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Author Topic: Sound advice please!  (Read 3272 times)

Offline crazypj

  • Posts: 12483
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Re: Sound advice please!
« Reply #70 on: Jul 22, 2017, 21:04:10 »
There are plenty of diagrams for modified pistons in CB750's. Majority have a 15 degree angle to top but personally I would measure combustion chamber  angle and probably make a cutter to machine them all equal (I did it with my CJ250 - 391cc motor, 14mm overbore  8) )
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Offline quadracer351

  • Posts: 526
Re: Sound advice please!
« Reply #71 on: Jul 24, 2017, 05:08:03 »
The problem is that the piston crowns are the wrong shape and don't match the combustion chamber shapes.  Adding thicker gaskets is not a very good solution to that.  better to rub bearing blue or even thick grease over the piston crowns and put the head on and turn it over to see where it is making contact and then machine teh pistons or head as appropriate to fix the problem.

You are looking for zero clearance without a head gasket and .040' with a gasket.  So test it without a head gasket and massage metal until it turns over without a clunk at TDC, then clean it up and fit a head gasket.
There are plenty of diagrams for modified pistons in CB750's. Majority have a 15 degree angle to top but personally I would measure combustion chamber  angle and probably make a cutter to machine them all equal (I did it with my CJ250 - 391cc motor, 14mm overbore  8) )
Help me out with this. Won't taking material off the piston make it weaker? And are you talking about machining the heads a few mm where the piston is making contact or the whole head? Like I said out only looks like it's cylinders 2 and 4 because of the spark plug orientation. Would putting a slightly thicker gasket on it have a negative effect or it's simply not the best fix for it?

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Offline hillsy

  • Posts: 4094
Re: Sound advice please!
« Reply #72 on: Jul 24, 2017, 06:58:46 »
Easiest fix is to add another base gasket. You will lose some compression but the goal is to stop metal bits hitting each other.

Offline teazer

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Re: Sound advice please!
« Reply #73 on: Jul 24, 2017, 12:02:22 »
It all depends where they are touching, but I would expect that they just need a touch off the "top corner" of the crown.  I have had many pistons machined to fit into combustion chambers they were never designed to fit.  The original CB750 64mm pistons were CB350 with valve pockets machined into them and crown reshaped. 

I suggested earlier wiping a thin layer of any dark grease over the crowns fit the head and look at the witness marks. It should be fairly clear where they clash and what needs to be done.  They can probably be touched up with a dremmel and sanding drum if the interference is small.  You want to be able to turn the motor over without a head gasket and without any clash. 

Online datadavid

  • Posts: 1120
Re: Sound advice please!
« Reply #74 on: Jul 24, 2017, 12:10:18 »
Help me out with this. Won't taking material off the piston make it weaker? And are you talking about machining the heads a few mm where the piston is making contact or the whole head? Like I said out only looks like it's cylinders 2 and 4 because of the spark plug orientation. Would putting a slightly thicker gasket on it have a negative effect or it's simply not the best fix for it?

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Hold on, did you say it is only touching 2 of the plugs? And you are 1000% sure you are using the correct length plugs?

Offline quadracer351

  • Posts: 526
Re: Sound advice please!
« Reply #75 on: Jul 24, 2017, 14:34:56 »
Hold on, did you say it is only touching 2 of the plugs? And you are 1000% sure you are using the correct length plugs?
It's not touching the plug. What I meant was it's only hitting the heads in cylinders 2 and 4 because of the orientation of those heads because of where the plug goes in. 1 and and 3 may possible be hitting the sleeve for the exhaust valves or that could be old. I'm going to do the grease test as you suggested and check everything else down the way that might have been damaged by this.

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Offline quadracer351

  • Posts: 526
Re: Sound advice please!
« Reply #77 on: Jul 24, 2017, 15:16:35 »
It might be a dumb question, but how do the bores on the head gasket fit?
.045 mm thick
2.59mm bore
2.50 mm cylinder bore

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Offline teazer

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Re: Sound advice please!
« Reply #78 on: Jul 24, 2017, 15:53:49 »
Plugs in that motor are long reach ie 3/4" but some plugs have projecting electrodes that protrude into the chamber.  What the photos appear to show is a clear interference on one cylinder and a less dramatic one on the other.

In theory the interference would be the same both sides of the chamber on all 4 heads but the bores are probably not concentric. 

I assume that your "mm" measurements are really Inches.  ie 64mm bore in barrel and .045" gasket with 66mm bore.
« Last Edit: Jul 24, 2017, 16:01:10 by teazer »

Offline 1fasgsxr

  • Posts: 1072
Re: Sound advice please!
« Reply #79 on: Jul 24, 2017, 16:12:09 »
Im following this..love to learn