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Offline Don R

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big bore 750 questions
« on: November 23, 2011, 11:18:57 AM »
I have my K0 apart for a freshening, paint and powder coat. I want to pop the head off and put on a fresh ported one with new valves etc. We don't know the bore size, the PO referred to it by different numbers, 836, 900, 936 so who knows? It dies run hard though.
 The question is what's the best head gasket for it? I'm planning on heavy studs and I have a drop out frame kit so I can re-torque later. I'm guessing I need to pull the cylinder also to replace the base gasket. I would like to have a look at the pistons too.
 Ideas or opinions?
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Offline Sam Green Racing

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Re: big bore 750 questions
« Reply #1 on: November 23, 2011, 12:27:09 PM »
Don, just measure your bores.

65mm=836
67mm=888
67.5mm=901
68mm=915
68.75=936

Sam. ;)
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Offline Don R

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Re: big bore 750 questions
« Reply #2 on: November 23, 2011, 01:15:19 PM »
Thanks for the chart, I'll measure it when I get the head off.  Experience has taught me not to tear it apart until I am ready to put it together.
« Last Edit: November 23, 2011, 01:25:14 PM by Don R »
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Offline Retro Rocket

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Re: big bore 750 questions
« Reply #3 on: November 23, 2011, 02:43:24 PM »
Quote
The question is what's the best head gasket for it?

As far as i know, the best head gaskets are the MLS gaskets that CycleX sell, the only problem is they are in limited sizes...

http://www.cyclexchange.net/High%20Performance.htm

Talk to Mike Rieck about them.... ;)
« Last Edit: November 23, 2011, 02:51:54 PM by Retro Rocket »
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Offline Retro Rocket

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Re: big bore 750 questions
« Reply #4 on: November 23, 2011, 02:59:15 PM »
Quote from: SamCB750A on November 23, 2011, 12:27:09 PM
Don, just measure your bores.

65mm=836
67mm=888
67.5mm=901
68mm=915
68.75=936

Sam. ;)

Hmmmm, i was told that 67mm was 900 and 67.5 was 915..

This is from Axels site..

61.00 (stock)    736
61.25 (1st oversize)    742
61.50 (2nd oversize)    749
61.75 (3rd oversize)    755
62.00 (4th oversize)    761
64.00    812
64.50    823
65.00    836
67.00    895
67.50    915
70.00    970
73.00    1060

http://www.satanicmechanic.org/bigbore.shtml
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Offline Sam Green Racing

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Re: big bore 750 questions
« Reply #5 on: November 23, 2011, 03:24:06 PM »
Well someone is wrong Mick........anybody ???........hope it's not me.  ;D ;D ;D ;D ;D

Sam. :-\
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Offline Sam Green Racing

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Re: big bore 750 questions
« Reply #6 on: November 23, 2011, 03:25:48 PM »
P.S. the calculator only cost £3.50 and it was made in China.  ;D ;D ;D ;D ;D ;D
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Offline MCRider

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Re: big bore 750 questions
« Reply #7 on: November 23, 2011, 03:35:07 PM »
This calculator is free. 67mm = 888.8, I round down to the crazy 8s (what i had/have.)

http://www.nightrider.com/biketech/calc_displacement.htm

stroke is 63mm.

Measure yours and get a made to fit gasket from Cometic.  http://www.cometic.com/

Other places make head cheeze as well. ASk around . MRieck forum memeber, Cycle X, etc.
« Last Edit: November 23, 2011, 03:38:08 PM by MCRider »
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Offline Retro Rocket

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Re: big bore 750 questions
« Reply #8 on: November 23, 2011, 03:40:17 PM »
Quote from: SamCB750A on November 23, 2011, 03:25:48 PM
P.S. the calculator only cost £3.50 and it was made in China.  ;D ;D ;D ;D ;D ;D

 ;D ;D ;)
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Offline mick7504

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Re: big bore 750 questions
« Reply #9 on: November 23, 2011, 05:20:26 PM »
Quote from: SamCB750A on November 23, 2011, 12:27:09 PM
Don, just measure your bores.

65mm=836
67mm=888
67.5mm=901
68mm=915
68.75=936

Sam. ;)
Quote from: SamCB750A on November 23, 2011, 03:24:06 PM
Well someone is wrong Mick........anybody ???........hope it's not me.  ;D ;D ;D ;D ;D

Sam. :-\
Quote from: SamCB750A on November 23, 2011, 03:25:48 PM
P.S. the calculator only cost £3.50 and it was made in China.  ;D ;D ;D ;D ;D ;D
That's a good Chinese calculator that you've got there Sam.
It's right on the money with those sizes.  :)
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Offline MRieck

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Re: big bore 750 questions
« Reply #10 on: November 23, 2011, 09:00:26 PM »
I believe 67.5 is 915cc. But that's what Wiseco has always said.
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Offline Retro Rocket

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Re: big bore 750 questions
« Reply #11 on: November 23, 2011, 09:05:19 PM »
Quote from: MRieck on November 23, 2011, 09:00:26 PM
I believe 67.5 is 915cc. But that's what Wiseco has always said.

Thats what i have always thought as well.... :o
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Offline MCRider

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Re: big bore 750 questions
« Reply #12 on: November 23, 2011, 09:10:00 PM »
67.5 = 902.156  per my calculator

Another on line calculator puts it at 901.8

I'd say the 915cc is rounding up.

