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MG Midget and Sprite Technical - Fill in the blanks

While trying to regain my motivation I thought some shiny new parts might help with that. So if anyone would like you could help me to fill in the blanks on what and what not to get.

A little about the engine so far:
Re-bored to 1293cc, 276 kentcam & lifters, hepolite/nural pistons, weber 45 on long manifold, Peter May's rear oil seal kit fitted, cylinderhead (not yet fitted but) machined from what's in wizards book and rebuild with bigger rimflow valves and valvesprings that can take high revs with maximum lift of 0,500" (I think I might have gone a litte over the top with the head) The engine mates with a type 9 that has hydraulic clutch concentric slave and machined flywheel to take a 7,5 inch ford clutch. A proper maniflow manifold is already on my wish list for Christmas.

I was about to fit the duplex chain and gear set to the engine but am not satisfied with not being able to fine tune the timing so

Blank 1: Adjustable duplex chain vs. Cam belt conversion or just fit the original duplex?

Feels like I'll want 1.5 Hi-lift rockers to go with that head

Blank 2: Minispares forged vs MiniSports roller tip and will I probably need new pushrods for this?

Due to the bigger clutch on the flywheel the stater doesn't quite fit.

Blank 3: Try and trim the starter to get it there or buy a new hi-torque starter that might have a closer fit?

And getting in to the very confusing world of dizzys. I've got a 45D and a 25D distributor (am going to negative ground the car and fit an alternator)

Blank 4: Should I convert any of these to electric with negative ground or just buy a new complete electronic distributor? Any reasonable priced kits and bits recommended?

Finally I want my engine to breath properly so..

Blank 5: Basically where should it breath? Should I connect the camcover breather to anything like carb, manifold or just put a small air filter on it? And I assume at least one additional breather is necessary so where to put it? Will I need a PVC, catchtank or anything like that?

Any input or feedback is much appreciated, thanks
//Alex
Alexander Sorby Wigstrom

definitely vernier adjustable timing gear, as that will get the most out of any engine (or, if you are clever, and patient, make an offset key to get the same result for less money - either way, dialing in the cam is #1 for getting the most out of it)


forged rockers are fine for most engines, and, as I recall, rollers are necessary only for super aggressive cams (whereas a 276 is a very street able cam, so forged should work very nicely)


a high torque starter provides a weight savings, which you'll need with the very heavy type 9 gearbox

for a highly modified engine, a 123 distributor will allow you the easiest job of finding the best fit advance curve for it on a rolling road, or you can have either the 25 or 45 re-curved by Distributor Dr., and use a solid state points for it (wait, I forget, is the 45 already electronic?)

fitting a Webber, for crank case ventilation, you'll want to install a PCV valve in the hose from your front cover (the oil separator can on the timing chain cover) and attach the other end of the hose directly to the intake manifold. The breather nipple on the valve cover is the "air intake" for the PCV system, so make sure its hole is 9/64" dia. If you fit a valve cover without the nipple, then use a vented filler cap.


You haven't mentioned compression ratio. the biggest single contributor to increased power (besides displacement). Everything else (cam, valve size, intake, carb adjustment, timing) is just adjustment to get the most out of whatever particular set up you've got (displacement, CR).


One other key thing that you need to tell us, besides compression ratio, is this: what are you trying to achieve? Racing? Normal, spirited road use? Those two extremes call for wildly different approaches to engine set up (racing is all WOT, so peak HP is key, without concern for anything below 3000rpm, but a road use car would be a major pain to drive if set up that way).
From your choice of a 276 cam, I am guessing that your plan is for road use, but it is on the threshold between the two, so I wanted to make sure.



Norm
Norm Kerr

blank 1
I have seen just to many broken belt drives recently so http://www.minispares.com/Product.aspx?ty=pb&pid=37852&title=
just go this way.

Blank 2
roler tiped rockers
As they will reduce side loading on the valve stems a lot reducing wear.
Normal push rods should be fine.
A shim under the rocker posts might be needed but this can only be checked with the head and rockes in hand.

Blank 3
std starter has always worked fine for me and if a normal flywheel is used I can not see how a cluth can interfear with it

Blank 4
Very dependant on the distributors you have
Always convert to negative earth this just works a lot easier, noting in the distributor has to be changed only the + and - have to be swoped on the coil.

