TY 250 A Crankshaft Repair/ Advice?

[Tillerman6]

So now it looks like the crankshaft is out of tolerance for "Deflection Tolerance".  Basically the crank is "spun" as the engine rebuilders like to say..  So in simpler terms, the left end of the crank is not lined up with the right end.

The tolerance in the shop manual is only 0.0012" per side and I've got .005"  of runout total.  

 

Does anyone have any experiance with "tapping" the crank halves back into alignment?

[bultobill]

Not  difficult to do just hold it in the palm of your hand by the weights and hit the appropriate weight with a copper hammer of a normal size. You dont  have to hit them that hard to move them. Occasinally they become "stretched" and need springing a little in a vise or similar at the opposite side to the crank pin to get the last little bit in that plane. A little patience goes a long way.

 

[Tillerman6]

Bultobill,

    That seems doable,but I don't have any confidence that it will stay in position unless the pins are welded or pinned in place, so I sent it off to be rebuilt.  The pins are just a press fit and the bike has a tendency to backfire when starting, so there is considerable strain on the pin to flyweight junction.  When I checked the runout, the specifications are very tight IMHO, so there was no wobble you could see just by looking, it was only when you put a dial test indicator against the flyweight circumference that you could see a .002" runout on one side and a .003" runout on the other side.  And the spec is .0012 left or right.

But I do want to ask you- what has been your experience with this sort of fix?   It's a lot of trouble to tear down the bike again if something slips.  Special tools, new gaskets, and possible bearing damage plus too much down time and expense. The case halves are also sealed up with a sealant after a repair like this, so there is that sealant to deal with as well

[feetupfun]

The proper sealant stays soft so it's no big deal splitting the cases again.

It's fairly common to weld the pin to the crankwheels but the person splitting the crank will be able to tell if that is necessary when they push it apart by how much force it takes.

[bultobill]

Honest answer is no experience of spun crankshaft, or 250 A, just changed a few crankpin/conrods and thats how I do it as thats how I was shown how to do it.   I do know the production racing guys used to weld crank pins as it was a common problem to them but never had to do it myself on a trials bike.     Happy to go with Feetupfuns comment an experienced guy would be able to tell if it was still tight enough from experience. A point to note here and I stand to be corrected but the DTI / clock shouldnt go on the weight it should be placed on the bearing journal as that is the bit that needs to be true and by putting it on the weight you are relying on the journal and weight diameters being concentric to each other (which in theory they should be but...) Just another point in passing ,this is why when stripping or rebuilding engines of this design its bad practice to tighten the flywheels by putting a spanner on the nut at the other end of the crank to hold it.

Good luck.

[greychapel]

Allen millyard has some videos on his you tube channel showing how he splits and builds up crankshafts for his multi cylinder specials

[Tillerman6]

On 5/7/2020 at 1:20 AM, bultobill said:

Honest answer is no experience of spun crankshaft, or 250 A, just changed a few crankpin/conrods and thats how I do it as thats how I was shown how to do it.   I do know the production racing guys used to weld crank pins as it was a common problem to them but never had to do it myself on a trials bike.     Happy to go with Feetupfuns comment an experienced guy would be able to tell if it was still tight enough from experience. A point to note here and I stand to be corrected but the DTI / clock shouldnt go on the weight it should be placed on the bearing journal as that is the bit that needs to be true and by putting it on the weight you are relying on the journal and weight diameters being concentric to each other (which in theory they should be but...) Just another point in passing ,this is why when stripping or rebuilding engines of this design its bad practice to tighten the flywheels by putting a spanner on the nut at the other end of the crank to hold it.

Good luck.

Bultobi

Well noted !  And I soon realized that this was way over my head, so I sent the crank out for an overhaul along with the piston, rings and pin and circlips.  In case they want to weigh them or if there is any question about the small end of the con rod.

 

The specs are in the DT250/DT360 1974 shop manual, and that was enough information to find out that the crank was spun.  To me this means that the two ends of the crank are not concentric.

But the manual was a little ambiguous.  They show the specs for each end as .0012" max runout and a - line for the runout  (reference ?) on the weight circumferences.   So you could theoretically zero your DTI on the weight circumference and take the reading off each bearing journal. But that would not neccessarily make any sense because like you said, the weights could be non-concentric with their own respective weights and that still would not give you a reading that would be useful.  So I just Zeroed the clutch end in my 4 jaw and took the reading on the other end.  I got .005" total runout which is a bit too much.

