Engine ID 2.5 or 3.0?

Mini Bike & Go-Kart Parts

#1
Hi bought a untagged engine. It was advertised as a never run NOS 2.5/3.0.
At the time I didn’t care but now I’d like to know what I have. What’s the best way to tell the difference? Thanks
 

SAT

Well-Known Member
#2
If I remember right, the bore is the same and the stroke is different by a lot, maybe an 1/8”, 0.125”. I would try a dial caliper depth measurement first. Then a depth gauge. Then pull the head and measure directly. Sorry, I don’t remember the stroke numbers.
Nice power plant.
 
#3
According the specifications, the only differences are the magneto bearing diameter (bushing) for H30 and 82 and earlier H35 are .9985 and .8735 respectively. The other spec that is different is the timing, which is .035 and .065 inches BTDC respectively. I've pondered this before when swapping parts around. Maybe [MENTION=5969]markus[/MENTION] can illuminate some more differences, or offer insight on why the timing is different. Given that the mag bushing is different, I'd wager the 30 and 35 use a different flywheel as well.
 

markus

Well-Known Member
#4
there really isnt a difference in the late 60's to mid 70's H25 and H30's, alot of the replacement "short blocks" from Tec crossed even. The H25 designation was dropped around 1975 so any of the stroke and timing changes that were made in the 80's dont apply there and also means it would have to be a small taper crankshaft/aluminum flywheel/rounded shroud to be an H25. Is it a complete engine or a short block, you would want to designate if its an H25/30 or an H35 and the bore size would indicate that. H25/30 were 2 5/16" and H35 were 2 1/2" Strokes are shown in the 1970's as 1 27/32".
 
#5
Wow, thanks for clarifying that Markus.

I went back in and looked at my manual and sure enough, I was spouting the wrong year. Sheesh. Sorry about that!
 
#6
Thanks everyone! So it’s bore size and it’s only going to tell me if I have a 3.5 or 2.5/3.0. It was a nos never used short block. I mounted a rupp side saddle tank and was going to use for a cub/rascal restore but as always I need cash so I sold the roller and now left with this engine. My problem was how to list it as.
 

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