusted Piston cb750
« Reply #2 on: February 09, 2008, 09:24:23 PM »
Well, you could try a gentle application of heat to the bottom of the cylinder liner hopefully expanding it to allow the piston to come out or you could lift the cylinders up enough to slide wood blocks in on both sides of the froze cylinder between the cylinder and the upper engine case and try tapping on the piston some more. Maybe you could try tapping the piston back up in the cylinder and spraying lube on the under side of the cylinder. If you can tap the piston back and forth you will eventually be able to remove it. It is a very frustrating process but it can be done. Good luck!
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Offline KB02

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Re: Rusted Piston cb750
« Reply #3 on: February 10, 2008, 08:48:59 AM »
With my bike (with a rusted and stuck piston) I ended up taking the lower case half off and disconnected the connecting rods. Like the guys said, with a rusted and stuck piston, you're looking at new pistons anyway, so there's no need to be too gentle with the old one.
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Offline TwoTired

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Re: Rusted Piston cb750
« Reply #4 on: February 10, 2008, 10:39:43 AM »
Have you tried letting it soak in penetrating oil for a day or two?

Mix Auto transmission fluid 50/50 with acetone.  Pretty good penetrating fluid.

Put a wood block on top of the piston and give it a couple of sharp blows to break it free.  If the piston is at BDC, also block up the cylinder block, so the crank can rotate once the piston rings have broken free of the cylinder. The takes stress off the con rod, wrist pin, etc.

Cheers,
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Offline crazypj

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Re: Rusted Piston cb750
« Reply #5 on: February 10, 2008, 10:55:33 AM »
If 3 are out is it number 1 still stuck? (far left)
 need to be careful not to bend rod.
 Heat, wood blocks and  BFH are going to be needed as previously mentioned.
PJ
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Online Terry in Australia

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Re: Rusted Piston cb750
« Reply #6 on: February 10, 2008, 12:13:06 PM »
Quote from: crazypj on February 10, 2008, 10:55:33 AM
If 3 are out is it number 1 still stuck? (far left)
 need to be careful not to bend rod.
 Heat, wood blocks and  BFH are going to be needed as previously mentioned.
PJ

Hey Peej, I haven't got a BFH, but I've got a BMFH, would that work OK? ;D
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I was feeling sorry for myself because I couldn't afford new bike boots, until I met a man with no legs.

So I said, "Hey mate, you haven't got any bike boots you don't need, do you?"

"Crazy is a very misunderstood term, it's a fine line that some of us can lean over and still keep our balance" (thanks RB550Four)

Bill

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Re: Rusted Piston cb750
« Reply #7 on: February 10, 2008, 10:25:11 PM »
It's out!!! ;D

I put wood under the cylinders to keep them level and used a porta power hydraulic spreader.  A friend had it. He paid $75 for it. It has several attachments for jacking things much larger apart. It just uses a small hand operated cylinder. Anyway, I would press up ward on the bottom of the cylinder then hammer on the piston from above with a rubber hammer and piece of wood. I did this on 3 sides (piston 4) and it gradually came out. Very minimal scoring of the case. I didn't use the max force.

Thanks to all for the help.

Bill
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