Bore flex - Myth or Math.
Turbo Mini Forums

Been getting conflicting info on this subject from diff sorses
Assuming flex caused on power stroke , at what sort of power does this start to happen on correct offset bored 73.5mm bore ?

What effect does the big increase in torque of a turbo motor have on this

Could it be that people running big bore turbo's are loosing comb. pressure , but just chucking in more to compensate ?

Would have thought that when looking at 200+ bhp it would def. be an issue

I think you can safely assume that the folks on here running 1380 turbo's are more qualified to speak on the matter than folks who've never run a 1380 turbo, or even (dare i say it) a 1380...

I haven't ever run a 1380, or a 1380 turbo. However, I have an arsehole - and hence meet the criteria to have an opinion. And that opinion is that this matter of bore flexure is waaaaaayyyyyy over-rated...

is it possible that this is more of a truth in the machine shop rather than when the engine is operating in the car?

just a thought...

bore flex is an old topic and mainly induced by the yellow bible, at this time A+ blocs were quite rare as was mpi bloc untill today, so they were using mainly pre A+ blocs, some told that the iron casting was better before and leyland had just strainghtened there new bloc just because they had to use poor quality iron......?????
this isn't logicall, pre A+ blocs had poor life, I had an innocenti bloc which needed to be rebored every 40000 kms,lot of A bloc I know are gold or silver seals , so most of them had to be rebuilt.
I think the A+ blocs are a great improvement in power capacity for A series.
regarding me , I would go for 1380, knowing that the main problem will be a gearbox one...
fab

well i had 1380 turbo, with cast omega pistons 15psi boost, built by OAP etc etc and the bore flexed, went oval and crushed the sides of the pistons.

since i HAVE tried it i would recomend everyone to avoid 1380 turbo unless running less power

at the end of the day its up to you, i can only say the problems i've had in the last 10 year or so of turboing minis. and personaly i dont like taking the engine out

Fab. I've read that someone in Aus did infact do a test on blocks. A ones are infact of a better material than A+ ones. You've been testing linered ones Remember that oils were not as good back then, neither was ring and piston material etc.

I don't think it matters to be honest.

Vegard, it's interesting me, do yu remember were you read this ?
linered blocs I had were mainly pre Aplus from silver seal, some aplus ones , but I generally don't reuse them for overbores, I'm completely on the Aplus side,never had a problem of bore, piston or crank with them ( appart from a bad quality oil pump bought from a wellknown part supplier).
the best exemple I have is a 1982 mety bloc which was on a knackered gearbox ( so about 80/90 000 km ), then reused without rebore to rebuild an innocenti A engine which was gold seal rebuilt in 1989(so with Aplus pistons), after some years the guy had much money and wanted a turbo as mine, so we took it appart , used std turbo pistons whith new rings (bloc haven't been rebored) , and this engine is still making a very good engine.that's the thing that made me prefering good Aplus blocs (not meaning all Aplus are goods but that some are better than others) completely agree with the oil stuf,but this was for old ones the rebuilt ones share the same oil as the Aplus ones.
I'm actually rebuilding the gearbox from a 1380 I built 6 years ago, it doen't have any bore wear, pistons are as new( mega 73.5), the bearings can be leaved and the crank is as I put it 6 years ago.
this said I just read again and 200 bhp+.... really 1380 is a very good road engine but isn't the best for high boost,and 200 need some :)
cheers
fab

carl talbo,

the A+ blocks with the upgraded meterial spec and the extra ribbing in the cast,

there are no issues with block flexture on 73.5mm bores

as the gents state above better pistons, ring packs ....etc

to but it broadley its one hel of a large peice of steel

it will take some flexing !!

IMHO its not an issue on the A+

Edited by BENROSS on 25th Oct, 2006.

It'd be interesting to take some slices off the top of a 1380 block to see just how thin the wall is.

Is it just me or are A and S blocks considerably heavier than an A+ block?

I've thought about this myself. I never thought about it concerning S blocks, but I seem to recall that I was pussled how light an A+ was compared to A a couple of years ago.

1380 offset bored with omega pistons, centre main strap, ultra light flywheel, modified rods professionaly Built by OAP and the bores went oval

the oval was measured and the only way this could happen is if the block flexed.

you can listen to the people who have never gone 1380. and you can listen to those that have done 1380 but with limited power or exagerated claims of power

the fact is A+ blocks do flex

I'm going to have mine apart in a while and see what's happened. It's getting a bit smoky and I'm not sure it's valve guides. If it's not worn, I'll give it 12 psi and if it is, I'll probably go +40 instead. Not in that block though :)

Bugger off, I'm getting there.

Im runninga 1380 turbo, home built running 181 bananas with 15 psi, never had a issue with bore flex. Jukka has run a 1380 turbo for years so maybe worth finding out if he has had any bore flex problems!!

-MINI CLUBMAN 1380 TURBO-

you can also wear bores oval......

Alex

AlexF

T3 tone & alex F have just sumed it up!

Edited by BENROSS on 25th Oct, 2006.

as has been saidbefore IMO it's not a big issue. Maybe years ago with the old non A+ blocks?

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Take it to 74mm and it does flex. MY machinist clocked the bores next to the one being bored and they were moving.

Its not 73.5mm though and 0.25mm is quite a lot more on the bore compared to the 74mm.

I presume this is one reason why 74mm pistons ceased.

Edited by Sprocket on 25th Oct, 2006.

yep 1380 bore flex is not a issue but may be on bigger bores evry one is going over to them now and the great vizard cant be right all the time.

personally 1380 has alot going for it, more low down grunt ect and extends the life of a block just 2 of them.

vizard once said that the esprit turbo used a t3 which was the same as the metty turbo

well it did but didnt say nowt about the trim differences.

so dont believe every thing you read in the great block as its only a opion.

as td said we all have arsholes and some bits out of the book as buy products of the said asholes.

always looking for them bigger bunches of bannanas

So - I think this proves that EVERY 73.5mm bore WON'T instantly flex. There are three cases (Jukka, Tony and Richard) above where problems have not been experiened, and one from Russ where he did have a problem.

That is not conclusive enough to say (IMO) that you WILL have a problem, but equally can't guarante you WON'T have a problem.

We can be sure that as Russ's example was an OAP motor, it is unlikely to be caused by a lousy build - so the 'excuses' for what could have caused the symptoms in this example are few - and certainly suggest that the hallowed 'bore flex' is a possibility, albeit a 1-in-4 (given above data) possibility.

Might be significant is the fact that there are a lot of different varieties of A+ block, i have counted 4 different castings to be precise not including MPI. I fail to belive that these are all the same, were all made from the same original/patern or that the moulds were all made by the same bloke. There must be differences in there somewhere.

What was the one like that failed with oval bores? Did it have no badge/mark on the back? BL badge/mark on the back? 1024 on the back? large 1024 on the front?

Are allegro blocks any different?

I've seen the future and tbh its Pie

Good point andeh...

And don't forget that some overheat more than others, and this can only be down to core-shift or maybe different cores altogether....

Any chance you 1380 boys can furnish us with the information? Andeh - perhaps you can mention where these markings can be found, so folks can take a look?

Edited by turbodave16v on 25th Oct, 2006.

WFM 1024 cannot be miss described. Circa 87 onwards??

Core shift is a big deal. Have you all tested your cylinder walls for thickness before the overbore?
Chevy 400-cubic-inch blocks are generally regarded as safe for 30-thousandths overbore only. But some have been able to go to 60 or 80 on a near-perfect core.

1275 with Back Door Turbo