One idea at a time in focus mode
What is known about porting, heads and big-bore kits, and what this site will not guess.
Before you start
Your own risk. Wrong jetting or timing can seize the engine. This is information, not advice.
How many steps: 3
Step 1 of 3
Port maps and squish are not published.
No port-timing diagram and no squish clearance for a Type 122 or a Type 123 were found in the sources used for this page. Both stay To verify. Send a manual page or two independent measurements if you have them. Head-bolt order is still not published. Type 122 head bolts are 30 Nm and Type 123 head nuts are 20 Nm, from Haynes, on the torque page. The order is To verify.
Step 2 of 3
Haynes compression is the stock figure, not a target.
Licence, MOT, insurance or a seizure.
Haynes 4298 prints Type 122 compression as 12.0:1 to 13.0:1, and Type 123 as 13.7:1 to 14.7:1. Those are stock service figures. They are not a target for a skimmed head. One exhaust comparison on Hartrusion used a VHM combustion chamber he quotes at 13.7:1, on an engine that was already modified in other ways. That is a description of a test engine. It is not a head specification for a Type 122, whose stock band is the lower Haynes figure. Copying 13.7:1 onto a 122 is To verify, and the squish that would have to go with it is not published.
Step 3 of 3
Big-bore kits exist. No gain figure is published.
Hartrusion’s VHSB chart cites a 2002 Polini instruction for a 154 cm³ kit on a Rotax 123, with its own main-jet range and, for a Superval reed version, a different needle. Those jets are the kit sheet. They are not a starting point for a standard 124.8 cm³ engine. A seller’s reed-block page says an Italkit 140 cm³ cylinder and a Polini 154 cm³ kit are the builds they would pair with a 34 mm flat-slide, and that the block also fits the original 125 cm³ engine. No power figure is attached. A gain is To verify.
This guide is done. Your place stays on this device.
No port-timing diagram and no squish clearance for a Type 122 or a Type 123 were found in the sources used for this page. Both stay To verify. Send a manual page or two independent measurements if you have them.
Head-bolt order is still not published. Type 122 head bolts are 30 Nm and Type 123 head nuts are 20 Nm, from Haynes, on the torque page. The order is To verify.
Source: Haynes manual 4298, torque pages already on this site
Haynes 4298 prints Type 122 compression as 12.0:1 to 13.0:1, and Type 123 as 13.7:1 to 14.7:1. Those are stock service figures. They are not a target for a skimmed head.
One exhaust comparison on Hartrusion used a VHM combustion chamber he quotes at 13.7:1, on an engine that was already modified in other ways. That is a description of a test engine. It is not a head specification for a Type 122, whose stock band is the lower Haynes figure. Copying 13.7:1 onto a 122 is To verify, and the squish that would have to go with it is not published.
Legal and safety. More compression without a known squish and a known fuel mixture is a way to seize or hole a piston. This page does not give a skim depth.
Hartrusion’s VHSB chart cites a 2002 Polini instruction for a 154 cm³ kit on a Rotax 123, with its own main-jet range and, for a Superval reed version, a different needle. Those jets are the kit sheet. They are not a starting point for a standard 124.8 cm³ engine.
A seller’s reed-block page says an Italkit 140 cm³ cylinder and a Polini 154 cm³ kit are the builds they would pair with a 34 mm flat-slide, and that the block also fits the original 125 cm³ engine. No power figure is attached. A gain is To verify.
Source: Hartrusion VHSB chart, citing Polini, and an Italkit manifold listing
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