Honda-SOHC
SOHC.co.uk Forums => Project Board => Topic started by: matt34 on January 02, 2021, 02:06:51 PM
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hi im currently building a cr 750 replica and have a problem with the head i bought an f2 head off ebay and sent it to a tuning company after porting etc they sent head to get the 5mm valve kit fitted i got from cycle x but said the valves are to big after a bit of resurch i think theres two f2heads one with smaller valves with i think i must off purchased is there a way of telling ie casting numbers before buying another head as ive got to have it tuned again before fitting of the race valve kit i think the inlets on the race kit were 34mm thanks matt
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F2 heads have 410 on the casting by the inlet side, there is only one valve size on the F2 ,
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All F2 head have the bigger valves as, the same as the K7 and K8 head. It has to have the bigger valves due to running on PD carbs so, no small valves on 750 SOHC with PD Carbs ;)
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My k7 head has same valve size as the earlier k series, not sure if I read your post correctly trigger, as far as I know F2 is the only one with larger valves, also different cam and re-worked head to accommodate the big valves
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As above, all F2 are same sized valves with the K7 using 2mm smaller inlets and 3mm smaller exhaust as far as I know.
Whether the casting is the same I'm not sure, Honda paired these two for production and service manuals.
Also, only the F2 head has the valves pushed back further into the head to get the distance between the two to clear each other. It's not,as often qouted, a different valve angle. But does put the valve tappet face higher in the head which alters the geometry of the rocker approach to the valves.
This makes the combustion chamber volume greater for the F2 which uses higher piston crown to compensate.
Valve springs went from 154 to 202 lb to cope with higher mass and increase in rpm limit.
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Apologies Pete, crossed over while typing there. :)
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http://anyflip.com/nfzj/jvui/basic
Shows workshop manual.
It's really cool if you flip the pages too, just like real paper. I may need a sit down after this ;D
Edit, you have to flip to page 6-13 for valve specification.
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My k7 head has same valve size as the earlier k series, not sure if I read your post correctly trigger, as far as I know F2 is the only one with larger valves, also different cam and re-worked head to accommodate the big valves
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Sorry, i was getting confused with the 392 head :o
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thanks for the replies ill contact the head shop see what casting no is on it cheers