Morris Minor Owners Club

Sorry! Easily confused these days with different engine builds - so 1098 engine, which of course is why it needs pockets of course. I appear to have 'chemo head' where my brain doesn't hold short term memory very well....... I told you at the start to simply sink the exhaust valves - it would have been SO much easier and safer. The pockets are stupidly deep - the top ring can't be far away. You'll still need to take the rings off I think -unless you think 'binding' them in place with tape will do while the tops are skimmed..... the gasket should be ok as shown, but do try to get a copper one....

I make that ~11.5 CR, and that's assuming the piston dish remains at 3cc - too high for a road engine. Probably need to take metal out of the combustion chambers with mounted points/carbide burrs (although they need high speed to be effective). Or chance it with high spec petrol with retarded ignition, but I think it will pink/detonate horribly.

[quote="bmcecosse"]Sorry! Easily confused these days with different engine builds - so 1098 engine, which of course is why it needs pockets of course. I appear to have 'chemo head' where my brain doesn't hold short term memory very well....... quote]
My sympathies Roy-I know the story very well. I had 604 hours of it last year but it appears to have worked!
Regards
Declan

Oh dear, I'm sorry to hear you're unwell Roy, i do hope you feel better soon.

I'll swap back to the Morris Minor piston, which has a dish of 7cc, although I will lose some when skimmed (assuming that they do need to be skimmed, should I dry build again or just measure the crown heights?). Hopefully this will be enough to lower CR even if I have .025 skimmed off the top of the piston.

Does that sound a good idea considering situation as it is?

Thanks,

Edd

Edd
You will need to install the Morris pistons and check the tops of the pistons relative to the top of the block.
They will still need to be a minimum of 0.010" below the top of the block.
They may or may not require skimming only a dry build will tell.
Phil

Yes - a dry build is always well worth doing. Time well spent. Alternatively you can take metal out of the chambers (gas flow around the inlet valves) or take even more off the tops of the pistons. I would get stuck into the head!

Thanks Phil,

Measurements for top ring clearance.

Distance from piston deck to top of top piston ring - .2"
Deepest pocket - .135" - I am struggling to measure this super-accurately. This is with steel rule and an ALDI calliper.
Highest protrusion of piston about deck - .015

So the clearance I have between block deck and top of piston ring in the worst case scenario is .050".

Which is what it is unless I buy a new block, or hacksaw the con-rods down and gaffa tape them back together.

BMC if you get a chance would you mind posting the formula to work out CR from the info that I posted before? So I can work out what my CR will be using the Minor Pistons with their dish of 7cc.

Many thanks,

Edd

It's worth considering the dynamic CR too - with a standard cam, which I think you've got (with relatively short duration compared to a more lairy one) the CR should definitely be kept reasonably low. If it really is 11:1 it will be impossible - I'd say definitely sub-10:1 if you want to stand a chance of running on normal fuel.

If you are going to do a dry build, do it without fitting the rings to the pistons it makes it a bit easier.

The pockets should have been 40 thou deep + the amount taken off the head of course, so actually 60 thou. Never mind -it will be fine I'm sure - just make sure the ring gaps are well away from the pockets when you fit the pistons. Surprised at the difference in piston protrusion. The ideal way of course is to measure the crank throws. measure the rod centre to centre lengths, and measure the piston gudgeon mount hole to crown.... Then match shortest rod to longest crank (limited in a 1098 because there are 2 different rods ) and then match the piston lengths to make as near you can the same distance below the block top below the deck, and machine each one to put it the chosen height you decide below the deck. Some do make them dead flush with the deck. And finally -balance the rod weights and the piston weights!!!!! However -I gather this is not a racing engine - so just do what you need to do to drop the pistons below the deck.

Definitely not a racing engine, I doubt it will do 500 road miles in a year!

I measured piston protrusion with a steel rule and then feeler gauges slid in the gap, so there is scope for error there. Definitely differences though.

If someone has the chance to look at the CR for me or let me know the formula as I don't have my yellow bible and the internet is not yielding much accessible info!

Hopefully the Morris pistons alone with the increase of 4cc to the combustion chamber, from 3cc dish to 7cc dish will be enough to lower the CR to sensible levels. I am fine running the posh fuel as I do so little miles and only for pleasure.

Many thanks all,

Edd

Or I may have done it...

Combustion chamber = 21.5cc cylinder head PLUS 1275 gasket 4cc PLUS piston dish 7cc (not affected by skimming as material removed is material removed just discount the 1cc from piston below deck) PLUS pockets 0.5cc EQUALS 33 cubic centimetres.

1098 +040 / 4 = 283cc Cylinder volume.

283cc / 33cc = 8.5 (+1) gives me a compression ratio of 9.5 - 1

Is this maths correct?

Many thanks,

Edd

I make it 10.2 assuming the 7cc piston dish - although it will reduce if you have to skim them, and so the CR may rise slightly. In this case I would be tempted to put the pistons flush with the top of the block to maximise the protection to the top ring. Can't see any harm in that. But check - because obviously these pistons may be slightly different length to the ones you show earlier.

Hi BMC,

Would you mind sharing how you worked that out? So I can understand where I have gone wrong.

Surely skimming the Pistons is always going to be material removed from the combustion chamber, so whilst the dish may be shallower, the piston material in the combustion chamber is actually less, and so the volume is greater?

Thanks,

Edd

It's just total volume at BDC divided by total volume at TDC. So add it all up at BDC, and then take away the swept volume (I haven't checked your figures on that ) and divide the result into the total figure at TDC. I'm a bit surprised the pockets are only 0.5cc, but that's the figure I have used..

Aaaah ok.

I was doing swept volume divided by cc volume, not adding the two together and then dividing.

Doing it your way I then get 10.5-1?

The figure of 1132cc came from Phil, as a 1098 +040.

I was also surprised how small the pockets were, I moulded the shape with blue-tac, feeler gauge to mimic cylinder curve and straight edge to flatten. I then dropped it in a 10ml syringe filled to 4ml and the water level only rose 0.5cc. Visually it is bigger than that.....

I may try and drip some oil in there, see how I get on. Could be tricky!

Thanks,

Edd

It's not going to make a significant difference, don't worry about it.

The formula for determining the compression ratio is:

C (total combustion chamber cc) plus V (swept volume cc) divided by C (total combustion chamber cc).

C + V
_____ = compression ratio.

V

Does anyone happen to know the part number for the standard HC 1098 piston? Or what the standard compression ratio of the HC engine is?

I only ask as the engine shop who are swapping my Pistons have listed a high comp engine and a low comp engine, but on their books the high comp piston also fits the midget 1098, ie the piston I already have which is too small a dish.

Is there any easy way to identify them? I will take along my old piston to compare.

Is the Morris minor low comp piston still available?

They will be county Pistons. I just don't want them to order the wrong ones!

Thanks,

Edd

It was 8.5:1 on the HC, 7.5:1 on the LC 1098. I think you'll be doing well to find LC pistons, but you never know...