Two Stroke Motocross

Two Stroke Motocross Forum => Technical => Topic started by: debun on April 30, 2013, 03:55:30 AM

Title: How much porting do you really need to do it you bore your cylinder over?
Post by: debun on April 30, 2013, 03:55:30 AM: 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?
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: 2T Institute on April 30, 2013, 12:39:47 PM: Just find a cheap +05 engine and bolt that in.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: debun on May 01, 2013, 05:00:54 AM: Why do you say +05?
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: 2T Institute on May 01, 2013, 08:52:07 AM: has a triple exhaust port and 110mm rod
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: debun on May 02, 2013, 12:30:26 AM: Definitely not a simple bolt on. Still doesn't answer my question about porting after you bore oversize.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: Jeram on May 02, 2013, 03:06:35 AM: Quote from: debun on May 02, 2013, 07:30:26 AM
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.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: Jeram on May 02, 2013, 03:08:16 AM: Quote from: debun on May 02, 2013, 07:30:26 AM
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.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: debun on May 06, 2013, 04:17:07 AM: The rod lengths are not the same.

Quote from: Jeram on May 02, 2013, 10:06:35 AM
Quote from: debun on May 02, 2013, 07:30:26 AM
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.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: Jeram on May 07, 2013, 01:20:24 PM: 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
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: SachsGS on May 07, 2013, 02:44:13 PM: Spacer plate to lift cylinder equal to longer rod length.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: Jeram on May 08, 2013, 07:20:37 AM: Quote from: SachsGS on May 07, 2013, 09:44:13 PM
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!
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: Jakes Dad 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.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: team499 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.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: TotalNZ on July 04, 2013, 05:44:16 AM: Quote from: team499 on July 04, 2013, 08:23:14 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.
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.
Title: How much porting do you really need to do it you bore your cylinder over?
Post by: Jakes Dad 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.