Front and rear pistons?

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StormyRob
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Front and rear pistons?

Post by StormyRob »

Need to replace a rear piston to have a second engine.

Had a look and seems the front and rear have different part numbers. I thought they would be the same size and dimensions etc. Is that incorrect?
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tony.mon
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Re: Front and rear pistons?

Post by tony.mon »

I've always thought that they were the same.
And the rings, are they different part numbers as well?
The pistons are very cheap though, std Honda ones, that is.
But whenever I've got that far into an engine I fit high comp ones, bit more spenny but lots more fun....
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StormyRob
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Re: Front and rear pistons?

Post by StormyRob »

Rings are the same part number. Yea they are about 25 quid. This engine is going into another bike at some point, not my main one so wont be getting the hi-comp ones...yet. I know of a set for about 150 or so used but just haven't made the decision yet as I cant find cams anywhere in the world for anything reasonable. I am hoping I get lucky some day :lol:

I would nearly gamble on the pistons being the same. Just cant understand the number difference.
If you don't try you never know if you can do it.
tony.mon
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Re: Front and rear pistons?

Post by tony.mon »

Thinking about it they will have oil drain holes in different positions, so are cylinder specific.
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StormyRob
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Re: Front and rear pistons?

Post by StormyRob »

Ah. Fair enough. Good thinking and will not gamble on one then!
As you say cheap enough. Fowlers in the morning it is
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fulviogsx
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Re: Front and rear pistons?

Post by fulviogsx »

tony.mon wrote: Sun Oct 01, 2017 8:41 pm Thinking about it they will have oil drain holes in different positions, so are cylinder specific.
...mumble mumble.. this should mean the cylinder are installed differently...

In other words: consider the vtr engine like 2 separate engines of 500 cc.Logically each one should have the same piston and components. If it don't, this is because some other part (which one? boh .. may be the cylinder)of the engine is not completely rotated of 180 degrees, then it needs a differrnt piston to compensate
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Red3446
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Re: Front and rear pistons?

Post by Red3446 »

The piston pin's offset should be different.
fulviogsx
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Re: Front and rear pistons?

Post by fulviogsx »

Red3446 wrote: Mon Oct 02, 2017 11:53 am The piston pin's offset should be different.
may be, but it means it hasn't been rotated of 180 degrees
tony.mon
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Re: Front and rear pistons?

Post by tony.mon »

There isn't any piston offset. This isn't a De saxe design.
The engine's rotation means that oil drain holes need to be in the right place so that oil drains into the back of the ring grooves after it has been flung off the crank during rotation.
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Red3446
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Re: Front and rear pistons?

Post by Red3446 »

Normally every Honda pistons have a pin offset, due to friction and vibrations reduction.....now I'm not sure for this in particular but I'll verify on pistons I have at home.

Other difference should be, as written before, on oil draining holes.
tony.mon
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Re: Front and rear pistons?

Post by tony.mon »

You could be right, I've never actually measured any. It'll be interesting to see.
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Red3446
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Re: Front and rear pistons?

Post by Red3446 »

As I supposed: piston pin offset is different (near to 1mm)

Piston one
Image

Piston two
Image

:thumbup: :thumbup: :wink:
Last edited by Red3446 on Thu Oct 05, 2017 12:32 pm, edited 1 time in total.
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StormyRob
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Re: Front and rear pistons?

Post by StormyRob »

Red3446 wrote: Thu Oct 05, 2017 10:27 am As I supposed: piston pin offset is different (near to 0,5mm)

Piston one
Image

Piston two
Image

:thumbup: :thumbup: :wink:

Thank you very much for that clarity. Do not think the question had been asked before
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freeridenick
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Re: Front and rear pistons?

Post by freeridenick »

Do you need a piston? I've got at least one you can have.
tony.mon
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Re: Front and rear pistons?

Post by tony.mon »

Red3446 wrote: Thu Oct 05, 2017 10:27 am As I supposed: piston pin offset is different (near to 1mm)

Piston one
Image

Piston two
Image

:thumbup: :thumbup: :wink:
Good info, well done for the evidence, too.
It's not falling off, it's an upgrade opportunity.
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