14 replies to this topic

[Coxie]

Something I've wanted to do for years but never got round to is build an engine. So this year I'm going to get stuck in.
I don't really need one but with a new baby in the house and a manic 4 year old I need an excuse to be out in the garage for some ear ache relief and I can always buy another mini to put it in when it's finished, if I can get away with it.
Anyway, picked up a 1275 a+ short engine yesterday with an engine number 12H694AAZ
Bit of research shows its possibly 1980-1982 ellegro.

I want to spend the next month or so reading, reading some more and get everything clear in my head before getting stuck in. Ordered the ultimate mini builder dvd too.
I'm also going to use this time to collect parts for the build.
The plan
Nothing Lairy just a nice road friendly engine that has a bit more power than standard, that will spend most of its life under 5000 rpm. My days of driving like a fruit cake are long gone.
I'm thinking
Big valve head( mildly ported)
Sw5 cam
How does this sound for road use?

Now the block looks in pretty good shape and looks to be on its standard bore, still has honing marks in the bores. I'll take it to my local machine shop and see what they think as it does have some very minor surface rust in one of the bores. I'd like to keep t 1275 if possible.


The pistons are marked m2210.
Is it worth keeping these or would there be a better option?

[gazza82]

I wouldn't worry about the pistons until it's all been properly measured. If the block needs reboring, those pistons are useless anyway. Strip it down, get it cleaned and properly measured, and then start thinking about what you want/need.

[Cooperman]

Once it is stripped & accurately measured you can decide what you want to do and how far to go.

The cam choice is good for what you want, but a 'big-valve' head doesn't define it. Ideally you need 35.6 mm inlet valves and 29.5 mm exhausts. Gas flow the head a bit, especially removing the 'steps' around the valve seats and smoothing out and reducing the valve guide bosses.

Measure and skim to get the pistons to come right to the tops of the bores at TDC and then measure the combustion chambers with the valves re-installed and lapped in. Calculate the compression ratio this will give and if it is not high enough have the head skimmed to bring the chamber volumes to what you need. A CR of around 9.75:1 to 10.2:1 works well on road cars and still allows 95 oct petrol to be used, although 97/98 is better.

Do a trial assembly to measure and adjust the crankshaft end-float and fit new main, big-end and thrust bearings together with a new oil pump.

 

Lots more advice available on here when you get a bit further into the work.

[Coxie]

Thanks gazza, lot to learn eh!

[Coxie]

Once it is stripped & accurately measured you can decide what you want to do and how far to go.
The cam choice is good for what you want, but a 'big-valve' head doesn't define it. Ideally you need 35.6 mm inlet valves and 29.5 mm exhausts. Gas flow the head a bit, especially removing the 'steps' around the valve seats and smoothing out and reducing the valve guide bosses.
Measure and skim to get the pistons to come right to the tops of the bores at TDC and then measure the combustion chambers with the valves re-installed and lapped in. Calculate the compression ratio this will give and if it is not high enough have the head skimmed to bring the chamber volumes to what you need. A CR of around 9.75:1 to 10.2:1 works well on road cars and still allows 95 oct petrol to be used, although 97/98 is better.
Do a trial assembly to measure and adjust the crankshaft end-float and fit new main, big-end and thrust bearings together with a new oil pump.
 
Lots more advice available on here when you get a bit further into the work.

Thanks cooperman, great easy to understand advice as usual.
I need to source a head at some point for it, as someone that doesn't really know much about them. What should I be looking for?
Are most 1275 a+ heads the same as in casting numbers

[Spider]

Just about any head for a 1275 you'll pick up will have a casting number 12G940, though there are a few others, but not too common these days. Look for one that hasn't been modified with 29 mm exhaust valves and 33 mm inlets (rounded to the nearest mm).

 

Just to expand a little on Cooperman's suggestions on modifying the ports, if you do grind it yourself, don't go berserk, just basically 'clean it up' and no doubt you'll go for an increase in valve size, then match up the area behind the valves , more or less, to the valve sizes.

[Coxie]

Just about any head for a 1275 you'll pick up will have a casting number 12G940, though there are a few others, but not too common these days. Look for one that hasn't been modified with 29 mm exhaust valves and 33 mm inlets (rounded to the nearest mm).
 
Just to expand a little on Cooperman's suggestions on modifying the ports, if you do grind it yourself, don't go berserk, just basically 'clean it up' and no doubt you'll go for an increase in valve size, then match up the area behind the valves , more or less, to the valve sizes.

