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MG Midget and Sprite Technical - spec-ing out a new engine

Hello everyone,

It's becoming clear that my Engine is getting very tired and will need rebuilding...

I'd like to do it myself, both for the money saving and for the experience. My plan is to buy and collect together all of the bits, build the engine in a more leisurely manner and then swap it out when it's ready. My car is a daily driver so I'm not looking for a fire-breathing monster, needs to be able to handle traffic jams and take me to work without too much hassle, but obviously a bit of extra get up and go would be nice.

Basic spec would be:

Something like 1310cc,
Grind crank, new shells.
New SW5 cam & 1.5 rockers.
Get the head re-worked
123 Dissy

Existing upgrades carried over:
HIF44 - which will shortly be fitted to my current set-up.
Maniflow ex manifold
Electric fan

So a couple of how-longs-a-piece-of-string questions that I've been ruminating on (remembering that this is a daily road engine so won't be stressed all that high but needs to be rock-solid reliable):

1. What is a sensible CR to aim for in a daily driver that will allow me to run the car on regular unleaded?

2. Lightening...

2A - I've got access to a lathe big enough to swing a flywheel. Can I do a bit of lightening myself? How much is a sensible amount for a daily driver? guess it would then need balancing?

2B - which other bits are worth spending the money on lightening?

3 - bolts, studs etc - which can I re-use and which should I be replacing

4 - what else do I need to look at to generate a bit extra from the engine for modest outlay?

5 - What's this going to cost me if I do the building myself, and what's the trade off between improved performance and more cost?

Be interested to hear the opinions of those of you who have been down this road many times...many thanks

Mark
MarkH1

When I was building my engine a couple of years back I came across the attached for flywheel lightening.

I now for the life of me cant recall the source (so if anyone recognises it shout!). I didn't have the attached done - I found it too late



Dean Smith ('73 RWA)

I doubt you'll save much, if any, cash in the long run but it does allow you to spend over time when affordable.
I'd re-use all bolts and studs after careful cleaning and inspection - if they look OK they usually are. CR 9.5:1 ?
Further items:
- LCB and ex system
- gasflowed head, 3-angle valve cut, bigger valves (Pete Burgess Econotune or Fast road) and ask Pete for best combination of rockers and cam to go with the chosen head.
David Smith

As regards 4) - Compression Ratio if you're having machining done anyway + Cam (though the latter seem to be getting pricier now as more are offered as new billets rather than regrinds)

Be honest about the rev range etc you intend it for. If you're not building a 7500 rpm screamer then you dont need a really fancy bottom end.

5) I have some figures from my build...but much will depend on the cost of the base engine you use and how much machining you do/have done.

Dean Smith ('73 RWA)

As A-series flywheels go, the 1275 is pretty light anyway.

I've got one ready to post at the moment and it weighs in at 7.9Kg with all of the packaging.

The earlier flywheels - 948 & 1098 - weren't relieved at the back, so could be lightened considerably.
Dave O'Neill2

Hi Dean

The source of your diagram is a Special Tuning document (as can be seen by the logo on the reverse) - I have a copy of the same somewhere... You can't go far wrong machining a flywheel to ST spec...


JB
James Bilsland

Mark, your basic spec seems pretty good to me for a fast road car.

Re your other questions if it were me:

1) 9.5 min 10.5 max

2)Don't bother

2a)Don't bother

2b)None for a road car

3)As David says

4)Concentrate on the head work - Peter B is your man

5)Depends but don't waste money on balancing, lightening, fancy pistons studs/bolts etc - it's expensive and you won't tell the difference

What you will have is basically the same as mine when I first built it and it's still going strong 6 years on. It cost me a lot less than £1k but things have gone up a lot since then.
John Payne

1- depends on the cam (would not go for the SW5 look at the RE13 266 of 276) but with a 276 10-1 should be fine

2- If budget is tight don't have anything lightend std weight flywheel will also improve idle and so make it easier to live with

3- minispares have a good cheap arp head stud alternative silly not to buy!
consider arp rod and flywheel bolts if budget alows otherwise at least get good rod nuts.

4- the trick imho is wildest cam you can live with the highest CR allowable with the fuel used combined with the best head you can buy(the pro's realy know best with heads)

5- If you considder dutch machining prices it is cheaper to buy a stage 2 engine from http://www.skiptune.com/engines.html
But iirc uk machine shops are a bit cheaper

With your wishes i'd consider the following.

std botom end only regrind when needed and only overbore what needed std fastners.
Good quality pistons (cast omega)
276 cam with venier timing gear (or a lot of messing with offset keys)
Get a good head from a reputable grinder fasten with minispares competition head studs (9 studs is plenty) and fit 1.5 roler rockers.
10-1 or 10.5-1 CR

And you ancileries should be fine.
Budget a roling road visit after it has been run in
Onno K

I got my flywheel lightened to that same ST diagram. At least I think I did, I copied the diagram and gave it to my engineer and he certainly lightened it but whether to that precise spec I couldn't tell. He also balanced the crank, flywheel and harmonic balancer as a unit which I believe is well worth doing. He then line-bored the crank. I am not sure why but that is what he chose to do. The shock is that the total cost was £120, and that included reboring to +30 but no assembly. (this was in 1996, but still remarkably cheap even then!)

Vernier timing gears are an expensive luxury only worth it is you are planning on swapping around your cams aor cam timing. Otherwise just using offset keys and a bit of extra time will get the same results at a fraction of the cost.

Mine is about 10:1 and I use a Kent 276 cam, HIF44 and 3 branch Maniflow manifold.
Guy

I did my flywheel also to ST spec, never regretted it. Balancing is a good way to prolong bottom end durability and smooth running.

And the minispares EVO camshaft is very nice, low down torque and highish revs, and very affordable. AND beats the SW5 IMHO (Had both in the same engine)
Alex G Matla

Totally agree with Onno the trick is the best cam with the highest CR

Personally I would be in the 286 range and 10.5 to 1 lthough I can see why people settle for a 276.

Agree also with David you save absolutely nothing by building the engine yourself, you still need to pay for machining stc so the cost of a couple of hours labour putting everything together is nothing.
Robert (Bob) Midget Turbo

I recognised the details Dean posted as from special tuning and that's what I used when I machined my flywheel although I used the 948 flywheel details as that's what I was using with a 1098 crank in a 1215cc. Crank, flywheel, damper all later ended up in a 1380cc giving 1420cc. They've done about 70k miles without issue and I never regretted the lightening. With lightening you may stall the engine a few times initially until you get used to it but after that no problems and I used the car as a daily driver with the 1420cc engine for about 9 years.

If you do machine the flywheel get it balanced.
David Billington

This thread was discussed on 11/01/2012

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