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| slarty-bart-farst |
 Posted: Fri Oct 21, 2005 5:08 pm |
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Rookie
Joined: 21 Oct 2005
Posts: 2
Location: Isle of Man
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| I have an early CB750 K0 which I have a spare engine for. The spare engine has just had an Wisesco piston kit taking the engine up to 836cc. The overbore was done by a very experianced company who explained that they were not happy with the clearance stated on the kit (0.002"). They explained that this was really tight for an aircooled engine and thought it would seize, it does seem close to me as well. They advised that the bores be honed out another 0.002" to take it to 0.004" clearance on the pistons as they had never heard of an aircooled engine running with such a small clearance. Anyone any experiance of these kits and what the correct clearance should be? |
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| Satanic Mechanic |
| Posted: Fri Oct 21, 2005 6:24 pm |
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Joined: 04 Sep 2005
Posts: 309
Location: Munich, Germany
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| The Wiseco pistons are forged, which means they need a bigger clearance than cast pistons as they will grow more with temp. To me, 0.004" sound more reasonable than 0.002". |
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| Satanic Mechanic |
| Posted: Sat Oct 22, 2005 9:56 am |
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Joined: 04 Sep 2005
Posts: 309
Location: Munich, Germany
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I did some more investigation. You may install the Wisecos with the recommended clearance of 0.002" but they will require a longer break in time, and need to be carefully broken in. You may go to 0.0025" or 0.003" to avoid that.
I dug out a spec sheet for an 836cc Arias kit, and they recommend 0.001" clearance! Of course the required clearance depends on the material, but OTOH who should know better than the manufacturer. |
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| Ron |
| Posted: Sun Oct 23, 2005 2:29 am |
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Joined: 15 Sep 2005
Posts: 30
Location: Indianapolis
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Satanic Mechanic wrote: I did some more investigation. You may install the Wisecos with the recommended clearance of 0.002" but they will require a longer break in time, and need to be carefully broken in. You may go to 0.0025" or 0.003" to avoid that.
I dug out a spec sheet for an 836cc Arias kit, and they recommend 0.001" clearance! Of course the required clearance depends on the material, but OTOH who should know better than the manufacturer.
I wouldn't know anymore than what you have already said. I would encourage a builder to always check their ring gaps before putting the ring on the piston. Push the ring into the cylinder with a piston and measure the gap. Occasionally they are too tight and the gap needs to be filed open. Certainly a tight piston with a tight ring gap would be a bad combo.
Additionally, if the gap is already at max limit and you hone the cylinder, you will have a gap that is too wide.
On another thread you mentioned the Yosh 2 ring slipper piston. I'm proud to say I have a set and they make really good power. They have a nice dome fly cut for a roadrace cam (moderate lift, long duration) They have a compression ring and an oil ring but no alowance for a middle scraper ring, to reduce friction. And the skirts are truly minimal, just about an inch wide in front and rear with no skirt at all below the pin bosses. I would think that if one could get some reasonably close 3 ring blanks a good machinist could nearly duplicate it and one could just leave off the middle ring in installation. |
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| slarty-bart-farst |
| Posted: Mon Oct 31, 2005 7:09 pm |
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Joined: 21 Oct 2005
Posts: 2
Location: Isle of Man
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Thanks for the info, I think I have to go for the manufactures recomended cearance and be carefull with the breakin. In my youth I came across a few 836 overbores and they always smoked, maybe they were 0.004" clear? Whatever I go for I will gap the rings real carefull, I know that this can give rise to a lot of problems so a few hours with needle files and feeler gauges will hopefully give me a smoke free motor.
Then I'll check the valve to piston clearance because i have a high lift cam from Dunstall |
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| Ron |
| Posted: Mon Oct 31, 2005 7:51 pm |
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Joined: 15 Sep 2005
Posts: 30
Location: Indianapolis
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slarty-bart-farst wrote: Thanks for the info, I think I have to go for the manufactures recomended cearance and be carefull with the breakin. In my youth I came across a few 836 overbores and they always smoked, maybe they were 0.004" clear? Whatever I go for I will gap the rings real carefull, I know that this can give rise to a lot of problems so a few hours with needle files and feeler gauges will hopefully give me a smoke free motor.
Then I'll check the valve to piston clearance because i have a high lift cam from Dunstall
In the olden days, we built drag race motors and left the valve stem oil seals off the exhaust side for better oiling of the valve. This would tend to make a motor smoke, even if (especially if) the pumping pressure was good (200+PSI) indicating good ring/piston fit. It did not affect power though as it was on the exhaust side.
The earliest stock motors did not have guide seals on the exhaust side. Only with the K1+ models were the exhaust guides introduced, though they could be retrofitted to sandcast motors. For a street bike, use the seals. I'm not really sure if there was any benefit leaving them off even on a racer. We did a lot of things on speculation back then, still do I guess.  |
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