42mm crank

adas

New Member
#1
Aloha, I am thinking about buying a 42mm full circle hi rpm
crank for my Minarelli Horizontal engine.

I figure it should fit with the minimum amount of "Carters"
grinding and with some alum base spacers to get a .6mm squish,
it should be an easy install. I also would get 5cc more +rpm+ higher
crankcase pressure.

The crank is advertised for Yamaha Zuma Jog, Eton, ATV's.

Anyone know if it is a direct replacement? Will the taper on the
magneto side of the crank be the same? Ideas anyone?

Or is it better to just do a 47-48mm crank? How about the spacers...easily
bought or fabricated. Whew....

thanks Frank

68cc MHR replica
Yasuni R
21 Delorto
Koso clutch
Polini Vario
 
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#2
you have to grind a lot of the case to make it fit. If it's listed as a 50cc conversion it should have the smaller splined side for the 50cc style variator plate and the correct taper for the flywheel. Not all of them are for the conversion. I assume you mean on ebay? I have one in my converted Yamaha Zuma now. It fit ok but it did take a lot of grinding and larger than 47mm cylinders will also require taking a lot of material from the engine cases for the spigot to fit. I've just been test riding and sorting the jetting but it's pretty fun!
 

adas

New Member
#3
Aloha, Brookespeed. I thought that the 42mm would fit with
minimum grinding. It is a full circle hi-rpm advertised as the
company who makes Malossi company crank.

If I have to grind much more, ..... I have seen samples of
Mexicano and others grinding Carters, then I will go to
a 47mm crank. If I go to 45-48mm (whatever is available),
what do I need along the lines of base gasket/spacer,
or do I need to buy cylinders that are for strokers and
get into grinding the spigot area? I have a LC Polini Evo,
DR. Tuning, and Malossi MHR that I currently have and
play around with and would not like to change. Plus the
crank is still a 10mm pin.

Yes on Ebay.

frank
 
#4
when you will place in the larger stroke crankshaft your port durations go through the roof !
and keep in mind the exhaust port needs to be open at BDC, with this new spacer under the cylider you will need to port the exhaust port downwards and this is only possible to do when you have an oval exhaust port, im pretty sure the MHR Replica has a butterfly exhaust port
 
#5
Your piston will travel about 1.4mm further in both directions. You can use base spacers (higher rpms). Or you can machine a deeper squish on the top (higher torque). And perhaps the best way to mantain a decent timing. A bit of both methods combined.
BTW. There is not much difference in lobe size between 42 and 47 cranks. It's all in the ofset of the big end pin.

A word of caution. I've heard that CMR which normally makes excellent cranks. Shipped a batch of 42mm cranks where they had forgoten to drill the balance cuttouts on the inside face of the lobes. Lots of distributors were very unhappy. Athough they should all have been returned for replacement. Try to ensure that you are not getting one of those.
 
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#7
adas said:
aloha, howfully I got one that is compatable with my engine.....correct taper, spline diameter, etc. Here is a photo of my ebay deal....let me know if it may be the deffective one. frank

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=4647374878
you do realise this isnt plug and play stuff?
does your cylinder have an oval exhaust port or is it a butterfly port?
if you place the spacer underneath you need to port some some 1.5mm downwards the exhaust port and transfer ports !
 

adas

New Member
#8
Aloha, this better be Plug and Play. I got a Microsoft MHR cylinder !!!
As you know the MHR Replica has an oval with 2 side ports not a butterfly like the older Polini Evo.
Here is a photo (1/2 way down the page) of the port layout from another biker.

http://www.apriliaforum.com/forums/showthread.php?t=71124&page=2&highlight=mhr+replica

As you can see the photo of the sample cylinder, there is a slight lip on the top of the exhaust port (1mm bump, maybe the nikisil?), and I have removed it on all ports, so to lower the rpm a bit, I am inclined to take the material off the head. Is there any recommended improvement for the squish angles? What do you think? Thanks for all your good advice.

Frank
 
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#9
adas said:
aloha, howfully I got one that is compatable with my engine.....correct taper, spline diameter, etc. Here is a photo of my ebay deal....let me know if it may be the deffective one. frank

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=4647374878

I zoomed the picture and at least that one has the balancing cuttout holes.
If you are going to machine the top. Keep the same squish angle. It's suposed to run almost parallel to the piston top.
 
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