Malossi BBK for People 50 2T

[chaz35]

Both crank bearings look good to me, very little axial play and no virtually no radial play.  The small crank bearing, on the stator side, is a little tight, has some debris in it, so I took it out for inspection and to clean.  Probably didn't have to take it out, but I wanted to see if I could get it out to replace if I wanted to.

I used heat gun to heat the case and drop out the bearing.  I got a non-contact infrared thermometer to check the temp while I was heating it.  The bearing dropped out at something over 200 degrees.  I didn't want to use the oven or outside grill because there are some rubber fittings pressed into the case on both sides where the engine case is mounted to the frame I didn't want to have to replace, and the side with the transmission has several seals I didn't want to damage.  Took awhile to heat up, but worked, I was proud.

To put the bearing back, I plan to cool the steel bearing in the freezer, and heat the alum case some with the heat gun, should just drop right in?

[Triesandluth]

Yeah,  that's the method I read of many using with success.  I've read that some try to put the bearings in the crank,  then put the crank in the freezer,  then heat the cases,  then install frozen crank with bearings.  Seems to be successful.  Things I've also read is that when you have it together it should spin freely.  If not you can gently tap it with a 2x4. Anything harder can knock the crank out of true.  I've watched videos on truing up a crank and checking the run out.  Seems like something I'd rather not have to mess with.  I feel like it's time for me to split my other tgb case so that I can have more of an experienced point of view on the subject.


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[chaz35]

Good comments on getting it back together.  If it doesn't spin freely after in the case with the case together, something has to be wrong?     All new to me, I did very little to my kart 2cycle engines except tune them:  pipe, carb, gearing.  Always took to engine builder for any major repairs.  I am learning, but has been painful so far.

I am sure you know, but the rod can be replaced by pressing the crank apart, so you can replace the rod and big end needle bearing.  Then you press it back together, keeping it straight as possible.  Then you true it up by hitting it with a big hammer on the appropriate spot to move it just a smidgen, like you probably saw on youtube.  Fortunately, you can get a complete crank already assembled for a relatively reasonable amount.

I am with you, don't want to mess with it, takes someone with the right tools, knowledge, and experience to do it right.  I think a properly assembled crank that is true is pretty hard to get out of alignment, unless you twist it, one end against the other.  For example, if you secured one side and applied torque to the other side, it will twist out of alignment for sure.

  I really like these engines, I love to hear them run, is music to my ears.  Cheers

[Triesandluth]

190mech on the 49ccscoot forums is the man to read up on with cranks.  He's the one I've robbed this info from.  Also with pipe building.  I gave him the outer dimensions of my next r pipe and he was able to tell me what kind of power it will give me.  Actually, he just told me it wasn't built to be a great performing pipe.  Not a tapered header,  angles of the cone wrong,  the baffles in it being restrictive,  there's welds that prohibit the wave from returning to the cylinder etc. 

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[chaz35]

Good info, I will checkout 190mech posts.

I dropped out other crank bearing just for grins and dropped out easy.  Bearings are maybe better than I thought, made by TPI in Taiwan.  Was looking at replacement bearings and ran across cool youtube 3 of 3 set on how to change out crank bearings on Super 9, pretty good info.  His main tool was a big plastic hammer, he didn't mind hitting the case and crank, made me glad I bought the case splitter, but you definitely don't have to have it.

Putting it back together, he installed the bearings into the case halves 1st.  He used heat gun and freezer routine.  I also figured out was lucky my crank bearings stayed in the case.  If they stay on the crankshaft you have problem getting them off.  You can buy a bearing puller, another $100 or so, or cut them off with a cut-off saw (ugly).  He was replacing his and had to cut one off.  1st he taped up the crankshaft and rod to protect the rod big bearing from debris, very good idea.

I did some testing putting it all back together.  I measured the best I could with my dial caliper and found bearing ID is approx .002" less than crank journal OD for both bearings, so definitely an interference fit.  I don't think I want to take down .002" on crank journal, so will just polish them some, that and the inside of the bearing with 2000 grit sandpaper.  Anyway, I heated up the bearings and put the crank in the freezer, then measured everything again.  It was amazing, the crank got smaller by approx .002" and the bearing ID got larger by approx .002".  I am thinking everything will go together easy, if I do some preparation.

