OS 70 four stroke rebuild - RC Groups
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May 16, 2017, 03:31 PM
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mistairjoe's Avatar
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OS 70 four stroke rebuild


I tore an OS 70 down for bearing replacement and everything went fine till I was seating the crankshaft in the new bearings where it would just seize at the very end of being tapped in place. Took the whole thing apart several times to make sure the bearings were properly seated and then tried to install the crank and again it would just seize and would not budge. What gives?
Thanks Joe
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May 16, 2017, 04:42 PM
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RCTodd's Avatar
Quote:
Originally Posted by mistairjoe
I tore an OS 70 down for bearing replacement and everything went fine till I was seating the crankshaft in the new bearings where it would just seize at the very end of being tapped in place. Took the whole thing apart several times to make sure the bearings were properly seated and then tried to install the crank and again it would just seize and would not budge. What gives?
Thanks Joe
I just rebuilt my OS 52. I cleaned the crank with 3M 7447.
I put the crank in the refrigerator while i installed the bearings.
Then with the crankcase warm and the crank lightly oiled it went in with a few light taps.


There's no way you installed the camshaft 1st, correct?
May 16, 2017, 05:24 PM
Dieselized User
gkamysz's Avatar
Will the crank not spin in the bearings after it's pushed in? Is this after you've assembled the rest of the engine? Or after installing a prop?
May 16, 2017, 05:34 PM
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mistairjoe's Avatar
Everything was apart except for the two bearings and the crank. The crank would turn in the bearings but for the last few taps when it would seize. The front bearing practically fell in the main was installed and taped in place while on the crank.
May 16, 2017, 07:26 PM
Registered User
Heat the case around the rear bearing to hot not just warm , then gentley push the shaft and bearing home . The bearing should be seated in the housing ,using heat , before fitting to the shaft . But other people have different ideas and that's ok .
May 16, 2017, 09:23 PM
Registered User
Is there a washer between the prop hub and the front bearing ? I cant remember if its like most 2strokes or if its different because it uses the woodroof key . Getting old
May 16, 2017, 10:36 PM
Registered User
I have a 70 pulled apart, and it has the washed, without it the prop-hub will scrape the case, like the crank isn't seated.
May 17, 2017, 03:24 AM
Registered User
Thats what I thought could be your issue . Misplaced washer .
May 17, 2017, 07:11 AM
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I TOBOR's Avatar
This guy, maybe: 46120000
May 17, 2017, 09:25 AM
Registered User
I could have sworn he said " nothing but the two bearings and the crankshaft " ?????
May 17, 2017, 09:29 AM
Registered User
mistairjoe's Avatar
Guys..sorry but I keep posting that there is nothing but the engine case two bearings and the crankshaft put together. So either there is something misaligned between the bearings or somehow the case got deformed.
May 17, 2017, 09:45 AM
Dieselized User
gkamysz's Avatar
Hammering the main bearing home while on the crank is a bad idea. Never apply load through the balls to install a bearing. That is, if the outer race must be pressed into a housing do not press on the inner race.

Chances are the outer race is skewed in the bore and not fully seating. Crankcase condition is a a concern. Another possibility is a bent crank, but I would think it would be visibly bent to cause binding like this.
May 17, 2017, 10:05 AM
the great Gassif´er
If the crank turned fine before the replacement, then it is something you do, not something that is wrong with the engine.

Warm the crankcase, and place the bearings. Make sure they are bottomed.

Leave to cool, and slide in the shaft. It should go in with some force, but no more a normal person can deliver by pushing it, if necessary using a pushing tool (like a stick of wood).
It should not need hammering, at best a light tap with a light object.

Actually, there is nothing that can go wrong that I am aware of, if you do it like above....
May 17, 2017, 10:15 AM
Registered User
Cougar429's Avatar
If the bearings were not fully seated the crank would still float when pressed in. I suspect your rear bearing may not be the correct width and the counterweight is contacting the housing.

If you can, remove the front bearing and install the crank normally. If the binding repeats or you can see the crank nose off center at the front you have the rear bearing installed skewed. It may also indicate if the crank is bent.

That will take removal and reinstallation, but a couple of concerns with that. First there may be some case shaving, contamination or other issue preventing the bearing from seating correctly. Second, I would suspect your initial installation and repeated removals may have damaged the bearing. May not show now, but durability will suffer.

RCTodd is correct the cam has to be removed during crank removal and installation. You definitely will bind and possibly case serious damage if not.
May 17, 2017, 10:59 AM
Registered User
All of the OS four strokes that I have had apart
have had the crankshaft slide easily into place.
None have ever needed tapping. This tells me
that the rear bearing in yours may be slightly
crooked in the case. If it slides in but seizes when
you tighten the prop drive, then one or both of
the bearings is not fully seated.

Jenny


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