 

There was also a tiny bit of squeeze to the weights.  I.E. the gap between the weights near the bottom end was different than the gap at the other side by about /004".  Hopefully that will all get sorted out.

I don't know exactly when the damage occured either. I am probably the 3rd owner, so no telling.  but I do know that this engine of mine likes to backfire on occasion.

 

I'm thinking of adding a diode into the wiring harness like the 75 models have..  That might make a difference.

 

I will be making or purchasing some tools to pull the crank back into the cases.  I can see now that the crank is not bullet proof, so it needs some TLC like everything else.

 

There is now a set of pusher and puller tools on the internet  that seem to be nearly an exact copy of the Yamaha tooling that is shown in the shop manual. These are made by TUSK and there are even demos on You tube where you can watch them being used.

 

I think the install tool is about 65.00 US and same for the puller. 

 

But I have a nice lathe, so I'll be doing it the hard way of course!

 

Tillerman6

 

 

 

 

[lineaway]

 Actually if you have it as straight as possible and it does not rub the cases, the halves will balance themselves out. Welding them and they cannot self correct. But what do I know, I learned it from an old flat tracker. Never had an issue as long as you are patient.

[Tillerman6]

Lineaway,

  It might be wishful thinking.  But if you are racing every weekend you probably have 3 or 4 bikes and at least one torn down to bare bones at all times. I'm not in that category.  This is my first and hopefully last case splitting party, so I have to get it right this time.  And I would respectfully disagree with your flat tracker friend about an engine "balancing itself out"  if everything else is as straight as possible.  What does that mean?

As far as I know, a reciprocating engine is never perfectly balanced.  And a single cylinder 2 stroke is probably one of the least smooth running engines out there.  Of course you can optimize things and make improvements with some expert balancing techniques, but those are also out of my area of expertise.  I'm just wanting this thing not to shake the handlebars so hard that it puts my hands to sleep or breaks another lug on the frame!

[feetupfun]

As a matter of interest in the vibration stakes, did you know that when Yamaha produced the second and later models TY250, they used a lighter conrod design, compared with the A model. This had a noticeable effect.

I have a few bikes with TY250 engines and the one with an untouched, original A model engine in it has enough vibration at one particular RPM that the vibration transferred to my body causes loss of vision. This doesn't happen when trials riding because the RPM never stays constant long enough and I'm standing up, but when I'm trail riding in 4th or 5th gear and find that particular RPM and am sitting down, the vision thing happens. When I do the same sort of riding on my other TY250 powered bikes, I don't get the vision effect. The other motors are TY250D model with original conrod and TY250B model with original conrod.

[thai-ty]

On 5/7/2020 at 8:50 AM, Tillerman6 said:

 It's a lot of trouble to tear down the bike again if something slips.  Special tools, new gaskets, and possible bearing damage plus too much down time and expense. The case halves are also sealed up with a sealant after a repair like this, so there is that sealant to deal with as well

Not really. A couple of special tools, good selection of normal quality hand tools, common sense and a shop manual, and your time is your labour.

They are not a difficult engine to work on.
IMO, a 2T single is one of the easiest motors to rebuild.

[Tillerman6]

On 5/8/2020 at 11:04 PM, feetupfun said:

As a matter of interest in the vibration stakes, did you know that when Yamaha produced the second and later models TY250, they used a lighter conrod design, compared with the A model. This had a noticeable effect.

I have a few bikes with TY250 engines and the one with an untouched, original A model engine in it has enough vibration at one particular RPM that the vibration transferred to my body causes loss of vision. This doesn't happen when trials riding because the RPM never stays constant long enough and I'm standing up, but when I'm trail riding in 4th or 5th gear and find that particular RPM and am sitting down, the vision thing happens. When I do the same sort of riding on my other TY250 powered bikes, I don't get the vision effect. The other motors are TY250D model with original conrod and TY250B model with original conrod.

