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74 TY 250A piston hitting flywheel on installation.


Tillerman6
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This might be a tough request to make, but I have a small dilemma.  I have a stock 74 TY 250A and I have had everything rebuilt on the engine.

But when I tried to install the piston, it touches the flywheel before the flywheel can rotate all the way around.

So either I have the wrong piston or the wrong Connecting rod installed.

My measurement for the piston skirt is 68.3 mm from the bottom of the lower piston ring to the bottom edge of the piston skirt.

Does this seem to be about right?  The piston is a genuine Yamaha, from CMS, part number 438-11631-01-96.

If anyone has a loose TY 250A -1974 model piston laying around loose they can measure- it would be a great help !!  Thanks in Advance!

Update- CMS has sent me the correct 438 piston now and I installed it this morning- fits perfect! 

Edited by Tillerman6
UPDATE status = corrected
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Came across this when I rebuilt my 250C, often a type 311 piston is listed as fitting the TY but there are a couple of minor differences.

The TY piston is made by ART but  the 311 type is made by Izumi, thickness of the skirt is thicker on the 311 piston but the main difference I found was the TY piston has a chamfer from the skirt to the transfer cutout that the 311 doesn't, that seems to be where the piston hits the crank. I assume that the DT2 which is what 311 model code relates to uses either a smaller diameter crank or a longer connecting rod so the piston doesn't make contact

If you have your old piston you should be able to see the differences I've mentioned, if you put wrist pin into both pistons at the same time you'll see that the overall height & pin position are the same. I didn't try altering the skirt on the 311 piston but I think copying the chamfer will fix the problem you have

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Tony27,

  Unfortunately I don't have the old piston.  I sent it off to my engine rebuilder and it got lost in the mail.  

But it sounds like you have run into this before.  I think it would be a big problem for anyone that has to replace pistons in these old bikes.

And there is a guy on Ebay selling a piston like it but he does not specify in his description which part number he is selling.

so he's listing the same piston with BOTH part numbers inclusively.  I.E.   311/438 11631-01-96. 

So I'm not sure if there is really any difference, or what is going on.  At least I have a con rod with 311 showing on it.

Maybe I do need the real 311 part number for a piston and not the 438???  Here is the ebay listing:

https://www.ebay.com/itm/NOS-Yamaha-Piston-Std-Size-1972-77-DT2-DT3-DT250-TY250-438-311-11631-01-96/264984342801?_trksid=p2485497.m4902.l9144

It might dissapear any minute now or get sold so don't wait if you want to see it.

 

 

 


 

 

 

 

 

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That is the 311 type piston, notice how the skirt is sharp where it meets the transfer, the 438 piston for a TY tapers in that area. The skirt on the TY piston is thinner as well

I'll see if I can find the photos I took at the time showing the differences

Try zooming in on the first picture in this listing, the taper is just noticeable & not very big which is why I think you should be able to modify your piston

Ty 250 Type 434/516 Yamaha Piston Full Side 0,75 | eBay

This is the guy I bought mine from but I don't think it was as expensive back then

Edited by tony27
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Tony27!

   That's the info I needed! But CMSNL still shows pictures of the 311 piston with a 438 part number on it.

https://mail.google.com/mail/u/0/#inbox/FMfcgxwLsSfJmSnbRtVsLJcFKlDWhnWR

So the next person that orders a 438 piston from them will get another 311 just like I did.

 

Either way- it looks fairly easy to knock off the corners near the transfer holes and Wallla!  It should clear the flywheel halves!

 

 

Edited by Tillerman6
correction of my mistakes
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On 2/12/2021 at 1:21 AM, tony27 said:

The used piston with the chamfers by the transfers is the original type 438 TY250 piston, the new piston that has sharp edges & the thicker skirt is the type 311 piston that contacts the crankshaft

Now I am straightend out.  And I agree with you 100%.  However CMSNL still has their documentation backwards.  Hopefully on Monday I will be able to contact them and see what they can do.  Here is the advert they sent me.  You can see the sharp corners in the photo and the 438-11631-01-96 part number there on it.  https://mail.google.com/mail/u/0/#inbox/FMfcgxwLsSfJmSnbRtVsLJcFKlDWhnWR

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On 2/12/2021 at 5:00 AM, guy53 said:

I posted a picture of 2 piston sold as original TY piston, one is from a DT model and the other a TY original.Gu

Guy53- I don't suppose you would be able to weigh both pistons and tell us all if there is any difference?  
I would think that the 311 with the sharp corners and thicker walls is heavier than the 438, but if it's just a couple of grams- no big deal.

 

Edited by Tillerman6
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On 2/10/2021 at 6:23 PM, tony27 said:

Came across this when I rebuilt my 250C, often a type 311 piston is listed as fitting the TY but there are a couple of minor differences.

The TY piston is made by ART but  the 311 type is made by Izumi, thickness of the skirt is thicker on the 311 piston but the main difference I found was the TY piston has a chamfer from the skirt to the transfer cutout that the 311 doesn't, that seems to be where the piston hits the crank. I assume that the DT2 which is what 311 model code relates to uses either a smaller diameter crank or a longer connecting rod so the piston doesn't make contact

If you have your old piston you should be able to see the differences I've mentioned, if you put wrist pin into both pistons at the same time you'll see that the overall height & pin position are the same. I didn't try altering the skirt on the 311 piston but I think copying the chamfer will fix the problem you have

Yes, the DT2 crank is smaller diameter than the TY's

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15 hours ago, jc2 said:

Yes, the DT2 crank is smaller diameter than the TY's

jc2,   That is the consensus on the Trials Central site.  You guys are the Guru's of the engine world.  Now the trick is to convince the people at CMSNL.  They have the 311 and 438 pictures and part numbers swapped around backwards which is how I came to recieve the 311 piston.

I was lucky enough to keep a picture of my old piston which is a 438 and sent it to them. But now they have the ball and it might take some time for them to respond to my emails.  I will get back on here when I get a response and let you all know what happened.  

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I have 2 X 71.00mm on hand one weigh 225g the other 240g and one 70.25 mm that weigh 224 g. I don't have any DT pistons on hand anymore. It is an easy job to cut the corner on the skirt of a DT piston to fit the TY crank.

Guy

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