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MG Midget and Sprite Technical - Rebuild recommendations?

My 1971 1275 was rebuilt at 172,000 miles and pistons and bearings went back standard. I have driven another 76,000 miles on this rebuild and it is getting tired. It runs hot, often just under 212°F, will not climb hills at speed in 5th (car was Rivergated) and compression is 110 straight across. It now runs without pinging or dieseling on regular gasoline (not a good sign).

It is my impression that the bore, with almost 250,000 miles on it, is probably the primary source of power loss. Idle is smooth so top end seems OK and oil pressure is 75 - 80 PSI at 3,700 RPM, same as it was when the car was new (I am the original owner). I assume that the cam is worn but at the 172K rebuild, it was fine and did not require regrinding.

My thought is to:

-Bore to next oversize.
-Replace bearings and grind crank if necessary
-New valves and guides even if not yet required
-Replace lifters, timing chain and sprockets
-Replace and upgrade cam

I would like to achieve additional midrange torque due to the overdrive and consider that a replacement cam with a mildly aggressive grind might be in order. The car lives at 5,000 ft above sea level so the weak spot has shown up driving in 5th gear between 3,500 and 4,000 RPM and performance in this range is progressively worse.

-Should I raise the compression ratio?
-Any other modifications that are in order?

The engine has always been run stock (including emissions) and it has been extremely reliable and durable. It is primarily driven on the highway so the only upgrade desired in performance is in the RPM 3,500 - 4,000 range under cruising load at high altitude.

Any recommendations would be appreciated. I will be pulling the engine this Saturday and off it all goes to the rebuilder.

Glenn Mallory

If you want to uprate the cam to gain some extra power then I would strongly suggest to have the head done. Take it 1 or 2 steps up. That means opening up the ports and fitting bigger valves. Together with the hotter cam you will release a lot of power.

Next step is considering a LCB exhaust and a single HS6 or HIF44 carb on a Titan manifold.

That would be a 20 to 25 Bhp gain. I dont know what that does for the Torque but with the right cam profile a considerable gain is to be expected too.
Bas Timmermans

Bas has got it almost right ;P
keep using the twin carbs or even replace with a twinchoke weber!

But now the serious stuff
For more torque you need the SW5 cam (made by swiftune)with high lift roler rockers.
It will give you more power and torque from low down to 6500 rpm (depended on the head)
and won't make it idle difficult or lumpy.

Keep the head work medium
i would say don't go up in valve size (or just a little bit) just have it flowed realy good.

raise the CR but don't go over board!
I am paying the price now for a stupid high CR and have to switch heads.
10 to 1 should be nice

and balance the rods and crank.

make sure the cam is timed verry good with ofset keys or a venier set.

And if it will not get you in to trouble ditch the emisions crap!!!!
Onno Könemann

Hmm at 5000 feet I would say get as much air in as possible.

And a twinport Weber is only favorable above 4500 RPM. Below the 44 MM SU will do as good or better. At least more fuel efficient.

Bas Timmermans

Ah Bas you stil do not get the point!

a single barel carb on a midget is a bit like one testical in your trousers!
sure one can do the work just fine but is just is wrong!!!! ;P

some times just take the hard road if it had to be easy we would not drive midgets

And if a weber is properly set up it is fantastic to drive
all the torque low down the engine can give an perfect fuel efficiency
the point is, properly set up!

Onno Könemann

Hey glenn

Im doing the swift tune set up with the SW5-07, if you go directly to the factory in the UK its fairly inexpensive...I got the kite with everything, so it was like getting the roller rockers for free...the shipping is really fast. From everyone I have heard you really need to have the high lift roller rockers to get the most out of the engine with a sw5 cam


theres also a kent 276 or is it the kent 266 thats thats considard vary nice also one is race and the other pure stock...cant remember

of the kents the 266 is the best but it does not beat the sw5 for torque and they are an even match for power
Onno Könemann

Build a solid bottom end then supercharge it.
There are plenty of ex bmw mini super chargers kicking around.
check out the austin (real) mini forums... I have beeen told it can be done for £400.00!
Tim Dalton

Thanks Onno,

I couldnt remember which kent was the fast road cam, I agree with getting the SW5...But there are only one place in the usa that carries it and its $400 for just the cam by itself (OUCH)...But the kent kits can be gotten easily here in the USA.

I got to agree with the 1st it seems extreme, complex and expensive...but having chatted with treavor and others that have done it...its definatly a do-able option and really not all that expensive as the SC set up uses mostly stock parts, the cam, the pistons, the carb (hif 6, SU 44)...but like you said you got to have a strong bottom. the one big plus, is it appears "they" have figured out the heat issues, that plagued the BMW SC set-up, I did look into it at one time, but backed out as I was to afraid of all that power under my foot...LOL

Prop...I miss the stories of Thermo Detenation"

but with the internet ordering direct at swiftune is no problem.

yeah sure you can build up a SC setup for 400 (not with machining pullys) but getting it runing properly and reliably like an normal tuned engine is a diffrent thing.

And i do not like bolting another thing to the engine.
i love the mantra KISS (keep it simple stupid!) anny thing extra is just something extra that can break
Onno Könemann

Thank you so much for the comments.

Prop, I was unable to find the 1.5 roller rocker assy that the Swiftune site mentions. Their SW5-07 kit sounds like the way to go but would like to include the roller rockers. Can you indicate the part number for the kit that includes the roller rockers and/or URL at their site for ordering this kit? The UK prices seem quite reasonable compared to Kent from U.S. sources.

Supercharging would be fun but is a little over the budget and is more than I require, which is reliability and improved performance in the 3200 - 3700 RPM range (3500 - 4000 in my original post was overstated).