I cleaned up the bearings with brake cleaner and compressed air and they feel better than new, they are broken in.  With compressed air, they would spin freely, had cool gyroscope effect, made me wonder how many rev's they were turning?  I am going to research the velocity of air coming out of an air gun at 60 psi and compute the estimated rev's lols.

I think ready to start putting it back together.  Cheers

[Triesandluth]

Awesome!  I'm glad to hear that the difference in hot vs cold is enough to make the install smoother.  Gives me a little peace of mind when I comes to me having to finally do it. 

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[Triesandluth]

Would the velocity be affected by the size of the end of the tool?  I assume it would be a different speed coming out of a 2mm hole than a 2 inch hole.  I guess that would be factored into the equation. 

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[chaz35]

Yes, there are many factors you can allow for, more than you or I want to know.  I initially found a physicsforum and they were arguing about the temp of the air as it change because the air pressure in the tank was dropping, the change in air pressure, whether the speed compressed the air or not, etc, etc, big long equations that allow for many variables.  I didn't understand most of it.  I went to another website and I got a rough estimate of air velocity coming out air nozzle at 70psi of approx 80mph, which seemed reasonable to me.  Then I computed an estimate of how many revolutions the OD of the bearing would have to make in a minute, if it travel 80 miles in one hour.  I came up with approx 15,000rpms.  That is, holding the bearing in my hand, blowing air on it?  Pretty crazy, I know, but seems plausible.

I know from experience with 2cycles, they are faster when the crankshaft gets a little loose side to side play, not up and down play.  Any up and down play is bad, should replace bearings.  If the bearing is tight on the crankshaft and bearing pocket in the case, the loose  side to side play is axial play in the bearing.  IMO, if the bearings are still good, they are better than new bearings because they have loosened up a little, a good thing if you are looking for more HP.  Cheers

[chaz35]

I chamfered the ports with my dremel and carbide bit.  This is the 3rd time I have done chamfering and I am feeling more comfortable doing it.  I did it with the 1st BBK, then practiced on the oem cylinder 2nd time, and 3rd time on the 2nd new BBK.  Before, I was setting the dremel for lots of revs thinking it would be smoother.  I found it hard to control and less forgiving.  If it jumped outside the port it was very easy to scratch up the cylinder.

This time I set the dremel for pretty slow revs and it worked a lot better.  I could feel the sharp edge on the port with my finder before I chamfered it, and the relatively smoother edge after.  I scratched up the bore a couple of times, but not too bad.

The big issue I have now is whether to run the flexhone through the cylinder or not.  The mystery is why the piston failed 2nd time.  I don't think it had anything to do with the flexhone, but don't know for sure.  After I chamfered the ports and ran the flexhone through the cylinder after the 1st disaster the cylinder looked really nice.  I am inclined to do it again, but makes me nervous.

Sure is a lot of work and expense if it blows again.  I am thinking I should have replaced the piston instead of just the rings.  That the piston was probably already mortally wounded, and was my fatal misstake to use it again.  Anyway, if I decide to use the flexhone I know have to really clean the cylinder and ports or will be failure, but I am confident I can clean it properly.  Cheers

[paceneedsstides]

Looking at my BBK it looks crosshatched like the honed it so while I will chamfer it I'm not gonna hone it.

[chaz35]

Yes, I think nicely honed from Malossi, not necessary, you are smarter than me.  I originally got the flexhone so I wouldn't have to chamfer the ports, flexhone would do that.  Now I figured out how to use the dremel and carbide bit to chamfer, so guess I really don't need to flexhone it now?  Cheers

[paceneedsstides]

No I'm not smarter than you. And Definitely don't have the experience you have on these but I know when to leave well enough alone lol.

[chaz35]

Got crank bearings installed on crank.  Put crank in freezer for hours, and heated up the bearings with heat gun to approx 200 plus degrees.  Easy, dropped onto crank no problems, except of course there was a problem.  1st one went on so easy, then it moved a little, and locked down out of position while I was putting on the the other side.  I ended up beating on it with a plastic hammer, bummer, but I think all is well.

