Casa 185 kit- new 66mm piston anyone?

[Pompeydave]

Yes, I've been trying to find a piston that looks as bear to the original as possible so a two ringer is what I've been after really.

[Pompeydave]

Hi all, update time. I had the Mahle 66mm piston adapted for use on the Casa 185 kit by John Madison (ScooterloopyJohn). He added the required port window as per the original 185cc piston, and removed some skirt area to match the original piston as near as possible. It was a cracking job, cheers John! It all went off to Langcourts, and came back fully repaired and nikasil lined. I fitted it all and settled on a squish on 1.1mm to start with. I ran it up and it was ok, but predictably the compression ratio was a bit high as it was mighty hard to kick over.

Anyway, I ran it in for about 500 miles even though it didn't really need it, and decided to change the squish to about 1.5mm. Jesus H Christ, I could NOT get the barrel off again! It seemed to be seized on the cylinder studs somehow. Eventually, out of desperation I ended up using brute force and although I managed to get it off, I broke the exhaust side of the skirt off . I had it re-welded back on but it wasn't lined up properly, and looked a bit of a mess. I thought it was fit for the bin only. Almost as an aside, I phoned Langcourts on the off chance, and they asked me to send to them to have a look. Amazingly, they managed to break the duff weld, re-line it up properly, build up the base area that had been filed down by my welder, and re-nikasil the whole bundle. Cost: £245, cheap for the amount of labour than must have gone into it, and way cheaper than a new kit.

I fitted it yesterday, and realised that when I first bought it (secondhand from a well known auction site) it came supplied with gudgeon pin circlips and exhaust studs that turned out to be for another engine/barrel entirely. The studs were fitted once the cylinder was on the engine, and were found to be too long by about 3mm. Accordingly, one of them had previously gone hard up against a cylinder stud, locking the barrel in place on the stud. No wonder I couldn't get it off again!

Moral of the story- when buying secondhand, check that all the bits supplied are correct for the rest of it . The engine is now running again with the correct squish and to recap, it's a Casa 185 which is now a homemade Casa 198. Compression ratio is still a bit too high as it's still hard to kick over, but John has kindly agreed to machine the head inside of the squish band a little. The Mahle piston crown is more domed that the Casa 185 one, hence the higher comp. Overall though it's fantastic to ride, and with bags of low down grunt, it's a real joy! Belt drive makes it phenominally smooth also .

[dscscotty]

The Mahle piston crown is more domed that the Casa 185 one, hence the higher comp

The cylinder head needs machining to match the dome height and shape of the piston, you may run into detonation/over heating problems if not matched. To High compression ratio with low octane/lead free fuel will cause issue., don't confuse comp ratio with squish clearance, sounds like the combustion chamber will may a little work.?

[Pompeydave]

Yep, the squish is perfect at 1.5mm so that outer band on the head won't be touched.

[dscscotty]

but the shape/curve of the head profile will need to match the piston, the CASA piston is flatter, Mahle/asso profile is relatively high @ 6mm its nothing to do with hitting the head and squish clearance.

[Pompeydave]

Compression ratio is still a bit too high as it's still hard to kick over, but John has kindly agreed to machine the head inside of the squish band a little. The Mahle piston crown is more domed that the Casa 185 one, hence the higher comp.

I know. That's why I mentioned above that John has kindly agreed to machine the head INSIDE of the squish band a little. Although the Mahle piston is more domed, at the outer edge (i.e. the part that comes up close to the squish band) it's almost a perfect match, profile to profile. This is backed up by the fact that when I checked the squish using twisted solder the thickness was a uniform 1.5mm all across it.

[Warkton Tornado No.1]

Sorry if I'm telling you what you already know, but this may serve as a reminder to others reading this Post.

As dscscotty intimated, a tight squish (1.0 mm practical minimum) is good & shouldn’t be increased as an easy route to lowering compression because the head does its job better of absorbing heat from the piston if they are on almost kissing terms...

[Pompeydave]

No, fair point, I agree that nearer to 1.0mm is better than 1.5mm. I don't think it made any difference to the compression ratio anyway, so it's a case of machining out the central bowl area and resetting the squish.