My '78 Mini has a 998 engine. About a year ago the head gasket blew. The shop that I have work on the Mini had a Cooper S head that had been rebuilt available so I bought it and they installed it. It ran great for a while... except since then the head gasket has blown 2 more times within about 7 - 8 months. The heads are flat and hold compression just fine. The shop can't figure out why this is happening.

Fast forward to February. I turned the car over to a gentleman that I met last year who I hired to do a bunch of work on my car. I finally got the car back last week and I think the head gasket blew again on Sunday.

My new guy thinks that because the head is upgraded, I need a different carb. He thinks that's why the head gasket is blowing.

For those that know, what do you think about this? As far as I know, the carb is the standard one for the 998 engine.

If I do need a different carb, which one should I get?

Thanks,
Mike

 

What head is on your car? What is your "Cooper S" head? There's a casting number on the top of it under the rocker assembly. Is it a 12G940 from a 1275cc? Or other number?

When exchanging heads you need to look at chamber volume and determine if the head needs milled to keep compression up (or conversely has smaller chambers, thus raising compression higher). Also, you need to use an appropriate head gasket. What gaskets have they been using? Small bore or large bore? Copper or composite material?

Examining the "blown" gasket should give evidence of where and what the problem may be. Is it blowing between cylinders? Which cylinders? Is it blowing between the water jacket and cylinders? Elsewhere?

What carb are you using? I assume a SU HS4 1 1/2" carb? It should be more than suffcient for your displacement. It may need re-jetting to get your mixture right. There are programs such as WinSU that can give you rough starting points for needle suggestions based on your engine configuration.

The head needs to be within 0.004" of flatness. Otherwise it needs re-surfaced. Has the mechanic ever checked the block for flatness? It also must be flat. If not, then it needs to come out & be decked for flatness. (a major dis-assembly job)

 

The 12G940 sounds right.

I'm not sure about the gaskets... I've never changed one myself.

The blown gaskets in the past have all blown cylinders. Not between the two middle ones but between an outside one an a middle one. No water or anything fouling the oil.

Again, not sure about the carb. I think it's just the standard one.

 

Ooops, forgot to mention I've been told the head and the block are flat.

 

When & how was it measured and what were the results? <- question I'd be asking By the mechanic or by a machine shop?

Blown between the same two cylinders each time?

Was the head crack tested? Do you have any overheating issues?

What is the compression ratio with the new head?

I suggest you take this discussion to Minimania.com where you can get more expert advice than here. Guys there have been racing & building Minis since the 60's and know their stuff. There are many possible causes and more minds can sort through this in short order.

 

Thanks. I'm not knowledgeable enough to really know what I'm talking about.

The head is actually a 12G295 for a 998.

 

That's what I run on my 1098cc. It is a Cooper (not S) head that was also used on the 1098 Midgets and Sprites in the mid 60s. It needed to be milled about 0.050"-0.060" to reduce combustion chamber volume to maintain compression ratio. It has a larger chamber than other small bore heads. You would be using an approprite small bore head gasket.

I forgot to ask. After replacing the head gasket, did the mechanics re-torque the cylinder head? After a heating/cooling cycle or two the cylinder head must be re-torqued to specification and the valves re-adjusted. Otherwise it will be "loose".