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PostPosted: Thu Apr 01, 2010 8:48 pm 
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Joined: Thu Apr 01, 2010 8:09 pm
Posts: 34
Hi,
I am 14 and have recently obtained my first motorcycle -- a 1976 Honda MR 250 for $250. The chain fell came off when the previous owner was riding it and of course, cracked a chunk of the case away. So he said that it would run fine until he went into a puddle and then water got on the chain, then into the flywheel/stator area and grounded out the points and it would stop running. He had purchased 2 nice case halves off ebay and just never got around to install them. He also had the cylinder off for some reason, and while it was off I think he let it sit outside so some water got in the crankcase and rusted up the left crank bearing. So, the day I got it I pulled the motor, and started to split the case. I got it split and this is where I am now. I have a few questions.

I measured the cylinder with my dads bore guage and it is about 0.007" to 0.010" bigger than the stock bore (70mm or 2.756") throughout the whole cylinder. I think the PO was honing it or something. The piston is also a little smaller that 2.756", I think it's around 2.752ish. This makes about .013" to .015" piston clearance. I think that is waaayyy to much, correct? What is it supposed to be?

We bought a gasket set off ebay, should be here in a few days. Babbitsonline.com lists both crank seals, so we will probably order those, but does anyone know anywhere else to get them?. I have found a few pistons on ebay (we are probably looking for the first oversize which I think is .020 or .025" correct?) and here is the big question.

Wiseco.com says wiseco part #338M07000 fits the following:
Honda Odyssey 250 (1981 - 1984)
Honda CR250M (1975 - 1977)
Honda MR250 (1975 - 1977)
Honda MT250 (1975 - 1977)

BUT on babbitsonline.com it says that the MT250 and MR250 pistons are different part numbers. I have found a .025 oversize piston on ebay that has the MT 250 part number. Will it fit the MR?

Thanks a lot!

-Ryan


Attachments:
File comment: Here it is!
059.JPG
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PostPosted: Fri Apr 02, 2010 6:30 am 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
Ryan,

Good luck with your new bike! Just take your time with it and you'll be fine.

You may be able to tell the bore size from stamped markings on the top ring faces, near the gap. If you find just an 'N', it's probably standard bore, and it you see a number in addition to the 'N', it's oversize. The Honda MT250 service manual shows a standard-bore service limit of 70.1mm (2.760"), so yours is either a well-worn standard bore or a fairly fresh first (.25mm) oversize (does the piston look good, ar really beat?). The manual also indicates a .05mm (.002") minimum piston/cylinder clearance, if you have the cylinder bored.

I would definitely get the OEM crank seals. These are special high-speed rotary seals designed to work with minimal lubrication.

I recently went through the whole MT/MR/CR/FL piston thing trying to find a piston for my friend's MT250. Depending on year, some of these bikes' cylinders have boost ports (small port directly opposite the exhaust port), and some do not. A piston designed for a boost-port cylinder will work in a non-boost-ported one, but the reverse probably is not true for OEM pistons. This is at least one reason why the MT250 piston has a different P/N. I couldn't even hazard a guess on the eBay piston without measurements from it and the MR cylinder, and even then it might be risky. I think you'd be good with a '75-'77 CR250M piston, but maybe someone here with more 250 experience could help with that.

Ray

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


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PostPosted: Fri Apr 02, 2010 7:33 am 
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Joined: Thu Apr 01, 2010 8:09 pm
Posts: 34
Thanks for all the info!

The rings that were on the piston are too big to even fit in the cylinder, but there is another ring in the box of parts that he gave me, so I will see if it fits in the cylinder and if there are any markings on it.

Yeah I will probably order those crank seals.

And I looked it up and the 76 CR250M has the same piston part # as the 76 MR250 so that would work, but I haven't found any pistons yet that fit the CR250M either, that are oversize and under $120, so I guess I will keep looking. I can get a picture of the inside of the cylinder and we can determine if it has boost ports or not? I think my MR does but I'm not 100% positive. I also am going to measure the piston and cylinder again to get some more acurate measurements. The piston isn't beat at all really, the PO said "it has a new piston and rings" but the rings are too big for the cylinder and the piston has a little crack in the lower rear skirt.

Thanks again,
Ryan


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PostPosted: Fri Apr 02, 2010 8:34 am 
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Joined: Tue Mar 30, 2010 4:51 pm
Posts: 13
Ryan
im 15 and i just finished a full racer build out of a 1975 cr125
the one i got was sitting on the farm outside for 10 years
i redid EVERYTHING
if i were you id buy those crank bearings and a full gasket kit
which you probably aready did
make sure your conrod bearings are good
otherwise there gonna get loose and that new piston of yours is gonna be givin the head a nice smackin
you got any questions just ask
-Sam


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PostPosted: Fri Apr 02, 2010 10:38 am 
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Joined: Thu Apr 01, 2010 8:09 pm
Posts: 34
Hello again,
I dont think I am going to get crank bearings as the ones in my new case halves turn very easily with no binding or anything, so I will just use those. I did order a gasket set that should be here soon.

