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Big bore 125m
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Author:  william edmonston [ Tue Oct 02, 2012 10:30 am ]
Post subject:  Big bore 125m
There is a guy running a cr125m big bore out to 178cc with a bridge port in a modified stock cylinder. His number plate number is 173. Does anybody have any information on him or how it was done, thanks Brad.
Author:  rayivers [ Tue Oct 02, 2012 4:14 pm ]
Post subject:  Re: Big bore 125m
No specific info on #173 or his big-bore CR125M, sorry. I think I can make a pretty good guess on how it was done, as I looked into doing this when I first got my CR running a few years back.

Assuming he's not using a stroker crank, the '76-'78 CR boost port cylinder would have to be bored at least 10mm. As the liner's only about 6mm thick, this leaves an extremely thin cast-iron bore, and the head gasket is really narrow (@ 2mm) inside the head studs. For these reasons I went with the MR175 motor, which I think is a much better solution as you get the beefier MR crank/rod and 171cc cylinder and cases, which also accept the CR 6-speed if desired.

I simulated the CR's boost port and bridged exhaust in the 171cc motor using the engmod2t software, and found they added about 2 hp (36 crank hp @ 9,500 rpm) as compared to the porting I'd already developed (34 crank hp @ 8,750 rpm). The MR crank and 5-speed are handling these power levels pretty well so far, I don't know how the smaller CR con rod and thinner 6-speed gears would hold up.

Ray
Author:  william edmonston [ Tue Oct 02, 2012 6:37 pm ]
Post subject:  Re: Big bore 125m
Ray
Sounds a little complicated. I am small and was looking for something light to run in the ahrma 6o+ group. I have 175mr and early cr125 frames and stuff. Any chance of getting a cylinder done.

Thanks Brad
Author:  rayivers [ Tue Oct 02, 2012 8:35 pm ]
Post subject:  Re: Big bore 125m
Brad,

Quote:
Sounds a little complicated.

Absolutely. Boring out a CR125M cylinder to 178cc is easy, but making a usable/reliable motor with it surely isn't.

Do you want to get an MR175 cylinder ported, or a CR125?

Ray
Author:  william edmonston [ Tue Oct 02, 2012 10:24 pm ]
Post subject:  Re: Big bore 125m
Ray
The 175 cylinder seems to be the best way to go.

Brad
Author:  rayivers [ Wed Oct 03, 2012 7:47 am ]
Post subject:  Re: Big bore 125m
Brad,

I agree about the 175 cylinder, which will require the 175 cases (the studs are different, the con rod's 9mm longer, and I think the crankcase volume's larger too).

A friend of mine recently had his MR175 cylinder ported by Harry Klemm at Klemm Vintage (http://www.klemmvintage.com), and he's quite pleased with the results. Here's the porting specs and info he used:

Image

This porting will give fairly modest power gains with the stock pipe / ignition / compression ratio. The MR175 motor is basically a bored-out MT125, despite Internet claims about it being 'based on the CR125M' or whatever. As you can see from the above, a fair amount of work was required to get truly high performance out of it.

Ray
Author:  Endo58L [ Wed Oct 03, 2012 5:34 pm ]
Post subject:  Re: Big bore 125m
Do a search of my (Endo58L) 4 posts under MR175 for another approach to building a CR175 vintage age class racer. Maybe you can help out here Ray. I don't know how to link the previous posts into this reply.

Coll
Author:  rayivers [ Wed Oct 03, 2012 5:52 pm ]
Post subject:  Re: Big bore 125m
Coll,

Hey, what's up? How's the bike running?

I right-clicked / copied / pasted the title of the other thread, hopefully it will work:

viewtopic.php?f=21&t=2948&hilit=cr175

Ray
Author:  william edmonston [ Thu Oct 04, 2012 12:08 pm ]
Post subject:  Re: Big bore 125m
Any and all help is appricated much from drawings to pictures ect. My email is wbe1@cox.net.

Thanks Brad
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