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Author Topic: 76 CB750f piston help  (Read 1176 times)

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stlal

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76 CB750f piston help
« on: January 19, 2009, 11:41:46 AM »
I dont have access to a camera right now but here is the story

I have a 76 CB750f.  I need help identifying the pistons

they all have a 100 on top of them and a 300 on the side.

They also have an Irt ir 1rt on the side I cant make it out that good.

The bike is said to have 14k and I can still see hone marks in the cylinder walls.  I want to re-hone it and install new rings but want to make sure it is a std bore.

I have a cheap dial caliper and am getting 2.439-2.440 all the way across.  The max in the manual is 2.406 but again it's a cheap dial caliper.

Everything looks in good shape and the engine seems to never been apart.

Any ideas?
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Offline crazypj

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Re: 76 CB750f piston help
« Reply #1 on: January 19, 2009, 12:16:49 PM »
100 probably means is been bored 1.00mm oversize.
The measurement you took would seem to verify this as 1mm is pretty close to 0.040" (forty thou)
PJ
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stlal

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Re: 76 CB750f piston help
« Reply #2 on: January 19, 2009, 12:35:20 PM »
Ok..so I am assuming it is safe to reassemble?

Is it common for some to bore it 1mm over with low miles?
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Offline crazypj

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Re: 76 CB750f piston help
« Reply #3 on: January 19, 2009, 01:44:34 PM »
No, its pretty unusual.
If someone wanted a bore increase usually it would be for 810 or 836 conversion
PJ
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Offline Clyde

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Re: 76 CB750f piston help
« Reply #4 on: January 19, 2009, 06:19:53 PM »
It is unusual, because aftermarket piston/ring sets are cheaper than original Honda ones. The 100 means 1.00mm oversize which is 62mm bore (vs 64.00 for 810 and 65mm for 836)
The 300 on the side means they are K pistons, which have a compression ratio of 9.0 vs 9.2 for the F. No big deal.
The writing on side may be Art??
Sounds like a light hone and new rings will give you a new engine.
Check the ring gap as well as checking the bore.
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SOHC4 #1909
Honda CB750 K0(original and unrestored), K1(in pieces), K2(restored), F1(restored), 76 750a (awaiting restoration), 1966 Honda CB72
Suzuki GT750 1972 (restored), Kawasaki Z1 1973 (restored)

stlal

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Re: 76 CB750f piston help
« Reply #5 on: January 19, 2009, 07:30:56 PM »
Great

Thanks for the input
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: 76 CB750f piston help
« Reply #6 on: January 19, 2009, 10:41:15 PM »
I checked both parts manuals I have for the 75/76 F. Both years used the same piston part number. The pistons would have 392 on the side as mine do. I agree with the assessment that you have K (300) pistons that are 1mm over size. Using my cheap digitial calipers your measurements are 1+ mm over my 392 pistons from 2 engines. I'd say for whatever reason you have either/or a fairly recent rebuild with low mileage or a replacement speedo and a recent rebuild.
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As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

stlal

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Re: 76 CB750f piston help
« Reply #7 on: January 20, 2009, 11:33:50 AM »
Thanks guys!!! Can't wait to get this thing running
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