Scooterotica

having just rebuilt my TS1, the new piston had a slightly different compression height (approx 1mm higher) so I put in a 1mm packing plate the squish ended up 1.51 + or - so I replaced the packing plate with a bottom gasket re-checked squish and got between 1,1 and 1,25, is the 1.1 too small ? if so what would be the consequences if any.

The clearance should be ok, the piston shouldn't hit the head with more than 0.9mm but make sure there are no high spots on the head which could reduce the squish to below this.

we tried .9mm squish on our racer,but at very high revs,the piston did start to kiss the head...
1.1/1.2mm is therefore good.
1.25/1.3 is well safe and still a good squish..

Thanks for the assurance, it just cost me 300 quid for a piston, cylinder repair and re-plate, and I don`t fancy having to spend more money if something happens again.

Make sure you check twice, I fitted a head I used on my scooter with a rapido 200 race barrel and ran it at 1.4mm. I've just refitted it to my SX on a ported SIL barrel and like you I am running a tight squish clearance at just over 1mm but on checking the clearance closely a section of the head was below 0.9 and the head had to be reworked to give the correct clearance. Don't just stuff solder down the plug and check once. Best make sure and either place a couple of strands of solder at different angles or use modeling clay Plasticene

Also note squish has a connection with Compression ratio

An example a std TS1 with 1.5mm squish and a head which gives 9.5CR
when changing to 1.1mm squish with same head will raise CR to 10:1 so could cause problems if wrong CR head


Smiffy

Yes I understand that, previously I wasn`t using a bottom gasket and I am now which would help the compression ratio wouldn`t it.

Stupid question no. 873.

I understand the need for a correct squish, but no standard set up scoot I have ever worked on ever had a squish that could be measured, but they still run fine, didn't overheat etc. The sqush definately did not seem to be of importance at the factory.
So why are we all hot and bothered about the squish? Just cause were tuning them a little?

dirtyhandslopez wrote:Stupid question no. 873.

I understand the need for a correct squish, but no standard set up scoot I have ever worked on ever had a squish that could be measured, but they still run fine, didn't overheat etc. The sqush definately did not seem to be of importance at the factory.
So why are we all hot and bothered about the squish? Just cause were tuning them a little?

Apart from fuel being unleaded and lower octane than when Lambrettas were designed, the squish type head that is fitted to a lammy has a cooling effect and did even at 2+mm as most left the factory with, but not as effective as a tighter squish.
If you are one of the tuning Guru's that makes expansion chambers squish is an important part of the design formula (MSV) the speed of the flame front is influenced by squish band area position and clearance plus combustion Chamber size has an effect, both depth & shape.
For some it's a piece of solder down the plug to check clearance for others it's an essential part of obtaining more power while keeping reliability (offseting detonation) and cooling the engine between power strokes.
Me I just sick a bit of solder down the plug and hope it's around 1.2-1.5mm.

So even a 2mm+ clearance helps with the cooling?
I've read, read and re-read and then read agian A.G. Bells tuning book, so I understand the theory behind the squish, But I have in the past built motors without checking squish and they have and do run fine. Just lucky or is there still a certain amount of lee way in that respect in the Lammy motor?