Thanks for your advice moke, as this is all new to me I'll leave any grinding to an expert. Baby steps and all that.
Think I have found a suitable head just round the corner from me so I'll go check it out.
Really appreciate all you guys advice.

[Cooperman]

Just about any head for a 1275 you'll pick up will have a casting number 12G940, though there are a few others, but not too common these days. Look for one that hasn't been modified with 29 mm exhaust valves and 33 mm inlets (rounded to the nearest mm).

 

Just to expand a little on Cooperman's suggestions on modifying the ports, if you do grind it yourself, don't go berserk, just basically 'clean it up' and no doubt you'll go for an increase in valve size, then match up the area behind the valves , more or less, to the valve sizes.

It is surprising how a small amount of cleaning up of the combustion chambers and throats gives a significant improvement in torque and top end power.

Another cam choice, one I usually choose for this type of application, is the MG Metro or Kent 266. They are very similar and give excellent mid-range torque coupled with max power at 5700 rpm.

[carbon]

Coxie,

 

If it is only light rust in the bores then careful use of a stainless steel pan scrubber (the one with coiled thin flat stainless wire) will clean it up. From the pictures the bores look good, using your fingernail check for a wear ridge about 8mm from deck - this is where the top ring is at tdc. If there is no wear ridge then you should get away without a rebore.

 

I would suggest taking out just one piston, to check that the rings are not gummed up. If these are free then you may not even need to re-ring the engine.

 

For cylinder head look out for an early MG Metro head, these have the larger 35.5mm inlet valve. If you go with the MG Metro cam as suggested by Cooperman you have the basis of a very decent road spec engine for a Mini.

[limby2000]

Did you put pistons back in to photograph, as their facing in different directions. Pick yourself up a mg metro head, I sold one on here a couple of month,s ago for 75 quid. It was,nt unleaded though.

[bikenutmart]

Minispares evo 001 cam is I think a good choice, and very well priced!! Mart.

[Cooperman]

There is little to choose between all the cams mentioned. Of course, those who may wish to sell you one won't necessarily agree, but it is the truth and the accuracy of build is  where the engine gets better or worse as a road car. You are not seeking a cam which will save 0.5 seconds per lap, just one which will drive nicely.

[Mrpeanut]

Minispares evo 001 cam is I think a good choice, and very well priced!! Mart.

Agreed. As great as the mg metro cam is I'd rather buy a new evo001 over an old and potentially worn mg metro cam every time. It's 266 ish with more lift built in and a superb fast road cam that pulls from down low.

[Cooperman]

 

Minispares evo 001 cam is I think a good choice, and very well priced!! Mart.

Agreed. As great as the mg metro cam is I'd rather buy a new evo001 over an old and potentially worn mg metro cam every time. It's 266 ish with more lift built in and a superb fast road cam that pulls from down low.

 

Good advice. Never fit a used cam to a fresh engine build unless it has been carefully inspected for wear on the case hardening on all the lobes. Considering the overall cost of a full build it is false economy to fit a used cam. New cam & new followers every time really for best results.

[tiger99]

Yes, new followers are essential. A while ago on this very forum I seem to recall someone that ruined a new cam by using old followers.

 

The thing to understand (because once you understand it, what follows becomes obvious) is that there is only line contact between the cam and follower (give or take some minor localised elastic deformation that spreads the width of the contact line so it is not quite zero), and the contact pressure is very high, in fact the highest of any engine parts. New can and followers are soft, and will gently work harden each other, which is partly why there may be instructions with the cam to run the engine at a certain rev range and not let it idle. But put used, hard followers on a soft new cam and, well, it might not be a high lift cam for long...If the cam manufacturer specifies special lubricant, be sure to apply it generously.

 

On modern engines with the latest materials and machining methods, running in is said to be unnecessary, but if you buy a new car, don't forget the poor camshaft. Don't let it idle for the first few hundred miles, keep it revving a bit, but not screaming its guts out. And don't forget the piston rings either. After a couple of hundred miles, accelerate hard at maybe 1500 to 2500 rpm up hill in a high gear, foot to the floor, to get maximum torque and combustion pressure, just for a few seconds. Don't rev it hard yet. Repeat periodically till you get to about 5000 miles. Then after the service and oil change, work it up gradually to full revs. You will be amply repaid for your troubles by reduced oil consumption and longer engine life.

 

A fully rebuilt Mini engine should be treated like a new engine.

 

Oh, and opinions will differ. Take all good suggestions into account.

Edited by tiger99, 02 January 2016 - 10:49 PM.