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MG Midget and Sprite Technical - 74mm Pistons?

Hello, first post to these forums. Two questions; I've bought a set of 74mm Jonspeed pistons for my 69 midget, are they likely to suffer any reliability issues in a 7500rpm engine? Secondly, my car is one of the 1400 ish assembled in Australia and so as a 69 model was probably manufactured in 1968 before being packed and shipped in CKD form. So... what oil pump drive is it going to have in it?

Cheers!


J M Mitting

74mm pistons will not leave a lot of meat between the bores, even after offset boring. Cooling could be an issue here. Wherabouts in WA are you?
Shawn

74mm is likely to give you troubles

why not go for a 73,5 mm bore giving you the more common 1380

oilpump drive is dependent on the camshaft an with the proposed bore i can't help to think that it wil not be std
Onno Könemann

Camshaft is a Kent MD276 winging its way to me now. Kent apparently run slot drives on all their billet cams, I was wondering if thats what I already had. I'm not familiar with offset boring, It's probably just what it sounds like, but how big is the offset from the centreline? Does anyone on the forum have experience with a 74mm bore?

The total package so far is; 5 port alloy head, weber 45, 1.3:1 roller rockers, extractors (local design) the pistons, duplex vernier cam drive and the cam. With some careful assembly and ensuring the block is true, I'm hoping for 120hp and a fair to good spread of torque over 1500 rpm. Being an Australian car it has an oil cooler fitted as standard and I've always found the A-series copes well with hot weather. An old mini 998 from long, long ago once lost all compression due to overheating while being given too much welly. It drove home after a little rest by the side of the road. None of my other A or B-series gave trouble due to overheating, unlike inumerable holdens and humbers in the family.


J M Mitting

methinks you need to do some reading. Start with Vizards book.
Shawn

indeed do some reading vizard still is the book if it concerns the A-series engine

new engine is new oil pump not something to save on

lets start from the begining
you want 120BHP and good torque from 1500 rpm

if so forget the engine you are planing now and go for a forced induction one supercharged being the best choice if you want torque that low down

you also mention regular visits to 7500rpm
is it a race engine?
or is it a street one?

no experiance with 74 mm bores here
with a 73,5 mm bore you already get verry close to the oilpump feed line at the back of cylinder nr4

so why risk it when you can still bore to 73,5
my preferance would be one of the normal oversizes

Onno Könemann

J M Mitting.

Firstly, WELCOME to this friendly and informative BBS.

My advice would follow that of Shawn and Onno's, unless you HAVE to, do not bother with the 74mm pistons. (I know, you've just bought them...). The 1275 block is NOT like the 998 (small bore engine) and can and will suffer over heating problems, so anything to help avoid this will be a blessing to you in the future.

In 1380 build (73.5mm bore) the engine copes adequately but still requires keeping a sharp eye on engine temperature. The rear cylinder may and will overheat causing seizure of the rings and the subsequent consequences that this will cause....

Also remember these cylinder blocks are no longer made and as such are NOT easy to find these days, so boring to 83.5mm is usually regarded as it's last life, (they don't liner well from that size).

As for the oil pump, use a NEW Metro Turbo one....

Hope this helps.

Mark.

Soon to be in Australia.... :-)
M T Boldry

Hi

On a non-comp engine I wouldn't go for 74mm unless forced to do so but there are plenty of 73.5mm (1380) road engines out there which don't give any hassle if properly built - Maybe Jonspeed will swap the pistons you bought for 73.5mm ones? Vizard will give you the offsets for boring.

I think 1500 rpm might be a little low if your're seriously wanting torque down there with a MD276 - its only a 1400 engine at the end of the day... but getting towards 120bhp should be acheivable as a reliable road engine if properly done - with balanced components you shouldn't have too many dramas running up to 7500rpm occasionally - but then again everything we use is at least 35 years old and you have to expect old metal to give up the ghost eventually...

I've never had any success fitting Metro Turbo pumps (the alloy fins always seem to hit the pump case on the backplate) but I find that 1300 Metro slot drive oil pumps are fine so long as you use the 2 fixing bolt ones with the steel backplate - I had 75psi oil pressure all year on my race engine - until the crank broke anyway...:)

Cheers

James
James Bilsland

Interesting point there James on the Turbo pump verses the 1300 Metro version. As you say the one you have used is more than adequate, the Turbo one is just that little better and the fitting troubles can be overcome.....

Mark.

Bloody good job we have spell check after a couple of pints of Jaipur... ;-)
M T Boldry

Thanks Guys, very informative. I'll look into the 74 v 73.5 thing and I will find a copy of this vizard characters tome. This not my first 1275, that was a 67 Cooper S in 1979, that said that was some time ago and the last of them was a while ago also and the memory may not be entirely trustworthy.

