Sprite Race Motor Build

I normally build classic Mini engines and transmissions, but have been known to build the occasional Sprite/Midget A series too, since they're mostly the same.

A friend asked me to build a hot motor for his Bugeye race car......this will be the second motor I've  built for him, and the third Sprite race motor I've done. The first one I built for this car made 108 at the wheels, we're looking to improve by at least 20 hp at the wheels (maybe more) with this build, should make lots of really good noises!

I started by compiling a list of parts we needed - and sent him off to find them from a couple of trusted sources. When the new pistons arrived the first order of business was to send the block off to the machine shop for a rough bore - the #1 cylinder was scored by a pin that came loose at some point and before we went any further with this block we wanted to be sure it would clean up with the amount of overbore we had available - we're at 60 over with these pistons. Just to be sure, we offset the bore by 4 thou, and the cylinder cleaned up nicely.

As most of the stuff is now here, I got started on the build in earnest today by setting the block up in the engine stand so I could fit the crank, one piston and rod and start doing some measuring so I would know what we can take off the deck and where the compression ratio will wind up. We're shooting for 13-14:1.....

I measured about 37 thou to the deck, so we'll be taking about 30 off, leaving a little for future cleanups if needed and a little extra gap jic. 

Next I need to see whether we have enough valve clearance or if we'll need to flycut the tops of the pistons. 

Just getting started on this build, so there will be lots more to come over the next few weeks. We're trying to make a race in Mid July - might be cutting it close as once it's all built and the cam run in it needs to go in the car then off to the chassis dyno for final tuning and break in.

Cool! Please continue.

wheelsmithy (Joe-with-an-L) said:

Cool! Please continue.

I second this. 

Interested, I have a spare 1275 on the shelf for my bugeye that I want to build into a hot street motor at some point

Are those MED rods?  I was interested in getting a dog gearset from them but they are NLA.

Note to any sprite racers - I'm on the lookout for some "special" 1275 heads - 12G1316 or 12G940 heads with no "X" mark near the PN.  I race in a class that does not allow serious head porting and these have the best flow around the valve guide...

Curious to see where the engine makes power. What is the life expectancy of an A series engine at this level?

My intro to LBCs was MG Midgets and the little A series engines were really fun to rev. When I moved to an MGB my expectations were that the B series would be the same only with more displacement. What followed was 40 years of disappointment with the B series; the thing is meant to be a plodder.

My mentor on this project is an 11 time national champion (in an MGA - using a B motor I think) who also builds race engines. He seems to be able to make his A/B go pretty well. His motors cost upwards of $16K tho, just FYI. Life expectancy is directly related to how many races you run, of course. But several of his engines are in their 5th or 6th season without an overhaul.

I may have a spare 12G940 head that's unmolested, I'll look at the ones I have later today. Is the 12G1316 the "smog port" head?

Yes, they are MED rods and pistons, I think MED does some really good stuff.

I'll have more pics later today...

I remember pulling a 1098 from my Midget without an engine hoist. Just two guys lifting.

That was many years ago. Will never do that again.

MiniDave said:

My mentor on this project is a 9 time national champion (in an MGA - using a B motor I think) who also builds race engines. He seems to be able to make his A/B go pretty well. His motors cost upwards of $16K tho, just FYI. Life expectancy is directly related to how many races you run, of course. But several of his engines are in their 5th or 6th season without an overhaul.

 

 

And hence the popularity of V8 engine swaps in MGBs. Much cheaper than build a B series hand grenade that would not run on the street. 

In the end, I found that the best B series engine was a factory stock  pre-73 version 5 main block of course. The means to keep it running happy were spelled out in the manual. Even the great Burgess head upgrade lead down a rabbit hole where it was up to the owner to guess at all the corresponding upgrades and tuning specs. Few got it right. Race engines are in some way simpler because they only run flat-out.

Measuring for thrust washer clearance....factory wants 3-4 thou, right now I have zero, tho the crank spins freely. I've noticed lately I'm having to machine a few thou off the backs of all the thrust washers in order to get the clearance right, doesn't seem to matter who I buy them from..... it's a PITA.

Second pic is a Mini engine I built, almost ready to go into the engine stand and fire up to run in the cam.

Wow that's pretty. I'm definitely following, I love seeing your engine builds. They're always immaculate

I'll pop up a run video once I get it mounted in the engine stand - waiting on one dang part so I can button it up, still hasn't shipped from England, after a week!

Thanks for the compliment. If you're not going to do it right, why bother to do it at all? That's my philosophy.....

I think your mentor must be Kent Prather. He certainly found ways to make those motors sing!

In for the inevitable shenanigans!

Oil gallery tapped for threaded plugs, also tapped a hole for a breather vent where the fuel pump would go on earlier versions of this engine. Later ones like this used an electric fuel pump so the boss is a perfect place to put a breather.

I also marked the block so we can drill and tap for two more cylinder head studs - I'll let the machine shop do this on their mill so they'll be both accurate and square to the deck.

The pistons and rods are marked and assembled, although I still have one or two more small mods to do to them before they go to be balanced.

The rotating assembly will go to the machine shop so he can balance everything, once it's all back from the various machine shops it will be time to put it together and make some noise!

Unfortunately due to health reasons I had to send this one back to the owner. It looks like Kent Prather is going to take it on and finish it as the owner is a friend, even tho Kent has officially retired from building race engines.

In reply to MiniDave :

Bummer. Hope you mend quickly!

Sorry to hear you are struggling with health. Hope things get better soon. 

Getting older is not for the faint of heart! I'll be OK, thanks.....just dealing with heart issues and the Doc sat me down for now.