Basically just looking for another trouble-free 250,000 miles but with better compatibility with the overdrive. I got the car new when I was 17 so I will probably out live me!
Glenn Mallory

just mail them
they still mention the high lift kit (the one you want) it just does not show
but they are real nice people so mailing with a question will not be a problem
Onno Könemann

Just be carefull to pay extra attention to the valve lift in relation to the valve seals / valve guides.
My sw5 / high lift trashes the valve seals....
So I'm about to take of the head once more.
Alex G Matla

Alex are you sure?!
never heard of that problem with the sw5
Onno Könemann

No-one seems to have congratulated you on the engine. 1/4 million miles in a Midget is pretty good going - for both you and the engine! As you have had it throughout its life it shows that you have taken good care of it and treated the engine well with your driving technique. Well done!

Guy Weller

Onno, ofcourse I'm sure.
Alex G Matla


Ive heard of that happening before...Onno is jealous that his head isnt getting ventalated like yours LOL.

I found and had installed in my new head a short bullet nosed brass valve guild thats made to clear any potential them from (seven enterises) here in the states, califoriacation... I havent seen them anywhere else...Might try a mini cooper shop tthere in the UK as thats what this place is

they make one set for the intake with the top hat seals, and they make exhauste without the seals...or you can use just all intake if you want seals across the board

my head is getting to see air on friday
compresion is just a bit to high......
so new head is waiting ;)
Onno Könemann

Oh please GUY,

A special pat on the back for a lousy 1/4 million miles? Nah!...If it was a million miles I could see that...LOL Sorry glenn, that is sorta of an accomplishment I guess, Ive done it on ALL my vehicals.

Hey Glenn,

Onno is correct, vary good to work with, I went in with a friend and we ordered 2 kits togather, but I think you have to email them direct, Im not sure the order form works here in the USA..something to do with zip codes and currency exchange as I recall

On the High lift rockers...the company was having an issue of some type getting there personal high lift roller rockers made (striking unions or something) so they where substituting this peticular set-up, which get high marks in reviews...not sure of there source. (see below) you will need to ask a them about that, we had to. BTW swiftune also owns Kent... I like the charts that show the SW5 really performs in that rev range where 80% of most of the driving takes place.

Id also recommend 21253 pistons they are a bit high compression without mod to the head...9.75 to 10 CR for un-molested head and block deck

Id also port out the blobs of metal that holds the guilds in the head, take that edge/lip off the short side of the of the ports inside the head, the lip inside the intake prots and the lip iside the carb that holds the jet..then install the cooper S vlave sizes...cant remember the size right off..then use the penny on a stick / rimflow / rimflow Knock offs valves...Also Id recommend Payne gaskets with the composite head gasket, these are the best trouble free ...theres alot of junk out there

In the end...what you really need 1st are 2 books 1st "The Bible" written by the 2nd comming himself (David vizard called Tuning the BL's A series engine...the yellow copy) and secound By our vary own Danial Stapleton ...How to powertune and modify the midget Both books will keep you on thee seat of your pants and can give you insight and new ideas, and whats avialable and what is doable.


You may have to copy paste to your web browser for some of these links to work


Cool Onno,

Which of the crossflows are going to get, the 7 port westlake head, or the Alden 8 sweet, Come on, you know you want to, its only money! Besides its not like Arie has one. (Or does he...AHhhh)

Instead of a new head, you could just get a solid cooper head gasket made for racing and glue it on the block deck, then put the payne composit over the top as usaual..... I had to do that on my work truck several years ago after muliple head shaves...still holding water..Knock on wood.


Thanks again for the comments.

The engine and transmission removal was fairly quick and entirely uneventful. Hopefully I will know this week what degree of machining is required.

I E-mailed Swiftune in the UK to inquire about the roller rocker setup that they show on their website. It would be nice to get whole package from one source and their prices for the cam kits are reasonable too. There are a number of roller rocker assemblies available with prices all over the place (mostly on the high side). Mini-Mania seems to have the broadest assortment in the U.S.

As for the engine lasting 1/4 million miles, I have been impressed by the robust reliability of this 3 main bearing engine. The first half of its life was also spent running with a 55 mile an hour speed limit (in reality 60 - 65 to avoid tickets) mandated in the U.S. after the 1973 oil embargo. Operating conditions with the current 75 mile an hour limit and routine traffic moving at 80 - 85 are clearly more demanding (hence the beauty and necessity of the overdrive).
Glenn Mallory

Prop no crosflow yet.....

I picked up a nice ported head with more chamber volume at silverstone last month
And i had some rimflow valves lying around from my old cracked oselli head
So put 2 and 2 together and had it reworked with new guides and the rimflows

i hate solutions like decompresionplates etc i have now rebuilt my complete engine and am not about to take a short cut here
Onno Könemann


That is really cool...I know what you mean about cutting corners. My head is set at 10.25 CR, so im hoping Im not out of bounds my self, luckly its not a daily runner as well, so I figure high end fuel at 91 octane, and a bottle of octane booster. should do it.

that and a splash of turbo blue racing fuel at 110 octane (RON)...fairly reasonable about 6 dollars a gallon.


Prop, not to envy you but do you know we gan get 98 octane at every gas station overhere?

Most of the times I also add some VSP actane booster as the cooper head on the 1098 block seems to like the octane as high as possible.

Its a shame they dont sell proper petrol overthere.
Bas Timmermans

Actually Bas,

Your octan is figured differantly then ours, so yours is a little higher, on avg. about 5 points...ours is figured by deviding Research Octane Number by the Motor Octane Number

The research is measured in a free running motor, the motor octan is measured in a motor running under load

where as most of the world uses one or the other


This thread was discussed between 13/08/2009 and 17/08/2009

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