With your engine spec it is very hard to say what std distributor curve will work.
An Aldon or preferable 123 (tune) would be more suited

If you want to try the std ones you have try http://www.simonbbc.com/ their electronic conversions are cheap and work perfectly

Blank 5
Now you have opened a can of worms!
This is almost a religious debate.

I'd say breather at the rocker cover and breather at the timing chain cover should suffice.
Either to open air or through a pvc valve to the manifold.

Onno Könemann

Just the type of answers I was hoping for!

Norm, You are correct that it's of road use, What I would like is a drivable car but still have that extra boost. I recently posted a thread in the general of not really know what I'm building and this all comes very clear now, hehe.

About the compression ratio of the pistons it is 10.0:1 but I won't really know what I'll end up getting until the head is on, right?

Onno, you are most certainly right about the "breathing religion" but since this is really something that I'm not at all familiar with I just want some opinions for a base to start on. If I choose to go with a PVC valve is there any particular one to choose or a place to scout one from?

Alexander Sorby Wigstrom

Any pvc vale will do
Minispares must have one in stock

You can (and must) calculate the compression ratio (CR) before you put every thing together!

You need to do some pre assembling tough
In an ideal world you measure the volume of piston dish, ringland and volume below deck with a burette
And also measure the volume of the head with a burette
Ad to those the compressed volume of the HG and with the swept volume of the cylinder you can calculate your CR.

Almost the same can be achieved with looking up or calculating seveal of these values

Vizard has writen an exelent chapter on it and with google you can find several CR calculators.

10:1 sounds like a CR matching the spec and your wishes.

If you build this engine properly you will get a nice road engine with a wider power band than std and plenty of grunt.

Result will be a midget that is a joy to drive!
Onno Könemann

Haven't used a burrete since chemistry-class in highscholl and don't own one. But my main problem is that the pistons and rings is already in the block. But still I'll do my homework and read up on the subject. And can hopefully get back to you with an accurate calculation.

But what can I gain from knowing the CR in advance?
Alexander Sorby Wigstrom

you're concentrating on the engine at the moment but you could also be thinking of what else keeps the car going quick like suspension, tyres ect.

I've not the first clue about engines but I can tell you I’ve had igniter heads and modules and on my present Midget an Aldon igniter head on the existing probably worn dissy only because the 123 dissys weren’t available for 1275s – as is my lucky a year or so later the 123 1275 dissys become available so I swapped over

I’m very pleased with the 123 fully electronic, no servicing, dissy as I put elsewhere the difference was particularly noticeable at motorway speeds giving the car a good clean pick up

You’d probably want a programmable 123 tune – Peter Burgess is your man to buy from and have him set the engine up on his rolling road, excellent value too - http://www.mgcars.org.uk/peterburgess/123.html

whoops, just realised your in Sweden but you might want to tour here, excellent roads in every county if you know where to look
Nigel Atkins

If the pistons and rings are in the block it is easy
Measure TDC for each piston and with a piece of plastic with 2 holes (parafin in air out) then use a burette (borrow one from the local school or ask the machine shop for help) and measure the volume.

Knowing in advance gives you the chance to adjust it.

If your CR appears to be to low (means you lose torque down low) you can eithe skim the head or the block to increase it.

If your CR appears to be to high it is even more important because this can cause pinking and detonation and ruin your block!
You can grind out the head some more to increase the volume and lower the CR
Onno Könemann

Hey Nigel, I covered the suspension questions a couple of posts back and are expecting and huge order from moss to drop in on Tuesday, also a front disc brake conversion is going on so that part's pretty much covered. (I might get back to that if I can't get the ride hight leveled though)

Everyone seems to recommend the 123 so I'll most definitely check it out, and I have a local shop that will probably be able to set both the weber and dissy up properly.

I actually hope that I at some point will be able to tour the UK in the sprite but that probably won't happen for a long time.. not until the midget 60's anniversary anyway.

Okay Onno thanks, you're a great help!
Alexander Sorby Wigstrom

This thread was discussed on 20/11/2011

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