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 70cc head on a 50cc bore 
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Post 70cc head on a 50cc bore
ok well those of you know what happened last week i now have my stock parts in hand
but i forgot to get a new bpr7hs plug :doh: so i may get her back together but wont be able to run her till after monday when the dealership or scooter shop is open again(for some reason the local parts stores dont have that plug)

and those of you that have had your head off know that the stock head has a funky design to it so i was wondering if any one has tried running a 70cc head on a 50cc bore my thoughts would be that it should benefit from the better combustion chamber design
but im just wondering if any one has already tried it and has any advice on it

well i have to get some yard work done first then gasket match the exhaust port before i can even think about tinkering with the scoot

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Sat May 02, 2009 11:55 am
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Post Re: 70cc head on a 50cc bore
Wouldn't it make the compression super low?

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Sat May 02, 2009 12:03 pm
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Post Re: 70cc head on a 50cc bore
The compression should be higher , not lower BUT I do not recommend doing it! The reason is that the 70cc head is cut for a larger bore than the 50cc and thus is bigger in diameter. This would give you a tiny shelf which may have some really weird results. If I were you I would just wait and set it up proper rather than run the risk.

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Sat May 02, 2009 4:46 pm
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Post Re: 70cc head on a 50cc bore
well i found out from some guys on another forum that as long as you use the 70cc gasket and you have to use the plug for the head that you are using it works great so i am going to give it a whirl
the increased compression is what i am looking for i dont have a scale or a ccing chamber but when i filled the stock head and then dumped it into the 70 cc head there was some flid that went over the edges but not a lot

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2008 zuma dual headlight mod koso mirrors liquid cooled polini corsa kit leo vince zx stage 6 variator rally kit custom red paintFUEL INJECTEDsoon to be 100cc
2008 stella olive custom ported 177 cc kit dechromed ford tear drop tail light alive and breathing and awaiting cosmtic surgery thanks to an donkey hind quater hole cager
1985 elite 150 how long can i leave it stock


Sat May 02, 2009 5:16 pm
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Post Re: 70cc head on a 50cc bore
um... the chamber volume would be higher with a bigger bore, so it would make the compression lower would it not?

I guess we're just talking out our asses with no hard numbers to look at though.

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Sat May 02, 2009 6:49 pm
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Post Re: 70cc head on a 50cc bore
its probably not the best idea to switch them.if the other guys were successful they might have done something else to make it work better


Sat May 02, 2009 6:58 pm
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Post Re: 70cc head on a 50cc bore
i dont see how that would work, like speed style said, the diameter on the head is going to be bigger then the stock bore.. i think the head is right about the compression ratio being lower as the there will be more volume at TDC, but if you hooked up a gauge pressure it would probably be lower then stock... but who knows I saw give it a whirl!

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Sat May 02, 2009 7:01 pm
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Post Re: 70cc head on a 50cc bore
The chamber in the head has a larger bore diameter, and possibly lower deck height, which would result in a loss for bore diameter and gain in lower deck height, but all in all, the bore diameter is going to have the larger effect, which would PROBABLY lower compression. The only true way to know would be to CC both heads with the same spark plug installed. (which ever has lower cc's would have higher compression) also, having the larger bore in the head would cause excess wear on the head gasket material, hurting longevity either way...


Mon May 04, 2009 2:48 pm
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Post Re: 70cc head on a 50cc bore
Csappo wrote:
having the larger bore in the head would cause excess wear on the head gasket material, hurting longevity either way...
thats why you use the head gasket to match the head you are using

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2008 zuma dual headlight mod koso mirrors liquid cooled polini corsa kit leo vince zx stage 6 variator rally kit custom red paintFUEL INJECTEDsoon to be 100cc
2008 stella olive custom ported 177 cc kit dechromed ford tear drop tail light alive and breathing and awaiting cosmtic surgery thanks to an donkey hind quater hole cager
1985 elite 150 how long can i leave it stock


Mon May 04, 2009 2:51 pm
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Post Re: 70cc head on a 50cc bore
Matching head gasket isn't going to matter, the compression of the cylinder will mushroom out on the head, and heat all the surfaces, creating expansion where there shouldn't be, between the larger head and top of the bore on the cylinder, over time this extra heated surface will effect the head gasket.


Mon May 04, 2009 9:51 pm
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Post Re: 70cc head on a 50cc bore
Csappo wrote:
Matching head gasket isn't going to matter, the compression of the cylinder will mushroom out on the head, and heat all the surfaces, creating expansion where there shouldn't be, between the larger head and top of the bore on the cylinder, over time this extra heated surface will effect the head gasket.

That kind of what I was thinking.

Now if you had a special gasket the engine would run fine even with that little bit of a shelf. The reason I say that is because of an old engine design called a side valve or more commonly called a "flathead". From the outside they actually look a lot like a 2-stroke but it is a 4-stroke. The design works but it has many drawbacks and was replaced for most uses long ago. There are a few still used today , most notably Briggs and Stratton lawnmower type engines.

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