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PostPosted: Mon Jul 29, 2013 2:09 pm 
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I am helping a friend put together a 1975 Honda MR175 engine that he has torn apart. Here is the issue:

When the cases split the Crankshaft and transmission stayed in the ride side case and the left case was empty. Transmission at this point had never been removed so no gears were swapped into the wrong place etc. Everything was turning freely and clearances were good. Put the left case on and tightened it down and everything was good. Began to put the bolts in that hold the cases together and suddenly the output/counter shaft will not spin. The crank spins freely but the transmission does not. I have taken it on and off and checked and rechecked placement of everything and it is correct. One of the times I put it back together i forgot the washer/shim on the end of the output shaft, got it all tightened up and bolted up and was so excited becuase the transmission spun freely and smooth as butter. However when I went to spin the crank it wouldnt budge as now it was binding. So I can get one or the other to spin but not both.

Can someone please offer any suggestions?

I have yet to take everything out and put it in the left side and try it that way but I don't see how that would solve anything.

Thanks


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PostPosted: Mon Jul 29, 2013 3:11 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
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Location: Connecticut, USA
I assume you're using a new center case gasket, preferably Honda P/N 11191-373-000?

Quote:
Put the left case on and tightened it down... Began to put the bolts in that hold the cases together...

Aren't these both the same thing? Or is there something I'm missing?

Make sure the cases are perfectly parallel to each other at all times during assembly, otherwise one or both shafts may bind. When I put my MR cases together, I carefully lined up the countershaft and crank through the bearing centers and lowered the left case down until it was almost perfectly level, and nearly together. Then I spent about 10 minutes tapping on the main bearing center race until I realized the case was about 1/16" lower at the back, and the CS was binding in the bearing race. I ilfted up the back of the case, and it fell right into place.

This is very unlikely, but if the transmission was in 2nd gear it's possible the CS 2nd gear could be sitting on top of the CS 5th-gear dogs; I doubt the cases would mate with the CS end washer in place, though.

Ray

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


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PostPosted: Tue Jul 30, 2013 1:06 pm 
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Yes a new center gasket, however it is an aftermarket gasket.

I mean I mated the left side of the engine cases to the right side then began to bolt the two together, as I tightened the bolts it got worse. Dont know if that answers your question.

So pretty much just make sure its level? Doesn't seem like there is any super secret method for putting it together or that having it in one side during assembly is really a big deal. I know some bikes have weird patterns for putting things together. Should there be any sort of washer or anything on the crank side?


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PostPosted: Tue Jul 30, 2013 3:01 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
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Location: Connecticut, USA
When the cases mate together prior to screw insertion, they should have 100% gasket contact and be basically in exactly the same position they'll be in after the bolts are tightened, within a few thousandths or so. It seems to me like your cases encountered some kind of obstruction and almost mated together, then tightening the screws clamped down hard on the obstruction and locked it up. Does this sound correct?

There are no washers on either side of the crank, but there are washers on both the LH CS and mainshaft ends. If the clutch and kickstart idler gear were not touched, the shafts could not have moved axially, so that's good. Before the cases are put together, obviously the CS, MS, shift fork shafts, shift drum, and crank must be aligned with their respectuve bearings or case bosses, and of course it's important to have the correct dowels on the cases, which are slightly longer than the clutch cover dowels. I made sure my gearbox was in neutral, just because it seemed like a good idea; I think the transmission should definitely not be in either 3rd or 4th gear.

Why were the cases split initially? I assume there was some kind of problem? Were the cases split with a case splitting tool, or some other way?

Ray

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


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PostPosted: Wed Jul 31, 2013 12:18 pm 
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The guy I'm helping took it apart. There was no good reason really to take the bottom end apart unfortunately. I didnt come into the picture until after it was apart and now am trying to help him put it back together.

You mentioned something about the dowels, do they seat fully in one side of the case? I seem to recall one of the dowels being accidentally pressed further into the hole than it should on one side. If it doesnt sit all the way in on one side I would have no clue the depth it is supposed to be though. May be a non event but I'm open to anything at the moment.


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PostPosted: Mon Aug 05, 2013 4:23 pm 
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Joined: Sun Jul 29, 2012 3:58 pm
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Hey Ray, I am in a similar bind. I purchased a 75 with a locked crank bearing and split the cases. I replaced all bearings and seals and reassembled everything, checking to make sure everything spun freely as I did so and shifted easily. That is until it was completely assembled. Now it won't shift, except what looks like second or first and neutral. I had an aftermarket gasket that I traced and cut out using 1/32" gasket material. This is .0065" thicker than what was in prior. I also have 1/64" material I could use, but it is .0085 thinner. Is it better to go with the thinner or thicker gaskets? Also, any guess what may cause the shifting bind?


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PostPosted: Mon Aug 05, 2013 5:15 pm 
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Location: Connecticut, USA
Re the dowels - they should go a little more than halfway into each case, as I recall. If you have any doubts at all about them, they're cheap and still available from Honda.

Any chance of learning the reason for splitting the cases, good, bad, or otherwise? There had to be some reason. If it's shifting related, there's a bunch of things you should check.

If the cases were split by someone else, I would assume the worst and pull both gearbox shafts, then check everything against the parts diagram. When you're sure it's correct, reinstall the shafts, shift forks etc, and drum, then try to shift into all 5 gears before reassembly, even if that means pulling the crank out. I believe some of the washers are the same I.D. but have different thicknesses, so they shouldn't be swapped around.

I dry-fitted my cases together several times with only the transmission installed, and not until I was 100% happy with the shifting did I install the crank and put the cases back together. I was using an engine stand, which was so much better than wrassling the cases around on the benchtop.

mbilder - first off, the tranny won't reliably shift above 2nd gear on the bench without both shafts turning; even then, the 4th/3rd downshift can still be hard. Try shifting it using the drum end only, without the pawl star or shifter shaft. Have you checked the shifter shaft and its sector gear for damage? One of my motors had taken a brutal hit to the shift lever; the sector gear was badly distorted and the shaft stopper pin had moved in the cases.

Ray

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


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PostPosted: Thu Dec 07, 2017 10:52 am 
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Joined: Mon Oct 31, 2016 10:09 am
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Bumping an old thread, but I'm having the same issue rebuilding a 1975 MR175.
The crank was seizing because the bearing in the left case wasn't fully seated.
I re-installed the bearing and now it is seated further than it was, but the top surface of the bearing is not quite flush with the inside of the case. It sticks up just a bit.
Any idea if this is an indication that the bearing still isn't completely seated? Or is the bearing meant to stick up slightly?
For what it's worth, I was able to get the case back together without the crank seizing up, but I don't want to finish rebuilding everything if there is a chance that bearing isn't fully seated.
Thanks.


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PostPosted: Thu Dec 07, 2017 12:05 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 970
Location: Connecticut, USA
On the NOS MR175 cases I have (all bearings factory-installed), the LH crank bearing is recessed slightly below flush, about at the middle of the bearing-pocket chamfer (which may be fully bottomed; I believe both gearbox bearings are fully bottomed into their pockets). On a related note - all the RH case bearings have to be installed for correct shaft & crank end play, since AFAIK none of them bottom into pockets.

Ray

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


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