Morris Minor Owners Club

wanting any number of .080 piston sets for 918cc sidevalve

I thought the largest overbore was .060?

The standard max rebore is 1.5mm =.063 thou but apparentely if you sonic test the block you can take it out to 080 thou. but you need a sound block to do it. A bit like taking an A Series out to 1380cc. I had an email about it from Robert Ball

Have had several sets of .080 pistons in recent years, and obtained them from the UK, anyone who thinks .060 is the maximum should do a little investigating, or maybe not believe everything you read. Several old timers who have raced these engines for many decades use factory .080 pistons, I have even found them in old engines that were being scrapped. I gave my son in law a phone call, he managed engine rebuilders in NZ where he did his apprenticeship on 8hp and A series, and in Australia where he did many older engines for restorers, the .080 was commonly used on badly worn motors.

Maximum oversize is .120, Bob must have been talking about off the shelf pistons, he is my partner in crime at the moment and we share all our information. Bob had a racing engine builder meaure the thickness of the bore walls and was told that .120 is fine unless supercharging, which we are both doing

Pistons up to .120 are made by an Australian company, but for my purposes they are too heavy, using .080, what is lost in capacity is more than compensated for in lower reciprocating weights, and the risk of the block being less rigid is an unappealing thought.

Is there an engine that uses that size piston as standard with the correct skirt length?
Did you find any double spring sets?

Hi Mike, Valve springs are in initial stages, will let you know by pm when finished, am hoping to get singles of the correct tension, doubles are really only necessary on a OHV, yet to have a report from the guy on this matter though

The main difficulty with using other pistons not specifically for the 8hp is the little end size, new con rods would need to be made.

After talking with Bob a few hours ago he told me the guy who sonically measured his block said it would go to +.160 but pistons would have to made, only some blocks have the concentricity in the cores to allow this. One problem encountered with overbored blocks is the head gasket between no's 2 and 3 blowing, but by using an extra long stepped centre head stud/cap screw, and threading into the metal in the lower part of the block casting this can be prevented

he also showed me photos of the supercharger mounted, its looking good, just the manifolding to be done now.

Kleinig head completed, will post a photo

Sorry, probably a dumb question but what is the resulting capacity increase from a +60 thou or +80 thou over bore?

.060 will give you 968.41cc
.080 gives 985.30cc

kotemaori wrote: One problem encountered with overbored blocks is the head gasket between no's 2 and 3 blowing

Would solid copper head gaskets solve the problem?
Do you have any spec. for strong single springs? I am looking for something which will allow the engine to rev to 6000 occassionally without putting too much load on the camshaft

Mike, I am not sure if they are still obtainable but I stiil have the box for my 'double spring set' I fitted to mine - Terry's 'AERO' valve springs part number V.S. 453

Was unable to get a response from Terrys, their valve springs needed slightly more pressure when used at 7000rpm

7000 rpm!!!