anyone know about 68.5cc & 66cc?

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crassius

crassius

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Long ago, there were H2 kits with sticker on motor saying 68.5cc, but seemed to have same bore & stroke as all the 66cc motors. Anyone know if there was a diff between them?

Seems like an almost unnoticeable diff in stroke *could* make a 2.5cc diff (just over 1mm I think).
 
Greg58

Greg58

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The 68.5 has a 40mm stroke, the 66 has a 38mm stroke. That Dax and I think Motorbicycle racing here on the forum both have sold the 40mm stroke engines.
 
crassius

crassius

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don't think so, as this is a 2009 motor with same piston as all 66cc motors - also, I think the 40mm is the rod rather than the stroke
 
Greg58

Greg58

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To measure the stroke of a engine remove the head and lower the piston to BDC and measure the distance from the top of the piston to the top of the cylinder. The displacement calculation is determined by the volume of liquid it would hold if no ports were present, so a 40mm stroke engines piston travels 2mm lower than the standard 38mm or 66cc engine. The link below may help.

http://motorbicycling.com/archive/index.php/t-19444.html
 
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crassius

crassius

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one of my degrees is in math, so I'm comfortable with the displacement calc

2mm extra stoke is more that a 2.5cc diff - that would be 66cc vs 69,5cc

I don't want to take this motor apart to measure (since it isn't mine) & don't have a skyhawk here to measure either.
 
Greg58

Greg58

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Sorry but I have no idea what you can do to find the displacement, maybe the 68.5 claim was some kind of marketing strategy. Over the years we've seen all kinds of numbers thrown out for these little engines.
 
crassius

crassius

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that's why I was wondering - this is the old bone-shaker crank with large mains rather than newer machined cranks with small mains

it belongs to one customer & another customer saw it & wants to buy it from the other - I can't find info to recommend this or not

I know the smoother running crank is better than an extra 2.5cc to me, but no real info otherwise.

just hoping an old-time dealer chimes in with actual info
 
S

Slogger

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Sep 8, 2014
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It's easy- determine your measuring increments, in this case centimeters.
First find the area of the piston crown, a circle.
PI X (R X R) R = Radius of piston crown- Mine measures 48 mm.. so it has an area of
3.1416 X (2.4 X 2.4) or PI X 5.76 = 18.09 square cm.
The stroke multiplies this area into cylinder displacement.
Mine = 38mm or 3.8 cm. 3.8 X 18.09 = 68.7 cc - don't tell the Fuzzzz ;)
This figure, times the number of cylinders (1) = displacement.

I got it from Duane at Dax. Good little engine, no leaks, no trouble and not much vibration.
 
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D

dtv5403

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A friend of mine had a 68.5 cc engine in 2011 that he ordered on eBay. It was a no-name motor. The H2 motor, I'm not 100% positive but if memory serves me correctly that was a Raw motor. Sometimes, engines are made but only so many are sold in a year, then they sit in a warehouse for awhile and sometimes they are sold to another supplier later on. And the fact that 66cc's are labeled as 80 is proof that the Chinese don't know how to measure displacement, so it's no surprise that 68.5 is also a mistake.
 
crassius

crassius

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Sep 30, 2012
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Slogger said:
It's easy- determine your measuring increments, in this case centimeters.
First find the area of the piston crown, a circle.
PI X (R X R) R = Radius of piston crown- Mine measures 48 mm.. so it has an area of
3.1416 X (2.4 X 2.4) or PI X 5.76 = 18.09 square cm.
The stroke multiplies this area into cylinder displacement.
Mine = 38mm or 3.8 cm. 3.8 X 18.09 = 68.7 cc - don't tell the Fuzzzz ;)
This figure, times the number of cylinders (1) = displacement.

I got it from Duane at Dax. Good little engine, no leaks, no trouble and not much vibration.
Click to expand...
never seen a 48mm piston - sure you didn't measure the carbon built up on it?
 
S

Slogger

Member
Sep 8, 2014
544
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nohio
I just used a small stainless steel office scale to measure across the bore and that's the number I came up with.
Next time I have the head off I'll check it again.
What sizes do they come in?

EDIT
I see what you're saying- the bore has to be a little bigger than the piston to allow for rings.
All that stuff above was based on the bore but I said piston crown..
My bad.
Displacement is the bore X stroke, though.
 
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