Saturday 8 December 2012

The Imp low compression piston?

For those who would like to find out about such an item, check out the link below .

The one on the left is the low compression one, the top deck height is misleading!

Read the two blogs on the same subject.

I have never seen this discussed before, just a mention the the Imp Commer van and some export cars being fitted with an 8:1 compression motor over the normal 10:1 compression engines.


From Imps 4 Ever, many thanks

Pistons are cast in LO-EX aluminium alloy and have two compression rings and one oil scraper ring. The piston runs in a nominal bore size of 2.6770 in. dia and the piston-to-bore clearance, measured at the skirt is, depending on tolerances, between 0.0017 in and 0.0010 in. These measurements are taken at right angles to the gudgeon pin hole.

On all except very, very early engines, of which only a few were built, the pistons have two cutouts machined in the top to provide clearance for the valves. Two compression ratios are available as far as the pistons are concerned. The one for use in the cars gives 10 : 1 cr, whilst the low compression for commercial vehicles such as the Imp van give 8 : 1 cr.

Pistons come in 2 types: small pin and large pin. Low or higher cost depends on what quality you are looking for.

Reboring of an 875cc engine up to 30 thou oversize is considered to be the maximum. But many people do go to 40 thou.60 thou pistons can be obtained.

  Note the lack of valve pockets in the pistons!
I rebuilt this very early engine block last week, I have to assume its dated from the first batch ever made. the sump was stamped July 7th 1963, just two months after the Imps release.


NB: If you want to remove seized Imp piston rings check this link