OHC 8V 1300 to 1400 Engine conversion - Looking for info

[Royston]

Just wondered if anyone has any magazine features or information on the above

I am considering upping the Sport engine using the age old 1.2 pistons (I have some new ones in stock)

Engine is now stripped and looking to prepare it fr the machine shop, needs reboring anyway

Main concern is the increases compression ratio, I can work it out the hard way but any information would be useful

Thanks in advance

[mowgli]

there wasn't a magazine article on how to do this Royston. there were a few pics (and some bullsh!t from PMC) in a couple of mags, but that was it.
when i did mine, i actually used a vernier to measure the pmc mag pics & scaled them up to get a decent idea of what needed removing from the dishes.

[thanks to bazzap8389]

[mowgli]

from memory, i used the original line of the dish as a start point, and i took it down about 5mm, with a radius on the edges.
it was in 1989 or thereabouts, so forgive me for being a bit hazy. i was aiming for 10.5:1cr.

i did end up doing the old fashioned method of actually measuring everything. i used perspex, cooking oil & an old pig syringe (my dad kept them) to measure the head cc, maths to do the gasket & bore volumes and then calculated what the piston top cc needed to be.

the one thing i remember most was the machinist refusing to accept payment for the piston machining as he was convinced it would blow up when i ran it..

[mowgli]

oh & as it was november, i built the engine up on my bedroom floor...... one of the daftest things i ever did with metal. it was sodding heavy to get down the stairs on my own

[Royston]

Thanks Mowgli, its a great start! You can see the depth of the piston crowns!

My SR has a PMC 32/24 DMTL weber conversion for years

[Andy]

Ya lucky you didnt go through the ceiling you silly sod

[mowgli]

my mum nearly did when she came home from slimming world & found me putting the sump pan on.

[Jonesfach]

my mum nearly did when she came home from slimming world & found me putting the sump pan on.

Class....

[mowgli]

sorry, my mistake... her slimming world class

[Royston]

Can anyone confirm that 1.2/1.3 are the same part number, combustion chamber size please.


Combustion chamber and piston crown volume(inc gasket) on 1.2 is 36.46cc and for the 1.3 it is 39.54cc working on 9.2:1 std CR

The swept volume for 1.2 is 299.02cc

The swept volume for 1.3 is 324.27cc

The swept volume for 1.4 is 348.9cc

From my rough calculations the comp ratio for 1.2 pistons/1.3 crank and conrods is 10.57:1 calc (348.9cc + 36.46cc)/36.46cc

assuming the 36.46 cc combustion chamber/piston crown (and gasket)


Will need to confirm though

Here are a few pics of the pistons 1.2 left, 1.3 +0.5mm right


Underside


Difference on piston crown


Engine details, if you want to have ago with the sums!