Vintage Dirt Bike Q & A

I have a Honda MR 175 that needs to be bored and oversize piston. Pistons for these bikes are very hard to find. A while back there was a post here about a Wiseco Piston # 2358M06600 that was a close replacement. The piston is for a Polaris 600XLT Snowmobile. It is 1mm diameter larger, .5mm higher deck (increasing compression), 4mm shorter skirt (increasing intake duration), and the same 16mm wrist pin compared to the standard MR 175 piston. Does anyone have experience with this Wiseco piston in a MR 175? Should I do this? The Wiseco unit is available from Dennis Kirk. What do you guys think?

Keep looking on here I read somewhere on here the other day not to long ago that someone on ebay had gotten some made from wiseco no std but a few different os it had his ebay name in the post so keep looking hopefully he still has some.

Found it

Glad to finally see other MR175's out there. I have a MR175 in a 74 CR125 that has been heavily modded. Just bought a brand new Wiseco piston kit ($159) on E-bay from a guy that had Wiseco do a custom batch for him. He has both .020 (.5mm) and .040 (1.0 mm). These are modern forged Wiseco pistons with modern thin rings that can handle high horsepower and RPMs. E-mail him at mailto:hbliss@aol.com I get copper head gaskets, gasket sets, and seals from a guy on E-bay elsinorestuff. Good luck with your projects. Coll

Which oversize are you looking for?

The standard Polaris 600XLT piston diameter is 65mm, so the 1mm oversize (66mm) is correct for the MR standard bore. The 2358M06650 would be .5mm (.020") oversize for the MR175.

As I mentioned in my other post, this piston really isn't an identical replacement for the OEM MR175 piston, but it should work. You may get some loss of low end, as the shorter skirt may allow blowback thru the carb at lower rpm's, and you will definitely get a lot more compression - maybe too much Using an OEM .050" head gasket with a .030" copper head gasket (yeah, I hate using multiple gaskets too), or converting the weird dual-squish-band head to a conventional dome w/single squish band (I'll be having this done to mine next week) would bring the compression back down again. You also may get more high-rpm power, especially in a ported cylinder and/or using a good expansion chamber. The piston pin, bearing, rod side clearance, and fit to the cylinder were all fine.

I've got a Wiseco 340 .5mm OS MR175 piston set here (piston, rings, bearing, pin, clips) - it came with a motor I bought off eBay. It looks to me like the piston was installed into the newly-bored cylinder dry, then they tried to start it like 50 times, then put the motor up for sale. The piston shows some very light vertical scoring & chafing, but no evidence of carbon or heat exposure. The ring faces are still mostly the dull gray as-new color, with some shiny areas. I wouldn't hesitate putting it in one of my motors, but as I have all standard-bore cylinders and a pretty good stock of standard and 1st-oversize (.25mm) pistons & rings, I really can't see myself ever using it - if you're interested, just let me know. The cylinder can even go with it, if you don't mind one cracked top fin (repairable) and one broken small bottom fin; the bore looks perfect. One thing about this piston; it uses one Dykes (top) ring and one Keystone ring. I don't know if Wiseco still makes rings to fit it.

Sorry for writing a novel here.

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: Could a taller spacer/ base gasket be made to help lower the compression when using the Polaris piston? This may also help with the shorter skirt.

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77 MR175, 89 HD Sportster
74 CR125, 00 Kawasaki ZG1000
79 XR185, 56 Cushman Eagle
03 XR650L, 79 Vespa P200E
72 Vespa Super150,
74 HD Z90

Ray, I'm interested in the .5mm OS Wiseco piston you have as a replacement for the .5mm OS for a MR175.

You're right - lifting the cylinder up from the bottom would help with the shorter skirt; it's a great idea. It would also bring the port timing back closer to stock. I would say that ending up with a piston deck height .020" higher than stock would be just about right; you'll have plenty of compression (mine has 165 psi cold with a head milled .020"), better squish-band action, and bottom-end-enhancing exhaust/transfer port timing combined with top-end-enhancing intake port timing; might be a good mix all round. Making up a metal spacer plate would be ideal, but an easier way might be to Permatex (or hi-temp epoxy) together several base gaskets to make up a spacer; if for some reason you didn't like the results, you wouldn't be much out of pocket. A new base gasket measures .023" (.58mm) uncompressed.

If you really wanted to max things out, you could also raise the transfers and exhaust .020", and end up with the stock port timing along with increased port areas - or raise them even more, at the risk of heading towards a road-racer sort of powerband (it would be hard to say for sure without trying it). You could also just raise the transfers; the porting specs I posted before basically raise/widen/angle the transfers, widen the exhaust, lower the intake, and open up the transfer passages. I lost zero low end with these mods, but then again I really don't know if the porting was done exactly to spec (I'm assuming it wasn't). Another mod I forgot to mention was opening up the MR exhaust manifold to 37mm, which improved both top end and throttle response slightly.

For what it's worth... I cut about 6mm from the bottom of my CR125M piston once. It definitely shifted the powerband upwards at least 1,000 rpm, along with more power up high / less down low. This was on a top-end-ported cylinder done by a real boob (myself); my game plan was simple - "raise all ports 1/8" - it looked real nice inside when I was done, though.

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

Be careful if you select the Polaris 600 piston. Check the ring-gap position, it may snag in a port, especially if you have widened the transfer ports.

There was a Suzuki oversize piston still available from Wiseco. Email their tech-support. There is very knowledgable guy there that may still remember the partnumber. It may have been from a PE175, but I'm not sure.

The Suzuki piston will fit, but the pin is located 2 mm lower (shifts the piston up). I cut a 1.5mm aluminum shimstock with the base-gasket as a template, used the shim with a gasket on either side. The rings are not keystone, so they don't rattle as the MR tends to do when they wear in a bit.The ring-gap position will clear any non-modified ports.

How did it do???

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77 MR175, 89 HD Sportster
74 CR125, 00 Kawasaki ZG1000
79 XR185, 56 Cushman Eagle
03 XR650L, 79 Vespa P200E
72 Vespa Super150,
74 HD Z90

That's a good point about the ring locator pins; I forgot to check that on the Polaris piston while I had it. I think that may have been the first time I ever considered using a non-OEM piston in one of my bikes, come to think of it. Oh, wait... there was that Harley Baja 100...

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