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Post by skilpadda on Jun 17, 2021 17:20:15 GMT 1
I measure roughly 64.88mm on the cylinders on my YPVS and about 64.45mm on the pistons, is it time for a rebore or not?
Are the pistons from Yambits any good or better to avoid...?
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Post by Robbieben on Jun 17, 2021 17:39:36 GMT 1
I measure roughly 64.88mm on one of the cylinders on my YPVS is it time for a rebore or not? Are the pistons from Yambits any good or better to avoid...? Measure the bore at top, where the top ring stops and just above it, middle and bottom, if there is a big difference across the bore then its worth considering a rebore as you will have excessive ring/piston wear. Worn bores tend to wear in the shape of a pint glass. |
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Post by jon on Jun 17, 2021 18:38:59 GMT 1
I measure roughly 64.88mm on one of the cylinders on my YPVS is it time for a rebore or not? Are the pistons from Yambits any good or better to avoid...? Measure the bore at top, where the top ring stops and just above it, middle and bottom, if there is a big difference across the bore then its worth considering a rebore as you will have excessive ring/piston wear. Worn bores tend to wear in the shape of a pint glass. +1 I have just had a set of standard bore 31k cylinders rebored to 0.75mm to get the wear from the piston skirt out. What made it worse was that as it was a 385 with forged pistons I had to go up another 0.25mm to 1mm oversize as they only go up in 0.5mm sizes. Jon |
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Post by midlifecrisisrd on Jun 17, 2021 18:43:22 GMT 1
If you are planning to buy new pistons I'd be tempted to just send them to Grampians or PJME and rebore/new pistons
As Jon said I've had std bore to 0.75 in one go
Amazing how oval and sloppy a bore can get and still run well
Steve
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Post by skilpadda on Jun 17, 2021 19:03:39 GMT 1
To send it from norway to uk is not an option. We have facilities at work to do the job though. But are these Mitaka pistons any good? www.ebay.com/itm/273443706219 |
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Post by midlifecrisisrd on Jun 17, 2021 19:15:46 GMT 1
Japanese Mitaka are good
One thing to note is that those are ypvs pistons. They do fit LC no probs but that starts the whole "tang removal" debate
Mitaka make LC pistons
Steve
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Post by midlifecrisisrd on Jun 17, 2021 19:21:14 GMT 1
PJME supply LC pistons (rear of skirt bottom is straight)
Also people say not to buy the Taiwan Mitaka pistons. All Mitaka pistons are cast in Taiwan. If it says Japanese on the box it just means the piston was cast in Taiwan and machined in Japan
Given the fear of the Taiwan ones cracking the casting is surely the problem there
Steve
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Post by oldelsieboy on Jun 17, 2021 20:07:36 GMT 1
Japanese Mitaka are good One thing to note is that those are ypvs pistons. They do fit LC no probs but that starts the whole "tang removal" debate Mitaka make LC pistons Steve Erm, the OP has a YPVS 
OEB
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Post by midlifecrisisrd on Jun 17, 2021 20:16:29 GMT 1
Japanese Mitaka are good One thing to note is that those are ypvs pistons. They do fit LC no probs but that starts the whole "tang removal" debate Mitaka make LC pistons Steve Erm, the OP has a YPVS OEB
I said LC as the ebay link he put up showed LC gaskets My attention span didn't allow for keeping info from the opening post 😂 Steve |
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Post by chippy348 on Jun 17, 2021 22:47:11 GMT 1
PJME supply LC pistons (rear of skirt bottom is straight) Also people say not to buy the Taiwan Mitaka pistons. All Mitaka pistons are cast in Taiwan. If it says Japanese on the box it just means the piston was cast in Taiwan and machined in Japan Given the fear of the Taiwan ones cracking the casting is surely the problem there Steve This is not True Steve, Mitaka motocross pistons are made In Taiwan, they do have some road bike pistons that are also Taiwan DT125R TZR125R and 350 pistons, but they will say made in Taiwan on the box, anything that says made in japan on the box is a FULLY Japan made piston, this i can assure you, well it is the case if you buy from me. As a side note there is a big difference between the Taiwan pistons, EG some of the DT125R ones like KHC have cast iron type rings, the Mitaka Taiwan has the better steel type rings, well they do if they remember to change them ! |
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Post by chippy348 on Jun 17, 2021 22:49:57 GMT 1
I measure roughly 64.88mm on the cylinders on my YPVS and about 64.45mm on the pistons, is it time for a rebore or not? Are the pistons from Yambits any good or better to avoid...? IF you really think the cylinders are 64.88mm then that could be a very worn 0.75 oversize. What is on top of your old pistons ? |
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Post by skilpadda on Jun 17, 2021 23:05:06 GMT 1
I measure roughly 64.88mm on the cylinders on my YPVS and about 64.45mm on the pistons, is it time for a rebore or not? Are the pistons from Yambits any good or better to avoid...? IF you really think the cylinders are 64.88mm then that could be a very worn 0.75 oversize. What is on top of your old pistons ? Carbon deposits what i can see... Shall there be any letters or numbers under the soot...? |
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Post by chippy348 on Jun 17, 2021 23:19:47 GMT 1
IF you really think the cylinders are 64.88mm then that could be a very worn 0.75 oversize. What is on top of your old pistons ? Carbon deposits what i can see... Shall there be any letters or numbers under the soot...? Yes there should be something on it, can i ask what you measured the cylinders with ? |
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Post by skilpadda on Jun 17, 2021 23:49:14 GMT 1
Yes. Digital vernier caliper at top and bottom of bore. Used the same for the piston. If it is a 64.75 piston it is very worn at 64.45 i think...
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Post by midlifecrisisrd on Jun 18, 2021 8:25:11 GMT 1
PJME supply LC pistons (rear of skirt bottom is straight) Also people say not to buy the Taiwan Mitaka pistons. All Mitaka pistons are cast in Taiwan. If it says Japanese on the box it just means the piston was cast in Taiwan and machined in Japan Given the fear of the Taiwan ones cracking the casting is surely the problem there Steve This is not True Steve, Mitaka motocross pistons are made In Taiwan, they do have some road bike pistons that are also Taiwan DT125R TZR125R and 350 pistons, but they will say made in Taiwan on the box, anything that says made in japan on the box is a FULLY Japan made piston, this i can assure you, well it is the case if you buy from me. As a side note there is a big difference between the Taiwan pistons, EG some of the DT125R ones like KHC have cast iron type rings, the Mitaka Taiwan has the better steel type rings, well they do if they remember to change them ! I'm only going on what Mitaka state on their website I just got my GT/TR750 top end back from BDK and they have supplied Mitaka pistons and they do indeed state made in Taiwan on them Not decided on them yet. BDK have had no problems with them 🤔 Steve  |
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Post by marrcel on Jun 18, 2021 8:56:32 GMT 1
Put a piston ring in the bore at different heights. Check ring gap. This will give you additional info😉
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Post by skilpadda on Jun 18, 2021 11:30:54 GMT 1
I have now ordered a set of 65 mm pistons...
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