Morris Minor Owners Club

I found no 4 cyl was a bit low on compression recently, so I am looking at a partial stripdown & rebuild. I haven't done anything yet, so I don't know any hard facts about why compression is low, whether I need a re-bore, new pistons etc, but before I start I have a couple of questions.

1) It says in my BMC Autobook 5 that pistons & con rods in a 1098 are matched and should be replaced together. I have not heard this elsewhere-is it true? Or can I just replace piston rings & pistons if need be & re-use the con rods?

2) Dave Vizards book suggests that a 1098 can be re-bored to about 1250cc with larger pistons & rings. He says that if you do this the bores need re-centreing. Would that mean a new crankshaft as well, because the c/l of the pistons would have changed in relation to the big ends?

3) Am I right in thinking that re-boring will raise the CR if I use the existing head with no mods?

cheers,
Neil

If it's only #4 pot that's down on compression - it's most likely just the valves - so check that before venturing further!
Con rods are certainly re-usable - just put them back in the same pots although it doesn't really matter provided they are the right way round! Standard re-bore will cost ~ £50 for the 4 pots + pistons/rings - check Bull Motif or others for best prices. Note some places won't do rebore unless they also get to supply the pistons - at high price!! Would be sensible to have the crank reground and new shells and oil pump/timing gears and chain etc too. You will be looking at ~ £250/300 for all the parts and machining - assuming you do the labouring yourself. Vizard's overboring is very extreme - v high cost for special pistons - high cost for 'off centre' boring and almost certainly the top of block would need machining too to suit the pistons. Standard crank remains - I think the rods may have to be slightly 'releived' to allow them to sit slightly off to one side - to be honest it's all a bit desperate. Also less reliable/more prone to overheating (thin cylinder walls) and hardly any power gain. Without a doubt - if you really want/need more power it's far far better to spend the cash on a 1275 rather than on a 1098.
And yes - any increase in cylinder capacity will v slightly increase the CR.

Before doing all that, remove the cylinder head and check for a burnt exhaust valve. If its just one cylinder down. I would have thought worn bores would affect all the cylinders.

Then re-grind all the valves and the engine will be much better (and more economical) for the top end overhaul.

One cylinder down on compression is most likely to be a valve problem- go for the easiest/ cheapest fix first!

chickenjohn wrote:Before doing all that, remove the cylinder head and check for a burnt exhaust valve.

I squirted a bit of oil into the pot then re-did the test-compression went from 60 to 150psi. Which says broken piston ring and/or worn bore to me. I have'nt started stripping it down yet though so it's all speculation.

Thanks for the advice on reboring, bmcecosse, I was just wondering what options are open to me as if I have to have it rebored now would it be a missed opportunity not to open it out a bit. I did suspect that a 1275 would be a better investment.

cheers,
Neil

Aye - well that test does rather confirm ring problem - but you may well get away with just fitting new rings or a new piston and rings to that bore - which of course can be done simply by popping the head and the sump! Someone had a 'set' of 3 pistons on ebay the other day!

OK, last and final question (until I think of another one!)

In the same BMC Autobook 5 it states that if the valve guides are replaced, the valve seats will have to be re-cut so they are concentric with the guide c/l. Is this true or is it another bit of over-engineering?

cheers,
Neil

Well - I have done it before without re-cutting-but recently I did a head with new guides and the valves were indeed slightly 'off' when I went to grind them in. Mind you - I had the (inlet) seats down to the narrowest possible with the throats ground away as much as I dared! This was only spotted when I put a very bright light up the ports - and noticed a tiny glimmer showing at the edge of some of the valves. So - I don't know if the original guides were slightly off centre bored - or the new ones - but it was enough to make a difference. Took quite a bit of hard grinding to get them right - since despite best efforts I have never manged to get hold of a valve seat cutting rig. So - think twice before launching in to new guides!

Here's the link to that 'set' of 3 new pistons however sadly they are only for the 948 engine. Could be a wise investment for a 948 owner!
http://cgi.ebay.co.uk/AUSTIN-A35-A40-HE ... dZViewItem

Now, what was that you said in another post about simply posting the link? ;)
http://cgi.ebay.co.uk/AUSTIN-A35-A40-HE ... dZViewItem

Aye - dunno what's happened there Andrew - it's always worked up to now!! I'll try and sort it out!!
Edit - Phew - worked that time - and i didn't seem to do anything different!! Just right click, copy - then paste, as normal!

You didn't leave a space between 'owner' and 'http'. I could have just put one in, but I was feeling cheeky

Ahhhhhhhhhhhhh - the space is important then - I guess, thanks!! Slowly getting the hang of these new fangled computer thingys !

Yeah, if there's no space, the server doesn't read the http which tells it that it's a link

Learned something new today then ! Thanks for the tip.