30 BHP Blog

Winter 2012/13 I stripped the engine down to do a rebuild just to check everything was ok. While it was apart I put what I thought was new piston rings in. I was rushing the build because i had left it late and needed to get it back working to use to get to the Scooterist Meltdown at Kalkar in Germany at the end of January.
For 2013 I had also been building a mat black Lambretta Eibar Winter Model which was to be powered by the H2 top end. Just in time, I got the Lambretta sorted and headed off for Harwich to catch the ferry to Holland. During the trip the engine ran fine but got increasingly hard to start, as can be seen in this video.


When I got back I decided to strip it down again to find out what was wrong with it. There was damage to the top edge of the exhaust port and the rings were stuck in their grooves. It then occurred to me to look in the box where I had got the rings from, it was then i realised I had used old worn out rings that i had changed a few year before, the new set of rings I knew I had were still in a bag in the box.
The worn rings must have snagged the exhaust port because they allowed the piston to rock more in the bore. The piston was ruined and the exhaust port needed some attention.


I decided that as I needed to do some work to the exhaust port anyway i may as well change it a little while I’m at it. The blow down on the barrel was only 25 degrees which isn’t enough to allow it to rev out properly so I decided to raise the port by 1mm which would give me 27 degrees blow down and widen the top 50% of the port to increase the blow down area. This was something I had wanted to do for a while but hadn’t because the engine had worked so well.

The first thing I did was order a new Wosnner piston in the same 71mm size, I didn’t want to re-bore the barrel even though it needed a re-bore due to the scoring that had been there for years. Then I set about making traces of the barrel to decide what metal I needed to take out to get the port shape I was after.


This is a comparison of the new port shape against the old shape. The top of the port is now 33mm below the top of the barrel!


Next I had to do the modifications to the piston

In the first image on the below you can see how the bottom skirt in the image impedes with the transfers on that side.
The next image shows the shape needed which needs to be cut from the skirt. I first use a small hacksaw to cut the majority of the shape away and the use the flexi drive to finish the cutting then make the edge nice and smooth.
Then I cut 2mm from the bottom of the inlet skirt, this makes the skirt 66mm long


the skirt length along with the measurement from the top of the barrel to the bottom of the inlet port (97mm), minus the measurement that the piston top edge sits below the top of the barrel at TDC (.3mm) gives an inlet duration of 163.43 degrees.

Once everything was cleaned the engine was put back together, The squish checked and the gaskets trimmed back.
While the engine was apart I changed the gear selector shaft as this had given me some problems, also the clutch steels needed filing to take the burrs off and a new set of strong springs added. The clutch is a standard basket with 5 Tino Sachi green plates and aluminium steels.