LCGB Forums

Hi all. Bit of advice needed please. On the return leg of Derby 150 ride out I had a sudden loss of power in 3rd and 4th. (Series 3, LI150). Managed to limp home and check it over. Air leak test showed head gasket failure. Removed head to find the piston is grooved up,and cylinder is very slightly scored, due to a broken piston ring. Advice is; new piston and rebore, or cylinder upgrade kit?? Any comments most welcome..

Quite often, piston rings break because they are worn.
Place the unbroken rings in the bore and measure with feeler gauges what end gap you get and get back to us.

Thanks for reply. One has snapped and the other is loose as f##k so can't get a true reading. Piston has only done 100 miles since rebuild so I'm assuming it was a crap or wrong replacement??

Don't forget you need to check what clearance existed between piston & bore before deciding whether the bore is worth salvaging.

My own feelings are that head gaskets should never be used, unless as a packer in a spigotted arrangement. Often, simply by measuring the squish & CR, the head gasket can be eliminated by juggling with base gasket/packing, especially as thin metal base packings are available these days.

If your existing top end is salvageable, then I would advise to do so, to get back up & running.

However, if you have a hankering for a kitted engine, 150 crankcases are fairly plentiful & not too expensive. You could gather the parts you desire & build it @ your own pace. There should be some parts fairs available soon & a glut of parts waiting to be off loaded, what with the World & His wife kitting their engines......I would avoid the Indian alloy cylinders as they are rough as (how you say?) f##k You can make them work very, very well if porting is your thing, because even I managed that. But you will need a right angle drive - say, on Dremel type tool - @ the very least & some very good burrs.

Much depends on your disposable income & whether your other half finds out.....

Markyraddy wrote:Thanks for reply. One has snapped and the other is loose as f##k so can't get a true reading. Piston has only done 100 miles since rebuild so I'm assuming it was a crap or wrong replacement??

If this was a new piston 100 miles ago....................could it be that an undersized piston was fitted in error?
You wouldn't be the first person to have suffered this.

I don’t understand the title of this thread, as there I no mention of a change of head in the text.

If you are thinking about an upgrade to a kit, as eluded to in the OP, then there are plenty to choose from. Sticky’s new Lambretta kit book has plenty of information and may help you decide if a new kit is for you. It is an excellent read.