The point of this engine is fast road with occasional semi-competative outings, hillclimbs etc, (assuming the car clubs can find a hill around here). The desire for 120 hp and torque is more a wish list than anything, I'd be quite happy with power.
J M Mitting

then go for the 73,5 mm pistons
nice high CR (looking at the max octane available)
expect torque from 2500rpm and be happy with anny thing over 100bhp

folow vizard, strap your center main bearing and have fun!
Onno Könemann

You still haven't told me where you are. if you are near Sorrento - come talk to me. I will take you out in my 1380 115hp example. in fact join our club, we do our own low key autokhana's, and we rent kwinana once a year to do drag races, sprints etc. my car is currently king in the Midgets, but only due to lack of decent competition! my 73.5's cost me $115 for 4 from minispares in South Australia. If you have not done any work yet, these would be perfect, as they are flush with the top of the block, thereby raising the comp ratio, and eliminating hot spots (Vizard). you do not mention the state of your ally head. Is it standard or decently modded? My car has to be in spot on tune so as not to overheat here. Even slightly retarded ignition pushes up the guage. I do not even drive it when the temps are much above 30, never mind mid 40's.
Shawn

Hi Shawn, I'm in Mandurah. Currently I'm also building a house (owner builder) and going back to sea in February as well as moving (locally) to facilitate the house (re)-building. So....I cant see me getting up to Sorrento anytime soon, however I'll be back locally in April/May and would love to see your Midget then. As for joining your club, would that be the MGCC of WA? If so I'm intending to join ASAP. The Head is a standard 5 port alloy job, about Longman GT6 standard or phase 3 (out of 5). CR should not be an issue by my calcs CR should be around 11.6:1 with the 74's and 10.3:1 as a 1275.


J M Mitting

i am currently in the process of building an A series that will rev when needed to 7500, but will be using 74mm pistons. 1475cc will be the output when finished. the gentleman building the engine has autotest/rally engines that have been going for 14 years without ever being opened! if it is done right it will last for a long time. However he himself, will admit that it is down to luck once you go to this size if the block will take the bore. His track record stands by itself so I am taking my chances. I currently have an extremely sharp 1380cc in my car. Having driven 14xx cars there is no comparison, so in order to remain competitive I am stepping up.

1380 is great, 1400+ is amazing. worst case you build a 1380 if the 1400 pops!

good luck on your decision
WDT Corry

You could always 'stroke' it for the extra cc's

Marina or Ital crank offset ground......

Mark.
M T Boldry

I understand that you have bought the pistons already, and they will be a sizable investment. All I am suggesting is that it is a huge risk, especially here, where blocks are not exactly two a penny. I personally would be ringing Jack at mini craft in midland, and seeing if he would swap the pistons for 73.5's. Still risky, but no where near as much. I have taken some risks in building my car, but I would not even consider that one. Your head is going to help shed heat from the top end, but the bore thickness will be too marginal for our temperatures. Every time I see the wall thickness between 2 and 3 on my block I worry.
Try and make the next club meeting (MGCC of WA)in January. They have plenty of books in the library to lend. We already have 2 events planned for next year. They have an autokhana at Midland, and have booked Kwinana again for drag racing etc. We are also looking at the driving centre at the airport using airport transfers sevenoaks for a track session. There are not many Midgets here tuned to our extent. The majority of fast spridgets here have transplants. Not always making them quicker I might add! send me your e-mail address, and I will send you footage of my drag race - sorry, I do not know how to post it on you tube.
shawn.dawn1ATbigpond.com @ not AT of course.
It was a 15.7 second run against a race midget. (All three runs were 15.7 secs)
We had everything there from an electric powered mgtf (60's, not one of those modern appliances) right up to a multi million $ Buggatti.
Shawn

I never thought to ask of the spec of the rest of the car. What suspension, brakes, gearbox do you use? I know where there is a recon ford type 9.
Shawn

Currently the car is bog standard, it hasn't been round the odo yet and is sitting just under 92000 miles. I am collecting bits and pieces to build a new engine to complement the head and carb combination. Unfortunately work has had other ideas about how I will be spending most of the first half of the year and I may not get the time to do the work I had planned until later in the year. Coincidentally it could use a new gearbox. I have been all over the MGCC website for a while now and I will try to attend the Jan meeting, see you there.


J M Mitting

I used an offset bored 1400 for a couple of years without problems and it was a very high CR engine to boot. Only removed it to fit my turbo engine, it is still under my bench waiting for a new owner/job :-)
Robert (Bob) Midget Turbo

Jm,

74mm pistons!!!

I got to tell ya, Id be heavely tempted to do it, if it works, then your a mortal god, if not, ahh then your just one of the countless no-named, masses.

I certianly agree there are some serious potholes in this build, but the reward is a huge treasure chest as well. Just the cool factor alone is worth the price of admission ( heck If I where an 18 year old lesbian monster titty hot blonde bomb shell babe, Id do you for a night, If you had the 74 mm pistons)

I really think this all comes down to your personality and the level of adventure you like in your daily routine. If you like an easy orderly preestablished routine from wake up to lights out, this anit the engine for you, If you love problem solving, can handle the swamps of dispare in the darkest parts of africa, and dance in your underware on the roof top when you overcome the drags of (It will never work sooth sayers) then definatly build this engine

If I can offer one point from my own personal experiance and something I never considard when building my engine, the more you mod, the more BS. you have to overcome, It WONT be like a stock build where all the pieces "Just" line up and and fall into place and the hayes manual makes perfect scence, An engine like this takes alot of mechanical creativity, some dumb luck, and an out standing BBS board like this one to see it fire up...trust me, It will be a mecnical struggle.

Prop...Sorry I anit gay, hope thats not offencive.
have you thouht about going to a EN40b crankshaft, and the Aeg 521 rods
Prop

At last! a rational response, sensitive yet commanding in its grasp of the facts and technical challenges. Thanks Prop, I'd pretty much made up my mind to go there all on my lonesome but now I'm convinced.

Cheers, Jeff.

(By lonesome I mean me, my financial controller and a top notch machine shop I've worked with before on some lurid 1275 cranks in the last 33 years.)


J M Mitting

This thread was discussed between 13/12/2009 and 25/12/2009

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