The Yamaha Yzf R125 Etc Engine Self Destruct Top End

Discussion in 'Technical Discussions' started by GSX1400Dave, Nov 24, 2016.

  1. The Yamaha YZF R125 and other bikes that have the same (Yamaha) Minarelli 4 valve 4 Stroke engine have a problem that results in the top end of the engine going into self-destruct.


    Showing piston from engine 1 the other is just about the same.

    I have 3 engines in for repair at the moment 2 are self-destruct motors and the other (engine 3) is for repair due to crash damage in both engine cases that can be welded (I have AC / DC TIG aluminium welding facility) but eBay provided 2 cases that were intact. So this was an easy solution.

    The other 2 engines:

    So, what do I find? The other 2 engines have self-destructed and have bent valves smashed piston and destroyed cylinder etc.

    Examination of both engines I find that the cam chain has come off of the sprockets at some stage, resulting in the piston colliding with the valves and piston and valves being destroyed along with the cylinder. Further examination I find that the small steel chain guide at the crank shaft sprocket shows that the cam chain has been rubbing on this and has pushed it out of position and bent it out of shape.

    Small chain guide without plastic chain guide fitted (as it should be).
    Small chain guide showing damage caused by cam chain.
    Showing that the guide can move if hit by the cam chain and allow the cam chain to come off of the sprocket. This is the most lightly cause of the cam chain coming off of crank shaft and then camshaft sprockets and setting off the self-destruct.

    Possible solution:
    The steel plate will move and allow the chain to come off so we should stop this.
    The chain can also come off by bending the plate out of the way due to high revs we should stop this also.
    What I have done is extend the metal chain guide both forward and backwards so extending the portion of the crankshaft sprocket that is covered by the chain guide. This also allows the chain guide to be fixed in place by the new extension (so that it can never move), also limits the amount that the cam chain can ride up the sprocket on the crank shaft and make it stronger.


    Prototype revised chain guide.

    A steel plate 0.5 mm thick cut to the same width as the guide and length to extend about 5-6 mm each end of the existing plate the new plate MUST be bronze welded to the existing plate as MIG or TIG welding will harden both metal parts and this is not desirable inside an engine for this type of application.


    This was a test fit and I am very happy with it (it is very usable as is) and just need to finish the part and get the shape and finish good.

    Further to this the plastic cam chain guides also get worn when the cam chain comes off the crank shaft sprocket. The following photos show this and please note that it can be on either side.


    If your guides are like this, left and centre (of photo) please scrap them.

    Right is ok as can as can be seen from the cam chain sprocket guide not touched by cam chain as this engine did not self-destruct.
    upload_2016-11-24_22-32-46.png upload_2016-11-24_22-33-50.png upload_2016-11-24_22-34-19.png upload_2016-11-24_22-40-4.png upload_2016-11-24_22-44-27.png upload_2016-11-24_22-44-59.png upload_2016-11-24_22-45-33.png
    About Moto recycle limited

    I have been rebuilding bikes and engines for 45 Years.

    My first engine strip and rebuild was a BSA 125 D1 Bantam when I was 15 years old.

    Since then then as a side line I have been engine re-manufacturing, re-building and renovating all sorts of bikes (and cars).

    My main employment is as a communications systems consultant and after being made redundant recently I have now taken on some extra engine rebuilds.

    Please review this document and make your comments.


    Old125Rider, Draft_ and Blazer like this.

  2. any to sell havnt got the experiance to do this would buy in heart beat bike blew with no warning rebuilt now but dont trust it

Share This Page