SYM T2 – Improve the Crankcase Breath Hole

This DIY was inspired by Taiwanese SYM T2 Club. A experience mechanic found the crankcase breath hole is too compact and the pressure in engine is not release efficiently. It can causes the engine temperature rise up, engine oil leaking and sluggish engine running.

He had no bad experience with the additional crankcase breath hole after running more than 10,000km.

Recently I found my SYM T2 is somewhat hotter near the engine area, however the coolant lamp is not light up (means coolant is good). The feeling of acceleration is ‘hard’ especially above 7000 rpm. Then this photo caught my attention the important of the crankcase breath hole. According to the owner, the engine case was cracked and the oil spills out. Many members was shocked because no one reported such incident yet. The other explanation also could link to the poor material or quality control of the crankcase. It doesn’t make sense to me that engine pressure able to damage the crankcase and blow it apart!

sym t2 crankcase cracked

The crack area suspect caused by tremendous engine pressure!


This DIY project is very simple, you will need a power drill machine, paper hole puncher, sand paper and wrench size 10. Please do this while your engine is cold! The crankcase cap is locate at the right side of engine, same side with the engine oil cap. After dismantled the cap, cover the crankcase with a piece of cloth to prevent foreign materials goes into the engine.

After removed the two screws, you will able to remove the diaphragm (a paper card) from the cap. This is to avoid damaging the diaphragm during drilling. Set the screws back and drill a hole opposite the small breath hole. I am using the 5mm drill to make a hole just as the Taiwanese expert did.

Then remove the screws again and polish the drilled surface with sand paper. Match the diaphragm with the new hole position, and use paper puncher to make a breath hole. Clean up, dry off and put everything back. Phew!

The verdict: Smoother engine rev, less ‘stress’ when it approaching high rpm. Engine braking seems weaker now, probably less pressure ‘hold back’ at the engine so beware of your new braking point. Engine sounds a little bit quiet (but who cares). Top speed no change but fun of riding +1 point 🙂


sym t2 crankcase cap

The center hole is the main breath hole, somehow the expert said it is not efficient due to tight clearance within the diaphragm piece.

sym t2 crankcase cap complete drilled

The hole at bottom right is new made.

sym t2 crankcase diapgraph

See that new candy hole on top right, because my hole puncher is special one 🙂

sym t2 crankcase cap complete drilled 3

Polishing using sand paper and make sure all dirt and residues are gone or it will enter into your engine!

sym t2 crankcase cap complete drilled 2

Last check before restore the crankcase cap. Top right is the new additional breath hole.


The ‘complete’ package with oil catcher and stuffs. (From the Taiwanese expert not mine!)


2 thoughts on “SYM T2 – Improve the Crankcase Breath Hole

  1. Hi, i really like your blog.
    I don’t have any problem with my t2 at 16.000 kmts everything is’t ok but this point of improvment the crankcase breath hole caught my attention. Do you really recommend this?
    My t2 is Completely original without modification.

    Greetings from Colombia

    1. Hi Anthony,

      This breath hole modification is one way ticket. Once done and no reversal unless you replace the cylinder head unit assy at price USD30.

      The major good point of this modification, you will feel smoother in acceleration and up shifting.
      The drawback is engine braking becomes weaker and you may need to brake early or adapt a new riding style.
      Personally I think it is a good change but I wish I had drilled a smaller hole, so I can have control to drill another one in future.
      Probably begins with 2-3mm hole drilling is fine. Cheers.

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