I anticipated problems and already had a piece of 2x4 with 1" hole drilled so I could support the bearing against it, if I needed to beat on the otherside of the crank.  Next time, I may install one side at a time:  freeze the crank, heat up one bearing and install it, then freeze the crank again and install the 2nd bearing.  However, you just have to be careful and know the bearing can move until it cools a little and locks down.

I decided to install the bearings on the crank 1st instead of installing the bearings in the engine case 1st because I figured it's easier.  It's easier to install a steel bearing into a alum case, than to install a steel bearing onto a steel crankshaft, but could go either way?  I know the steel bearings will drop out of the alum case with some heat, but are very difficult to get off the steel crankshaft (need an expensive bearing puller or grind off with cut-off saw).

After I was done, I started worrying about the crank being true.  I put the crank in a vise holding it by the bearing and could spin it.  I don't have a dial indicator, so didn't measure roll-out, but looked true.  I think when the crank has run-out, it's usually easy to see some wobble, and if you can see it, probably too much.

Also, I still think the bearings are good, but in the vise I could really tell the right side bearing/ smaller bearing on the stator side had a lot more play than the other bearing.  In the youtube I watched on how to change crank bearings, as I recall, the same bearing went bad on his engine.  I wonder if that side usually goes 1st?  He had a lot of up and down play on the end of the crank and was able to show it on the vid, that bearing was trash.  It was an Airsal BBK engine and still running.  However, the bearing was disintegrating and had maybe previously damaged the cylinder, but he was just now figuring that out.  Anyway, I am ordering some spare crank bearings.

Next, I plan to put the case halves together, should be an adventure.  Cheers

[chaz35]

Have new crankshaft installed in crankcase, went together easy, no problems.  I didn't bother to freeze the crankshaft this time and just used the heat gun to heat up the alum cases.  Cases are slow to heat up and probably something around 150 degrees is sufficient.  I installed the crankshaft in the right side 1st on the floor.  I had the longer left side in a vise on the workbench so it was secure and easy to work with.  I used Permatex that's like Hylomar Universal Blue to seal the cases together, there is no gasket.  I read up on Hylomar, is very good for sealing a case.

A few items to watch out for:  per the service manual the right crank seal in approx 1/2" down into the bore; there are 6 bolts that hold the case together, torque them 6 to 8 ft pounds; 3 bolts are longer, with 2 long ones going with the dowel pins, and the 3rd long one is on top where the cylinder bolts down.

I had to pop the ends of the crankshaft both sides with a hammer to get it to loosen up after I tightened up bolts, but that loosened it up nice.  Crank is nice and tight, no play side to side or up and down, spins easy, no worries about the bearings.

I have new theory about why the piston failed, think it was most likely too much rod side to side play.  I know my original rod was way over spec and should have been replaced.  If the rod has enough side to side play, it may be able to bind the wrist pin bearing enough to freeze the small end of the rod to the wrist pin.  When that happens, the small end of the rod becomes a lever that forces the side of the piston against the bore as it is going up and down. 

I think that is what happened and why the piston failed.  The largest port is the exhaust port, so most likely place to fail, and that is where it failed.  I installed a new crankshaft assembly, so should be OK now.  I don't think it had anything to do with using flexhone to smooth the ports.  I will probably use the flexhone again before it install the cylinder.

Would finish it up tomorrow, but weather is turning cold and wet, should have it running early next week.

One other item:  my Ford mechanic friend got onto me for free spinning the crank bearings, not a good idea, and hard to imagine but could be dangerous.  OK to use air to blow out bearings, just don't free spin them to the moon.  Cheers

[Triesandluth]

Yes, I think nicely honed from Malossi, not necessary, you are smarter than me.  I originally got the flexhone so I wouldn't have to chamfer the ports, flexhone would do that.  Now I figured out how to use the dremel and carbide bit to chamfer, so guess I really don't need to flexhone it now?  Cheers

Brand new kit,  smoothed the port edges,  shouldn't have to hone it.  That extra hone might make it less taxing on the rings while it breaks in and the get properly seated,  but if it ain't broke....
Congrats on the successful install,  glad it went rather smooth.  And that's a good possibility for your failures,  the reasons it seized at the ex port. 
What's the big deal with spinning the bearing?  Not good to dry run? 

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