Polini Head Gasket

I’m putting together a Polini 207 kit for my Rally 200 and am about to buy a SIP head for it as I’ve heard that the compression is too high otherwise. Should I use a head gasket with it? Anyone else have one of these heads?

Thanks

My needle goes right off the clock, but I always hear that seizing sound and have to pull the clutch in and coast to a stop.

Well the head is bored to 69mm, a Polini needs it to be 68.5mm as the bore is 68mm, that head is designed for a malossi. On a polini the compression is raised higher than the malossi because the exhaust port is slightly lower. To combat this the head volume needs to be increased and the combustion chamber modified to accomodate this. The squish band and combustion chamber need to be modified exactly to suit the cylinder, on a malossi the powerband is a lot higher up so needs the head to suit. The polini powerband is more mid range so it needs a head to suit. Im not exactly sure of the math but it is a combination of the width of the squish band and the volume and shape of the combustion chamber that makes the difference. It is better to get advice form an expert or you are in for a few months of pinking just like me.

The Polini would be a truly great kit if only they supplies a head with it and sorted out the thermal problems.

Malossi = No pinking no seizing 75mph all day! [:bounce:]

I was looking at a centrally plug sip head the other day and it certainly did have a modified combustion chamber (quite tall and thin), maybe they have a new one since you tried it? They also claim that the squish band is exactly the same as the bore? What do you think?

Thanks for the help…my head is in mail to you-know-who [;)]

Just a note: When I first put the SIP head on the compression was so high that the starter motor was not powerfull enough to turn the engine over and it was really hard to kick over. This is when I first discoverd something was wrong. It did however start first kick and ran fine up until about 45-50 when a little pinking set in.

SIP heads do not have the combustion chamber modified and have a large squish band which means you will get loads of pinking and heat siezes. The best thing to do is to get a standard PX200 (late type) head and have it modified by MB Developments, these guys really know what to do to a head, squish band and combustion chamber is modified and costs around £20. Since I had mine done I can pull 70mph on the motorway all day long. With the SIP head doing 60mph for more than a couple of miles resulted in a mass of pinking and then a seize.