The rod bearings are really tight but we might get a crankshaft off ebay anyway because the flywhel nut threads were stripped, I fixed them, it would probably work, but If we can get that crank for cheap we will buy it.

Anyway now time for the stupid part :? . When I measured the cylinder before I had the micrometer set wrong, I had it set to 2.751 instead of 2.756. So I measured it again and now it is only .003 to .006" wore depending on where I measure it. I think that will be OK for standard bore even if it is out of spec a little. So now my questions are:

I need a set of standard rings for this thing. I found a Wiseco set on ebay. Is that only for Wiseco pistons or will it fit my honda piston too?

I also measured the piston again with the micrometer and it measured 2.744" at the bottom of the skirt. But then I checked the clearance with a feeler guage(attached pic) and and the 0.006" guage fit snug. I think that is good enough??

Also attached is a picture of the crack in the lower piston skirt. Im afraid if I leave it it might break off and screw something up? But if I broke the little piece off and filed the edge smooth would it affect porting, etc.?

Thanks,
Ryan
Attachment:
File comment: Piston Clearance.
027.JPG
027.JPG [ 135.35 KiB | Viewed 3402 times ]


Attachments:
File comment: Cracked piston skirt.
020.JPG
020.JPG [ 126.67 KiB | Viewed 3402 times ]
File comment: Cylinder.
009.JPG
009.JPG [ 166.63 KiB | Viewed 3402 times ]
File comment: Here is the broken case.
003.JPG
003.JPG [ 201.28 KiB | Viewed 3402 times ]
File comment: Rusty Cases.
001.JPG
001.JPG [ 208.85 KiB | Viewed 3402 times ]
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PostPosted: Fri Apr 02, 2010 12:36 pm 
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Joined: Thu Jan 05, 2006 1:35 pm
Posts: 344
OK, wiseco rings will not fit oem pistons. Pistons for the '75 and '76 MR, CR and MTs are the same. The '73-'74 pistons are different. The ring pins are placed differently because of extra ports on the '75-'76 MR and CRs. Do not use ebay pistons unless they are clearly for '75-'76 bikes. I've done this and ended up having to get the correct '75-'76 piston.
Seals are available from any honda dealer. Mine had them in stock. you might have to wait a couple of days if you order them. If you have the engine apart, don't skimp on main bearings. Go to any bearing supply company and spend the $45-$65 for new ones. I've also tried to skimp in the past and paid for it later. I prefer wiseco pistons because they are forged and not cast like the oem ART pistons. Do not reuse this piston. cast pistons shatter when they exceed their life limit and tear up your engine. Yours is halfway there already. I'd bore out your cylinder and go to at least a .020 over. Local guy charges $40 around here to bore and prepare a cylinder. Wiseco will be in .020 oversize increments (.5mm) I had my head off the other day and I snapped a photo. This is a .040 over wiseco piston (71mm bore now). Here is a link to the photo (scroll down--it was easier to just post this link than to resize the photo and post it here):
http://www.dirtrider.net/forums3/showth ... p?t=178293

send me an e-mail and I'll scan and send you an MR Honda shop manual
mmaurer@mcgivernlaw.com
-Mitch


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PostPosted: Fri Apr 02, 2010 12:58 pm 
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Joined: Sat Dec 01, 2007 3:03 pm
Posts: 265
I bought a mr250 motor for the tranny and clutch I have no need for the crank if you want it you can have it for shipping costs.


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PostPosted: Fri Apr 02, 2010 2:00 pm 
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Joined: Tue Mar 30, 2010 4:51 pm
Posts: 13
ya i no its a pain but get wiseco everything and bore the cylinder
thats what i did
also listen to mitch he knows what hes talking about
and by the way you got an extra shop manual for a cr125 mitch?
-Sam


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PostPosted: Fri Apr 02, 2010 3:06 pm 
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Joined: Thu Apr 01, 2010 8:09 pm
Posts: 34
Mitch,
Thanks for all the help and info, I sent you an email.

First of all I talked to my dad and he said that we are just going to bore it anyways because it is apart and wore a little and the piston we have now is cracked. I will call the honda dealer and see if they have the crank seals.

So the 76 MR and MT 250 pistons are the same? On ronniesmailorder.com, and a few other web sites, it says that the standard MR piston is part # 13101-395-000 and the MT is part # 13101-358-000. Are they interchangeable anyways though?

