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Author Topic: How much porting do you really need to do it you bore your cylinder over?  (Read 4596 times)

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Offline debun

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I picked up a 2001 RM 125 and the cylinder has some scoring.  I'm debating about doing a 144 kit, but I keep having to remind myself that it's a 2001 and I will never get back out what I sink into it.   I can just repair the cylinder but for slightly more I can have them bore it to a 134 or 144.   I heard from an engine builder that most of the modern 125's come pretty good from the factory and even if you bore them oversize there isn't too much in terms of port timing needed.  Yes you can get more power by porting but most of it would come from the boring i.e. you don't really need to port it.  So does anyone have concrete knowledge on how good/bad of an idea is is to do a big bore without porting?
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline 2T Institute

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How much porting do you really need to do it you bore your cylinder over?
« Reply #1 on: April 30, 2013, 12:39:47 PM »
Just find a cheap +05 engine and bolt that in.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline debun

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Why do you say +05?
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline 2T Institute

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has a triple exhaust port and 110mm rod
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline debun

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Definitely not a simple bolt on.  Still doesn't answer my question about porting after you bore oversize.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline Jeram

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Definitely not a simple bolt on.  Still doesn't answer my question about porting after you bore oversize.

Why wouldn't it be a straight swap? the crank cases are the same!

The answer to your question is that there is no point doing either (porting or boring) to an early RM125 engine because you can get far better gains from installing the newer style RM motor.

« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline Jeram

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Still doesn't answer my question about porting after you bore oversize.

and no, you dont HAVE to do porting when you perform an overbore.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline debun

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The rod lengths are not the same.

Definitely not a simple bolt on.  Still doesn't answer my question about porting after you bore oversize.

Why wouldn't it be a straight swap? the crank cases are the same!

The answer to your question is that there is no point doing either (porting or boring) to an early RM125 engine because you can get far better gains from installing the newer style RM motor.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline Jeram

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yes... and?
The frame mounts dont care how long the rod is when your doing an engine swap ;)


But if you wanted to do just a cylinder swap, a longer rod never gets in the way.
there are many ways around that.

1.cylinder mods
2. install the rod, piston and cylinder from the later model RM

« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline SachsGS

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Spacer plate to lift cylinder equal to longer rod length.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline Jeram

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Spacer plate to lift cylinder equal to longer rod length.

Doesnt the later RM have a longer rod and therfore a taller cylinder?

or did they balance the increased rod length via a shortened piston compression height so that no mods are required?

Im not sure.

But the engine or cylinder swap is definitely doable and Im sure many have done this mod too!
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline Jakes Dad

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How much porting do you really need to do it you bore your cylinder over?
« Reply #11 on: July 02, 2013, 01:18:28 AM »
Find a funnel cut off about 10% of the length from top. What happened to the opening? Now remember you have a larger capacity air pump trying to breath through this "new" opening.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline team499

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How much porting do you really need to do it you bore your cylinder over?
« Reply #12 on: July 04, 2013, 01:23:14 AM »
You dont bore cylinders these days its a nickaseal plated sleeve. In order to go to a bigger piston you half to have a new sleeve installed. You only bore cylinders with steel sleeves.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline TotalNZ

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How much porting do you really need to do it you bore your cylinder over?
« Reply #13 on: July 04, 2013, 05:44:16 AM »
You dont bore cylinders these days its a nickaseal plated sleeve. In order to go to a bigger piston you half to have a new sleeve installed. You only bore cylinders with steel sleeves.
Firstly it's nikasil secondly bikes these days don't have a nikasil plated sleeve as you put it. The nikasil is a very thin coating straight on the alloy barrel, no sleeve. sleeves are steel usually and old school. He want's to go from 125 too 144 or 150 so he's gonna have to bore the cylinder, then either replate or steel sleeve.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »

Offline Jakes Dad

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How much porting do you really need to do it you bore your cylinder over?
« Reply #14 on: July 16, 2013, 12:54:36 AM »
To follow up with my previous answer and to address the guys original question. Yes you should address the porting because the port openings are now smaller and port timing has changed.
« Last Edit: December 31, 1969, 04:00:00 PM by ' »