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I came on here to look for the same thing Fenderfreak, and thanks Doode for the link.

I'm interested too in knowing whether I can simply bolt on a 350 head and jugs to my 71' 250K3 engine. The link mainly talks about the problem with the 360 head being chamfered. Not sure that's the case with the 350 or not (it doesn't appear to be).

Fenderfreak, I'd be interested in knowing how you get on (and likewise I'll keep you posted on any progress I make)

Cheers
 

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Discussion Starter · #6 ·
Thanks guys, it would seem that they may not be compatible due to the squish area which would probably reduce compression ratio. The 250 and 360 heads are not compatible at all.

But then I have heard so many differing opinions�� so will stick to fitting a 250 head
 

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Not sure if this helps but old 650 triumphs are usually "upgraded" to a 750 by changing the top end (pistons, jugs, head). Not sure if this would apply to Honda architecture... I'm sure rod to stroke ratio and rod clearence may apply..
Honestly, I would bet someone somewhere tried to fit different motor parts that were never designed to work together..
as for the results... not finding much info should give a good indication ..
 

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Wouldn't you also have to change the cylinder has as well because the larger cylinder bore would not match the combustion chamber diameter of the 650 heads?
 

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Combustion chamber of the 250 is a lot smaller so it is definitely not plug and play. I have a 250 head on my 350 so I was actually doing it the other way around. In the image you can see the difference in the combustion chamber. The right side is modified to 350 and the left one is standard 250.
Auto part Engine Tool accessory Automotive engine part Metal

By the way... 350 jugs won't fit on a standard 250 crankcase because it has smaller holes. You can enlarge the holes.
 
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