Vintage Dirt Bike Q & A

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.

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

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

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

Brad,


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

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray
The 175 cylinder seems to be the best way to go.

Brad

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:



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

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

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

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

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Any and all help is appricated much from drawings to pictures ect. My email is wbe1@cox.net.

Thanks Brad