68mm is 915.1
« Last Edit: November 23, 2011, 09:19:03 PM by MCRider »
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Offline mick7504

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Re: big bore 750 questions
« Reply #13 on: November 23, 2011, 09:23:04 PM »
Quote from: Retro Rocket on November 23, 2011, 09:05:19 PM
Quote from: MRieck on November 23, 2011, 09:00:26 PM
I believe 67.5 is 915cc. But that's what Wiseco has always said.

Thats what i have always thought as well.... :o
Well, using the formula to calculate the volume of a cylinder, 901 is the answer.
(Unless I've got a dodgy Chinese calculator like Sam has.)  ;D

67.5 /2 = 33.75 (Radius)
33.75 x 33.75 = 1139.06 (Radius squared)
Radius squared x Pi x Stroke x Cylinders (4)
1139.06 x 3.14 (22/7) x 63 x 4 = 901.........


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Re: big bore 750 questions
« Reply #14 on: November 23, 2011, 09:26:30 PM »
Oh well, looks like my 900 kit just shrank..... ;D 888 it is.....
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Offline MCRider

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Re: big bore 750 questions
« Reply #15 on: November 23, 2011, 09:38:17 PM »
Quote from: Retro Rocket on November 23, 2011, 09:26:30 PM
Oh well, looks like my 900 kit just shrank..... ;D 888 it is.....
Yeah the 888s were often marketed as 900s. Its OK you can round up. Honda did (736 = 750)
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Offline mick7504

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Re: big bore 750 questions
« Reply #16 on: November 23, 2011, 09:40:59 PM »
Quote from: Retro Rocket on November 23, 2011, 09:26:30 PM
Oh well, looks like my 900 kit just shrank..... ;D 888 it is.....
It'll still kick butt mate.  ;D
I've always liked the 900/888 engines.
I look at it like this.
They can be like a little terrier sneaking in for a bite at the ankles and then taking off before you can catch 'em.  8)
« Last Edit: November 23, 2011, 10:35:13 PM by mick7504 »
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Re: big bore 750 questions
« Reply #17 on: November 23, 2011, 09:51:18 PM »
Its ok guys, still got the 70 {970} and the 71 [1000}, they seem to be a little more resilient and don't shrink much.... ;D ;)
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Offline Sam Green Racing

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Re: big bore 750 questions
« Reply #18 on: November 24, 2011, 01:24:54 AM »
Quote from: Retro Rocket on November 23, 2011, 09:26:30 PM
Oh well, looks like my 900 kit just shrank..... ;D 888 it is.....

Well I'm glad I've convinced you Auzzies, now I just need to work a bit on young Murphy up in Boston. ;D ;D ;D

Sam. ;) ;D
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Offline Big Jay

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Re: big bore 750 questions
« Reply #19 on: November 27, 2011, 07:40:41 PM »
I don't have my notes available, but when I did the original 915 kits, we used a piston that Hap Jones had that was a big bore for a CB350 twin. I believe it was 68mm.
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Offline Sam Green Racing

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Re: big bore 750 questions
« Reply #20 on: November 28, 2011, 03:08:32 AM »
Well it must have been a 68 if it was a 915 kit. ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D

Sam. ;)
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Offline fang

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Re: big bore 750 questions
« Reply #21 on: November 28, 2011, 04:45:52 PM »
I have a few old Henry Abe "900cc Kits."  They are supposed to be an actual 895cc's, and my pistons measure approximately 66.8mm.

By my math, a 66.8mm bore would be closer to 882.7cc, but I'm a lover, not a mathematician. 

Peace and grease,
Fang
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Offline MCRider

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Re: big bore 750 questions
« Reply #22 on: November 28, 2011, 04:53:49 PM »
Quote from: fang on November 28, 2011, 04:45:52 PM
I have a few old Henry Abe "900cc Kits."  They are supposed to be an actual 895cc's, and my pistons measure approximately 66.8mm.

By my math, a 66.8mm bore would be closer to 882.7cc, but I'm a lover, not a mathematician. 

Peace and grease,
Fang
Did you measure them in the direction of the pin, or 90° out? One way should be slightly larger than the other, pistons are not round. Anyway, 895 doesn't divide down into anything if we restrict ourselves to .5mm increments. 67mm is 888.9, which was commonly called a 900 kit.

What a few ccs between friends?  ;)
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Offline fang

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Re: big bore 750 questions
« Reply #23 on: November 28, 2011, 05:49:33 PM »
Quote from: MCRider on November 28, 2011, 04:53:49 PM
Quote from: fang on November 28, 2011, 04:45:52 PM
I have a few old Henry Abe "900cc Kits."  They are supposed to be an actual 895cc's, and my pistons measure approximately 66.8mm.

By my math, a 66.8mm bore would be closer to 882.7cc, but I'm a lover, not a mathematician. 

Peace and grease,
Fang
Did you measure them in the direction of the pin, or 90° out? One way should be slightly larger than the other, pistons are not round. Anyway, 895 doesn't divide down into anything if we restrict ourselves to .5mm increments. 67mm is 888.9, which was commonly called a 900 kit.

What a few ccs between friends?  ;)

I measured the actual cylinder bore with a crappy digital micrometer, and it said 66.8mm. 
Then I went back and re-measured with a reasonably nice Fowler bore gauge, and got 2.6275", or 66.73mm. 
I imagine the Honest Abe pistons are a smidgin smaller than that.   ;)

Between friends you are 100% right about 895 not dividing into anything.  An 895cc would have 67.26mm pistons.
« Last Edit: November 28, 2011, 05:54:00 PM by fang »
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