Sound advice please!

Sorry. My brain was thinking pushrod when you said connecting rod. You do have connecting rods.
 
irk miller said:
Sorry. My brain was thinking pushrod when you said connecting rod. You do have connecting rods.
Lol no problem does it sound like it could be the con rods?

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Have the motor pulled again we are going to be checking the piston clearances and the con rods. The valve cover came off again and again everything up there was triple checked and everything was fine

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Ticking problem solved! Wrong gasket was put on, .040'm was on when we're thinking I need a .060~.070I'm because of the profile of the Pistons. Pistons 4 and 2 were making contact with the sides of the dome and the sleeves around the exhaust valves. I have some plastiguage in the mail and need to see if anyone on this forum is making custom thickness gaskets if not I'll just get one online
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quadracer351 said:
Ticking problem solved! Wrong gasket was put on, .040'm was on when we're thinking I need a .060~.070I'm because of the profile of the Pistons. Pistons 4 and 2 were making contact with the sides of the dome and the sleeves around the exhaust valves. I have some plastiguage in the mail and need to see if anyone on this forum is making custom thickness gaskets if not I'll just get one online

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Contact Lani at Copper Gaskets Unlimited... http://www.coppergaskets.us/ . He can make them in a plethora of thicknesses. Give him a call and talk through it. I put his gaskets on all of my machines, especially the ones I have bored out.
 
irk miller said:
Contact Lani at Copper Gaskets Unlimited... http://www.coppergaskets.us/ . He can make them in a plethora of thicknesses. Give him a call and talk through it. I put his gaskets on all of my machines, especially the ones I have bored out.
Love it, thank you!

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irk miller said:
Contact Lani at Copper Gaskets Unlimited... http://www.coppergaskets.us/ . He can make them in a plethora of thicknesses. Give him a call and talk through it. I put his gaskets on all of my machines, especially the ones I have bored out.
Love it, thank you!

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there should be no touching with the head gasket out. I don't understand why 2 pistons are touching the head.Does the combustion chamber need to be opened up to suit the larger pistons or are the bores and head not concentric? That's not as uncommon as you might think.

With zero clearance, a .040" (1.0mm) gasket should be fine. If there is "negative clearance", that should be fixed and not fudged with a thick gasket. Bikes usually have zero to 1.0mm negative deck height. In other words the edge of the piston is at or below the deck at TDC. If yours are too high, investigate why. Conrods rarely have that much difference and pistons made in the last decade or more should close to identical.

If the combustion chambers are non concentric with the bores, they can be relieved gently with a dremmel on the side that hangs over the bore.
 
teazer said:
Conrods rarely have that much difference and pistons made in the last decade or more should close to identical.
DOHC rods interchange, but they're 1mm shorter. CB900 and CB1100 rods interchange, but they're 1.5mm longer. All three cranks interchange as well, but with different strokes. I don't know what work was done to the motor, but there are interchanges that effect those numbers. Is it possible that the wrong combo of rods and pistons is at play here?
 
Now that makes sense. If they replaced a rod or two with 900 or 1100 rods, thinking they are the same length, that would produce this result. If so, they need to make it right. And not with ridiculously thick gaskets.
 
If you need to run a thicker gasket it should be the base gasket, not the head gasket.


But as teazer said, I'd be worried as to why only 2 cylinders have this issue and fix the problem from that aspect.
 
All the cylinders show signs of interference. The con rods were all checked and are all stock. The piston kit I purchased is 3 mm bigger than stock with a different head profile. The Pistons are trw 811. All the cylinders show contact around the sleeves for the exhaust valves with cylinders 2/4 showing a little more contact along the side of the dome. But those cylinders are the same shape because of where the plug is situated in them.
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Those are odd looking pistons. A SOHC 750 has inlets and exhaust valves offset and as a result the valve pockets are offset from the center line. The crown on most 811 kit pistons is more of a gentle curve. Maybe because teh original 64mm pistons were machined out of CB350 OEM stocks.

The crown shape and valve cutouts look wrong, but I don't claim to recognize ever piston ever made. Those look like they were manufactured a while ago. The head profile doesn't look as if it matches a SOHC head, so it's a bit of a puzzle as to what they were supposed to fit. TRW don't seem to list anything in their current (Federal-Mogul) catalog for a SOHC.

The crowns look tall compared to an average big bore SOHC piston so maybe they were a very high comp race piston or something.

What do we know about those pistons?
 
teazer said:
Those are odd looking pistons. A SOHC 750 has inlets and exhaust valves offset and as a result the valve pockets are offset from the center line. The crown on most 811 kit pistons is more of a gentle curve. Maybe because teh original 64mm pistons were machined out of CB350 OEM stocks.

The crown shape and valve cutouts look wrong, but I don't claim to recognize ever piston ever made. Those look like they were manufactured a while ago. The head profile doesn't look as if it matches a SOHC head, so it's a bit of a puzzle as to what they were supposed to fit. TRW don't seem to list anything in their current (Federal-Mogul) catalog for a SOHC.

The crowns look tall compared to an average big bore SOHC piston so maybe they were a very high comp race piston or something.

What do we know about those pistons?
They are no longer made found them online a few years ago when looking for Pistons. They were manufactured in the late 70s early 80s I believe. Up until last year I was still finding them randomly for sale online. I'll see if I can find anything else about them
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A little Google-fu shows that although TRW brand pistons are no longer produced, the company SpeedPro took over. Maybe you can contact them for some more info.
 
irk miller said:
Your CB750 doesn't have connecting rods.

What connects the pistons to crank? ;)
The piston height should have been checked before the head was fitted, one of the guys doing a CB378cc conversion is having similar problem due to production variations. I've done a few 378 motors and didn't need to get 'special' gaskets but obviosly there are dimensional differences between various year motors.
You either machine piston tops or fit thicker base gasket to get a total clearance of 0.038"~0.040" piston to head clearance. CB750 motor weighs 210lbs and IS a PITA to get in and out of frame all the time (no way could I lift one out on my own like I used to at 21 :( )
 
LOL, your welcome ;) 8)
I deleted about 1300 posts not too long after I joined
 
crazypj said:
What connects the pistons to crank? ;)
The piston height should have been checked before the head was fitted, one of the guys doing a CB378cc conversion is having similar problem due to production variations. I've done a few 378 motors and didn't need to get 'special' gaskets but obviosly there are dimensional differences between various year motors.
You either machine piston tops or fit thicker base gasket to get a total clearance of 0.038"~0.040" piston to head clearance. CB750 motor weighs 210lbs and IS a PITA to get in and out of frame all the time (no way could I lift one out on my own like I used to at 21 :( )
This motor has been out 3 times this month. I measured all the cylinders at tdc across the head and I got the same measurement for all. I honestly think it's the way the domes are situated in 2 and 4 that is causing a little interference. I had the plastguage come today but won't be trying it till later this week. Hopefully that will give me a better idea of the clearances through out the piston head. As far as the gasket goes it seems the consensus is the base gasket will be the better of the two to go a little thicker on. Guess I'll be needing two in the end since the head one is also custom for the larger piston diameter.

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