Morris Minor Owners Club

I took my dismantled engine along to the local engine shop to get the bores honed. I've been told the block needs reboring. There appears to be a choice of oversize pistons including 8.8:1 types at about £20 each and 9.4:1 at about £40.
My question is...will I notice a major difference with the more expensive ones or will the cheaper ones be OK.
Also, when I run my finger along the bore there is a very subtle undulation but not a definite ridge. Is a rebore my only option?

cheers,

Tom

Is a rebore my only option?

no, but it is certainly the best option.
A very heavily worn A series will have around 20 to 40 thou ridge in the bores!! therefore it is all relative.

I'm not sure if the honing can be 'done well' without removing the ridge and therefore you may as well go for a rebore to get everything correct.
Also if you replace the pistons, there's no guarantee that the top ring is exactly in the same place and if it runs into the ridge that is not a good thing.

Personally I'd get it rebored and buy the higher CR pistons. The lower ones might be OK but you'll get better economy and power from the higher CR ones.

But you could get the CR back up by having the cylinder head skimmed (about 80 thou) - this would be a cheaper option. I am running a Mini 1275 engine which I 'hand' honed with emery paper, and then fitted a set of pistons I took out a 1300 engine about 30 years ago. It runs fine - good compressions and burns very little oil - but it's not a car i do huge mileage with. Shop around for the pistons - I'm sure they can be less expensive than that for the higher CR pistons - try Minisport - or ask engine tuner since he has started coming on here.

The pistons you refere to are 21251 & 21253,

The 21251 pistons sit 0.070" down the bore on a STD height block, I & many other tuning companys use these pistons in fast road engines. But to get the compression up to 10.0:1 you skim the block and leave the head as STD.
E.G if it was bored +060 then you would deck the block by 0.030" to leave the pistons 0.040" down, using a STD head this gives you 10.0:1.


The 21253 pistons sit 0.010" down the bore, so if it was bored +060 and the head was STD it would give you 10.75:1

I've seen these on ebay (#4621365663).
I was only planning to rebore to 20 thou but would it be worth going for a 60 thou bore with these pistons (provided they don't go for silly money).

thanks

Well some blocks can go porous at 60 thou - and the 'land' between cylinders 2 and 3 becomes quite thin - so there is sometimes a head gasket blowing issue. Take engine tuners advice and use the cheaper pistons (although ACD disagrees !!) at 20 thou. To skim the block as he suggests rather commits you for the future, whereas skimming the head gives the easy option later to make changes. But i suppose the block is in a mess anyway during the reboring - and since it's at the machine shop anyway skimming the top may not cost that much more if done at the same time.

I see Minisport want an extra £100 or so for the higher comp pistons !!