That's very interesting!  I had a similar problem with mine, but I found a couple of things that (on my bike) were bad.  #1 was I had a twisted crank shaft, and #2 I had an engine mount broken loose from the frame.  I fixed the frame, but I was told by an old timer that the mounting bolts could use beefing up on these bikes- like go to an english sized grade 8 bolt that is stronger.  And the factory book has you torque the frame to engine mounting bolts - I doubt if mine were torqued to specs.  Note that the front bolt is a different size than the two rear ones.

so quoting from the DT250A/360A factory repair manual - the 8mm bolts get 217-251 inch pounds, and the 10mm bolts get 390.6 to 477.4 inch pounds.

 

But how did you find out that the 311-11651-00 from the 74 was heavier than the 431-11651-00 from the 75 TY250B?

 

Could be the pistons are also a different part number and weight- but now you're talking unobtanium.

 

 

[feetupfun]

2 hours ago, Tillerman6 said:

But how did you find out that the 311-11651-00 from the 74 was heavier than the 431-11651-00 from the 75 TY250B?

The thing that got me interested was a magazine test I read in 1975 when the B came out. It listed some differences between the A and the B models.

Then over the years I collected motorbikes and motorbike parts and one day was able to compare the dimensions of the two conrods. I could see that the B conrod has a smaller beam section of the conrod so I finally believed the motorbike article. I haven't weighed them though.

I suspect that Yamaha changed the conrod design on all their similar motors at the same time as the TY250 changed, much like the primary drive gear went from spline to key drive with the B model at the same time as the Yamaha MX and trail bikes changed. The same happened with the fork sliders design and the clutch design.

[feetupfun]

3 hours ago, Tillerman6 said:

That's very interesting!  I had a similar problem with mine, but I found a couple of things that (on my bike) were bad.  #1 was I had a twisted crank shaft, and #2 I had an engine mount broken loose from the frame.  I fixed the frame, but I was told by an old timer that the mounting bolts could use beefing up on these bikes- like go to an english sized grade 8 bolt that is stronger.  And the factory book has you torque the frame to engine mounting bolts - I doubt if mine were torqued to specs.  Note that the front bolt is a different size than the two rear ones.

so quoting from the DT250A/360A factory repair manual - the 8mm bolts get 217-251 inch pounds, and the 10mm bolts get 390.6 to 477.4 inch pounds.

Broken engine mounts are very common on twinshock TY250s. Usually one of the top rear mounts but the latest one I've repaired also had a crack in the steel cross piece between the front engine mounts. It had been used for sidecar trials though which is a tough ask of any frame.

One TY250B I bought had been ridden for some time with all the engine mount bolts loose. How could I tell it had been like that a while? The holes in the engine casings were elongated and had cracks at the ends of the holes in the casings (and the bolts were loose). It had lots of other problems too but it was a good parts bike for the price.

I've never seen a TY250 broken engine mount bolt but have seen the threads worn off from movement.

Edited May 24, 2020 by feetupfun

[Tillerman6]

 I have been trial fitting the cylinder to the empty engine cases to see how the fit is working now that I have a new cylinder.  And it went on over the studs but only just. If you put a few pounds of pressure towards the rear it will just slide down over the studs.  I spent some time measuring everything and it turns out that the bolt pattern is not rectangular at all.  The holes in the cylinder are oversized by .020" and the bolt pattern is actually out of square.  None of the distances between the studs are the same.  Anyway that's Ok because the holes in the cylinder are out of square the same way.

 

I ordered all new parts for the engine except the piston.  It will get all new seals and bearings, new rod and upper and lower needle bearings, washerrs and o ring. Also new pins top and bottom.  I's only money and what else could i do to stimulate the economy?

 

i also have been sanding off the rough outside burrs and flashing edges of the castings to improve the appearance slightly. There were several places with sharp edges that are now rounded down to a dull roar.  I'm not too worried about the sanding dust getting into the castings now because it wiil get all new bearings later on.

I noticed a little corrosion starting inside the front case ears where the front mouninting bolt passes thru. Fortunately this was cleaned out without much problem. 

 

The aluminum is stained pretty deep in several places. not sure if it's worth sanding it all out because intend to be riding it more than polishing it.

There is quite a bit of cleanup to do to get all the old case sealer off the edges of the castings.  I'm using Goof off and a rag for that.  Acetone eats up the rubber gloves too fast.

I still did not hear from you about how you knew that the weights of the con rods are different between the 74 and 75's?

 

Is it riding weather over there now? Having any fun at all?  Probably still no trials meets any time soon?

 

Stay safe!