Using a Sprite 1275 block in a Mini

Started by MiniDave, January 04, 2015, 12:32:28 PM

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MiniDave

One of the reasons I consider myself an "expert" on this subject is that I did exactly that for my Mini!

My block was worn so far that I would need to sleeve it and I wanted to save some money on my rebuild by re-using my old +60 pistons. A friend had a Sprite block that was already 40 over, so it seemed a natural fit, so I bought it.

Obviously, there are some difference between the years too - you can get a 1275 with tappet covers, one without tappet covers (pre-A), and then there's the A+ blocks which are completely different, and the SPI and MPI blocks, different yet again.

My engine is a Pre-A, so that was common with the Sprite engines.

There is no difference between the basic Sprite block and Mini block save those differences to adapt the 1275 to rear drive use vs the Mini's front drive, and these differences are largely extra holes drilled to bolt stuff on - depending on the years. Sprites were made until 1969, and MG Midgets till 79, but towards the end the MG used a Triumph engine (75 on I think) and those are completely different of course.

Here are some pics showing the differences between the blocks. In all of these pics the Sprite block is on the left and the Mini on the right.

From the top....



The only difference you might see here is if you have a Cooper S, or you've done the modification to add the two extra head studs found on the original Cooper S. You can easily do that modification to the Sprite block if needed. Simply bolt the head on and mark the two extra stud locations, then put it in the drill press and drill and thread the holes.....any machine shop can do this for you. For that matter this modification can be done to most 1275s, but I question the need unless you're building a high output or race engine. I was building for highway cruising and good fuel mileage so I didn't do it.

The back or flywheel end...



Here you can see many small differences....the suction hole for the oil pump is considerably larger on the Mini block - I drilled mine out to match, and I drilled out the same passage where it comes up from the transmission. My engine builder didn't think it would make any difference to the oil pressure, but I did it any way. You can also see an extra hole between the top two bolt holes that has been filled in - just a stud locktited in and ground down flush. The two oil galleys usually just have plugs pressed in them, but in order to clean those galleys you need to remove them - so most people thread those holes and use a pipe plug to close them off. The Sprite block already had that done. Other than that there are extra holes all around the face that are used to bolt the transmission on the Sprite that aren't needed on the Mini. they don't hurt anything and you can simply disregard them. Also there are three holes around the crank opening..... the rear main seal bolts on here on the Sprite, however this are is open on the Mini so there is nothing needed to be done with them either.

From the front.....



Here you can see the other end of the oil galleys - the opening on the right is one. Again, normally these have a plug pressed in but the racers thread them and use a pipe plug to make them easier to clean out. Other than that, no differences. However, if you cut the deck down to increase the compression ratio, sometimes you need to grind a little off the top of the water pump or it will hit the head and keep it from sealing properly.

Front side, or distributor side if you will.....



Everything is the same, unless you have an A+, SPI or MPI, those blocks are significantly different and represent the evolution of the engine, and as Sprite and Midget production was well over by the time they were used, they will not easily interchange, and may not at all.

From the right, or back side on a Mini.



Virtually no difference here....except - early cars had tappet covers on the back, these later engines don't as you can see. Makes little to no difference, to install the lifters you just turn the engine over and drop them in the holes, then slide the cam in to hold them in. You can see the Mini block has an extra hole threaded in the middle of the block (red paint) I don't know what this is used for on a Mini as mine was not used, but I think it may be for a support bracket for the exhaust. The cast boss  is there, so to use it you simply need to drill and thread the hole.

Bottom side.....



There are a few small differences here, the first being the oil pickup hole. on the Sprite it's threaded for the pickup tube. On the Mini it's just a straight hole, and this is the second part of the suction passage that I drilled out to match the Mini as it was larger - the engine builder said it wouldn't make any difference, but as the Mini has to pull the oil up from further away - as the transmission case is deeper on a Mini than the oil pan is on a Sprite - I decided to drill it out. There are locating dowels that locate the transmission case to the engine block, be sure to use those.

Here's a closeup of the oil pickup hole on the Sprite



And the same hole on the Mini



Lastly, the main bearing caps on the Mini are bigger than those on the Sprite, but unless you have the block line bored you have to use the ones that came with the block.

Sprite on the left, Mini on the right....





I had mine line bored and used the MINI caps. If you use the Sprite main caps, the rear one has a boss to accept the rear main seal, you simply grind this off to match the Mini design.

That's it, if you have questions, please post away and I'll try to answer them.



Complete failure at retirement

1989 Cooper Racing Green
2009 Clubman S
2014 Audi Allroad

MtyMous

Fantastic write-up. If I wasn't determined to use the A+ that I already bought years ago, I might consider tackling this one.

Mudhen

Bookmarking this post for future reference - thanks a lot for doing it!!!

Pat

Tim

Perhaps I've missed it, but what do you do about the crankshaft?  Do you just substitute a Mini one, or can the sprite one be modified in some way?  (I've never looked at a Sprite one).

Tim.

MiniDave

You have to use a Mini crankshaft, everything after the back of the engine is completely different - the Sprite is rear drive.

However, I did see a place in England that sells a kit that bolts on an adaptor to a Sprite crank that would let you use it with a Mini....However #2 - the Sprite cranks had smaller rod bearing journals than the later Minis (like my 1980) had, so a Mini crank in good shape would be the obvious choice.

I posted this for those who might need a new block like I did, as another place to get one other than another Mini. Blocks are still a dime a dozen in England, not so much here in the US.
Complete failure at retirement

1989 Cooper Racing Green
2009 Clubman S
2014 Audi Allroad

Lucas

I would like to compliment Minidave for the write-up. I would like to ask a related question Can one swap the crankshaft of a Metro mini with a standard 1275 Crankshaft? in order to use the metro Mini engine in my Sprite frogeye?? Best regards Lucas

MiniDave

#6
You want to use a Mini block in a Sprite then?

There are several issues - the first is the oil pickup - the boss is threaded on the Sprite block, you would have to thread the hole so the pickup would attach to the block. Next you'll find there are several more holes in the Sprite block, to attach the transmission and the rear main cap - you'd have to locate, drill and tap those holes.

Also, the rod bearing journals on the Metro crank are bigger than those on the Sprite, your rods may not fit.....

It's do-able, but there are far more Sprite blocks around than Mini - I'd just find another Sprite engine.
Complete failure at retirement

1989 Cooper Racing Green
2009 Clubman S
2014 Audi Allroad

Lucas

Thanks MiniDave, that's a clear answer. Best regards Lucas

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