We would like to get the NOS honda piston on ebay because it is so much cheaper than a wiseco, I am not going to be racing this thing or anything, so hopefully a Honda piston will be good enough. Do you think a .25mm oversize piston would be enough? The guy with the ebay piston says that it is part # 13102-358-000. Which is .25mm oversize or .010 oversize that fits the 76 MT250 I believe. Will this fit the MR?

We also have a local guy that bores cylinders for about the same money.

Thanks again,
Ryan


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PostPosted: Fri Apr 02, 2010 3:09 pm 
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Joined: Thu Apr 01, 2010 8:09 pm
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And Woosh I sent you a PM. Thanks a lot!


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PostPosted: Fri Apr 02, 2010 6:10 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
Ryan,

Here's a link to a guy on eBay selling the Sudco ART FL250 pistons - I offered him $40 for the standard piston and he accepted (these are complete kits including rings and wrist pin - bear in mind the Honda rings are over $30):

http://cgi.ebay.com/ebaymotors/HONDA-FL ... ccessories

Here's the ART piston info:

http://www.sudco.com/art.html

I can take measurements of my piston if you want. It seems like the ring retaining pins are quite far apart and it might work with the MR cylinder, but I'd need the boost port width to be sure.

The FL250 motor was nearly identical to the MT250, by the way.

Ray

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


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PostPosted: Fri Apr 02, 2010 9:05 pm 
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Joined: Thu Apr 01, 2010 8:09 pm
Posts: 34
Measurements would be great! You guys sure are awsome doing all this stuff for me!

And that would be a lot cheaper and better.

Attached are some pics and measurements of my piston and cylinder for reference.


Attachments:
File comment: distance between outside marks (inside edges of boost ports) is 1 5/8" and distance between inside marks (ring locating pins) is 1 1/8"
029.JPG
029.JPG [ 261.6 KiB | Viewed 3367 times ]
File comment: inner marks are the ring pins and the outer marks are the inside edges of the boost ports
026.JPG
026.JPG [ 184.53 KiB | Viewed 3367 times ]
File comment: here are the ring locating pins in relation to the boost ports.
024.JPG
024.JPG [ 161.53 KiB | Viewed 3367 times ]
File comment: measurement from the 2" to almost the 4" is 1 13/16"
020.JPG
020.JPG [ 66.5 KiB | Viewed 3367 times ]
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PostPosted: Sat Apr 03, 2010 12:36 am 
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Joined: Mon Mar 08, 2010 11:06 pm
Posts: 50
Location: Seattle, WA
It looks good to me. I would just soak the cylinder and piston in marvel mystery oil for 20min., wipe it down, run a ball hone through the bore, put on fresh and gapped rings and run it. I have fabbed up rings out of HO scale model train track rails before while I was waiting on oem parts. Make sure you de-carbon the ports before assembly and set your jetting before the first start. I like to heat cycle 3 times of five minutes each, making sure to let the motor cool down completely between cycles. Then ride it below half throttle for 20 minutes and you're set.

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osu_cowboy


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PostPosted: Sat Apr 03, 2010 5:51 am 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 1010
Location: Connecticut, USA
Ryan,

OK, here goes:

Ring retaining pins: 1.26"/32mm apart (as measured on flexible ruler)
Skirt cutout height: 1.983"/50mm (not super critical)
Skirt height (max): 2.735"/69.5mm
Pin centerline to piston top edge: 1.27"/32.2mm

The retaining pins appear to be slightly farther apart than on the MR piston, possibly to clear the wider (?) CR boost port and allow this piston to be used on the CR/MR/MT/FL motors. I would suggest taking a measurement around the bore between the inner edges of the rear transfer ports, on each side of the boost port; if it's 36mm/1.42" or greater, you should be fine.

The wrist pins and small-end bearings have the same P/N's. so that's good.

The ART piston has a really cool machined surface (different from the OEM ART's I've seen) and should retain oil well even after a lot of use. It looks like a quality part, especially for the price.

Ray

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


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PostPosted: Sat Apr 03, 2010 8:07 am 
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Joined: Thu Apr 01, 2010 8:09 pm
Posts: 34
Well we bought a piston!!!

It is a NOS honda .25mm oversize. It was on ebay. In the auction there were piston and rings, both with the correct part numbers. Here it is: http://cgi.ebay.com/ebaymotors/ws/eBayI ... 0347249518

We were going to buy that ART piston but my dad said it is a risk because it might have a different weight than the stock piston and that could cause problems over time. And this piston wasn't all that much more anyways AND it is a honda so I think we did pretty good.

So when my piston gets here (it is in my same state even, ND!) and my gasket set gets here, and if I get that from ebay, I can start re assembly.

Thanks for all the help so far!

Oh and does anyone know the torque specs for the head, cylinder base, etc?

Thanks,
Ryan


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