954 big bore kit - Honda Motorcycles - FireBlades.org
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post #1 of 8 Old 07-01-2019, 12:07 PM Thread Starter
 
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954 big bore kit

Hello again guys!
After I disassembled my engine due to lack of oil pressure, I found that cylinder 3 and 4 were burning oil (massive scratches on cylinder walls) and my crankshaft bearings were destroyed.
I am looking at increasing the bore to 76mm instead of the original 75, but does that mean that I have to get the crank balanced to the weight of the new pistons?
Also as this will increase the compression ratio to 13.5:1 from 11.5:1 do I have to get some kind of reinforced head gasket?
What other modifications will I have to be doing?
Any help would be much appreciated!
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post #2 of 8 Old 07-01-2019, 12:14 PM
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Re: 954 big bore kit

Hi, I am not a scientist, but doesn't a larger bore DECREASE the compression ratio?

My reasoning is this: There is a greater cylinder volume, but still the same amount of air/fuel flowing into the chamber. Using the same domed pistons and same size valves means that you now have the same volume of air/fuel being compressed in a larger space. That, to me, gives the effect of 'diluting' the mixture with the consequent LOSS of compression.

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post #3 of 8 Old 07-01-2019, 12:18 PM Thread Starter
 
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Re: 954 big bore kit

Well, to be honest I have no idea. I just red the description from the wiseco kit.
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post #4 of 8 Old 07-01-2019, 12:27 PM
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Re: 954 big bore kit

Quote:
Originally Posted by Dealia View Post
Well, to be honest I have no idea. I just red the description from the wiseco kit.
Hi again.

I'm sure we'll get more input on this during the evening.

I base my thesis on my racing experience from the mid-70s. To illustrate, I bored a CB750 from the original 736cc to 820cc and used CB350 domed pistons as opposed to the flat top 750. (A popular mod in those times) That raised the compression significantly, even with the use of larger valves.

Of course I do not know what is in your Wiseco kit. Perhaps the pistons are 'domed' different than OEM.

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post #5 of 8 Old 07-01-2019, 12:39 PM Thread Starter
 
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Re: 954 big bore kit

Thank you!
I do not know if you will get any useful information, but I am looking at these pistons https://www.ebay.com/itm/152165176920
I am guessing that I also will need more fuel sprayed into the cylinders. It would not be a problem if it had a carb as I could just re-jet it, but how do I solve this issue on a FI bike. (this is my first bike with fuel injection)
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post #6 of 8 Old 07-01-2019, 2:58 PM
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Re: 954 big bore kit

Hello again.

I've looked further. Obviously I'm in no position to challenge Wiseco's specs, so we'll assume you get the 13.5 to 1.

I would email Wiseco for advice on head gasket (one with a good copper or similar seal around the ring) and I would also seek advice on perhaps increasing cylinder head torque by a couple of pounds.

http://www.wiseco.com/ProductDetail....02&AppID=24491

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Last edited by nigelrb; 07-01-2019 at 3:25 PM. Reason: added link
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post #7 of 8 Old 07-01-2019, 7:09 PM
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Re: 954 big bore kit

@mika_u should be able to answer any 954 overbore question. He was around in June. Wealth of knowledge.

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post #8 of 8 Old 07-02-2019, 2:20 AM Thread Starter
 
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Re: 954 big bore kit

Thank you very much guys!
Do you know what modifications I need to do to my fuel delivery system and also are the OEM rods and crankshaft strong enough to be used in this build?
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