To follow on from my last thread on bore and stroke options.

I am looking at getting a couple of 1275’s for the Twini.
Something between 80 & 100 HP that can be used with ease on the road.
I was planning on purchasing complete 1275 A+ with box and doing them up.
Stroke the crank with oversized pistons (73mm) for 1412cc’s and 10:1 compression to run on premium unleaded.
Lightened and balanced flywheel, crank, rods, etc.
I would get a centre main strap and ARP bolts to hold it together.
Do the head up to a little better than Cooper S spec.
Road / rally cam, duplex timing chain and 1.5 roller rockers.
Single 1.75 inch SU with extractors.
Standard A+ gear box with 4 pin diff and centre oil pickup.

Couple of questions:
Does the large journal crank need a bit of work to bring the weight down? What is involved in this?
What is the best option for 1 & 5/8 journal con rods in this application (Cooper S?) – It is not a racing engine.
Is it cost effective to machine the standard A+ head to Cooper S type spec, or should I buy separate heads (from the UK? MiniSport or Mini Spares).
Can the A+ head & block be converted from 9 to 11 head studs with ease or are there problems in doing this?
Is it easy to convert the vetro clutch to standard, or is vetro better. Can the vetro be lightened for the same results?
Are there any issues changing the cam on the A+ and going to a duplex chain?

In summary, is the 1275 A+ a good place to start for an engine of this type, or should I start with the old 1100S blocks and find all the other bits?

What have I missed that may be a problem?

The A+ is a good start yes.
The standard crank is a bit heavy you could wedge the crank to save weight but it will cost
If grinding the crank for extra stroke Cooper S rods or small any small journal rod is the only option unless spending big $ on billet rods.
A head will cost about $1000 (min) by the time you fit S size valves, reseat valves and port depends on cost of a UK one.
The block can be drilled to 11 stud easily.
Duplex chain can be fitted easily just need to counter sink bottom 2 screws, cam no problems.
I like verto clutches they don't put as much thrust load on to the crank as the old diaphragm type but you can't make them as light. Standard they are lighter than a diaphragm.

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4efte powered Clubman
supercharged 998 race car

Crank-
Large journal cranks are poorly counterweighted but the A+ ones are a bit better then the old 1100S ones.
I would wedge the crank, this not only lightens it but increases the balance factor, reducing load and wear on the main bearings.
It lightens the crank on the crankpin side, this has the same effect as making counterweights bigger would (but lightens rotating mass also).

Conrods-
Unless spending silly money you just have a choice of Cooper S or MG Midget. Same rod really, only the material spec is different (S better). Either rod needs narrowing slightly to fit the stroked big journal cranks.
Standard A+ head is fine, needs hardly any work to be S spec.
100+HP at crank is easily possible with a stock head, if using a decent cam. Gerg got 102HP from the 1360 that GR built him, with single HS6 carby. And that was a stock 1100S head, which flow worse than the A+ ones.
Here's what a wedged big journal crank looks like after milling, and before linishing etc. Took me 6 hours on my little mill, GR does it much faster.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Another option for conrods are these:

http://www.specialist-components.co.uk/ ... egoryID=24

Not to expensive for 'H' beam rods - depends on how cheap you can get x8 good 's' rods + the cost of having them lightened + arp bolts.

edit: 'feralspirit' was telling me you could order a set for a ford V8 that were the same (but cheaper price per rod)

A+ cranks are better than the 1100s type, not as prone to cracking (although cracking isn't a huge issue if you crank was ground properly).

Unless your motor is going to be reving at 6500+ 80% of the time, your standard main caps will be fine. Don't normally worry about it untill your producing 105hp+ and reving over 6500-7000 rpm constantly.

I only send my heads out for decking and valve seats. I normally manage to do a complete head for about $190-$300 depending on valves and rocker gear. If you can keep it in house you'll save big $.

Changing over to a duplex isn't an issue, there are a couple of bolts under one of the sprockets that you will need to change for counter sunk screws so that there is room overwise it is straight forward and worth every penny.

Only problem with the A+ over the 1100s is the material, (and only sometimes). I don't know why (i'm sure someone here can explain it) but some of the earlier A+ castings can be almost porous and the quality of the material doesn't seem to be up to scratch. I have heard of (although I haven't seen it my self) problems with gases leaking into the water jackets on some of the really really early ones. The A+ cast shape is a stronger one with thick flanges and extra webbing. The 1100S's are good in thick flange (but I have had one which was bored off square from the factory ). The A+ and 1100S have the same HP potential and potential for reliablity, you will always find dogs in both forms from time to time but they are both pretty much on par.

Thats my 2c anyway

Many/most A+ blocks were factory sleeved due to porosity. You can overbore these to +.020" no problem.
You can bore the sleeves right out for say 1380 pistons but until you do, you don't know how porous the casting is...
Also, the cylinder walls are usually thicker one side than the other.
Personally, I prefer A series `1100S style' blocks. More meat there to bore holes into...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I`m with the Doc on this,,,couldn`t have said it better myself

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

when you decide on the type of head and if you still want to go to un-leaded fuel, I have just tooled/geared up to get back into cyl heads ,[mainly 16 valve bike heads and V8's]
I am getting a supply of V/Vs, seats and guides[both liner insert and full silicon bronze].
To keep the costs down I can help out with the basic machining and you can do all the fiddly bits ie the old spit and polish.
As for roller rockers I am knocking up a couple of sets of roller rockers next week[ for a ford V8 race eng]but mini ones as just as easy to make. These have precision hi-load needle roller brgs on the shaft and oil fed needle rollers for the tips . They are all 6061 alloy with new alloy posts.

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

oh ok, cool thanks Doc and MiniMan,

I have encounted sleved blocks before, but not all of the A+'s i have seen are sleved. I also prefer the thick flange 1100s if I have the choice but the A+ (at least for me) is very a close second (woow close second when there only a 3 to choose from). But yeah I agree. Not that the A+'s are bad so don't worry IndigoBlue. They are still just fine and I'm sure your you'll be very happy with your end result

Sorry to interupt David..