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MG Midget and Sprite Technical - 74.7mm bore with Triumph pistons

Having pulled my AH Sprite out of storage last year for the 50th celebration, I'm enjoying a bit of an A-Series renaissance.

Dusting off my Vizard I can't help but return to the 'over bore' chart and think of the bragging rights that an offset ground Marina crank and a set of 74.7mm pistons, giving ~1475cc, would give me.

My aim would be a torquey fast road motor running on a flowed HIF6, 285 deg cam, flowed metro head with 1.4" intakes, and thick flange midget block. Car is regularly used for short trips up to 30 miles, but is not a dailey driver.

So question to the Forum, does anyone have experince of the 74.7mm bore using standard Triumph pistons please. I searched '74.7' in the archives but didn't manage to find thread on the subject.

J E G Eastwood 1

you are scaring me already
S.A. Jones

You might be better going for a stroked engine to get the cc's; 1430 is fairly easy (ask Dave B!)

Toby Anscombe

You'll certainly know all about secondary vibration with 74.7s and a long stroke... will vibrate the std starter-motor and dynamo to bits...

A 1400K series would be less trouble if you don't mind a little more extra work up front.

A
Anthony Cutler

and here i am worrying about boring to 73,5mm :P
Onno Könemann

With 74 mm pistons and stroking using a marina crank (Reducing to 1 5/8 bigends) you end up up with 1460, which is plenty IMO plus 74mm pistons are available in Omega so how much better is that?
Robert (Bob) Midget Turbo

Anthony - I would suggest lightening the rods, lightening the pins with an internal tapered bore, and possibly lightening the pistons (I have a friend at Federal Mogul who can advise) to combat, if not erradicate, the increased 2nd order vibrations. What experince have you had with the longer strokes damaging components?

Rob - it's also a question of cost, a set of standard Triumph pistons are going to cost ~£130, where as I imagine the 74mm Omegas are going to be nearer £250+.

I made some calculations as follows; Minimum 'acceptable' intra bore width is a 1380/73.5mm at 2.836mm (between cylinders 2 and 3). A 74.7mm bore offset as per Vizard 15 thou, has a cyl 2-3 intra bore width of 2.398mm. If this is deemed as too little, a compromise of an 2.6mm intra bore (on all cyls) can be created by offsetting cyl 2 and 3 by 0.5mm, and 1 and 4 by 0.7mm.

However it is not clear from the various 74.7mm bore write ups, whether the problem lies in a minimal intra bore causing flex and blow by or by thrust side bore flex, or both. Vizard also recommends offsetting all cylinders 10 thou away from the thrust side, was this possibly to maintain thrust side bore structure?

I'm still looking for some one who has actually built a 74.7mm engine, come on there must be some one out there!

James
J E G Eastwood 1

Toby,

I you mean me, I'm not the only Dave B that posts on the board, I used a big mains 1098 crank to take the 1380 bore to 1420. Mainly because I had a fully prepped ie wedged, balanced, and tuftrided 1098 crank which it seemed a shame to waste. Not given me any troubles in about 65k miles, the only thing done to fit it was the thrust bearings needed thinning by 0.005" or 0.010" each.
David Billington

I,ve a 1480 ready to put back together for my mini that was built by Dave Mountune a good few years ago.

Howie.
Howard Wright

I am listening because I've just bought a spare 1275 with Marina crank and I am in the mood for something....

Bas Timmermans

Hi James

Agreed that lightening of components at the 'small' end of the rod would reduce vibrations (and stresses generally).

Experience on a friend's 1420. E.g. the bolts holding the end-plates would come loose on a regular basis, possibly caused by engine being cruised at or near period.

Fixed by hi-torque starter and alt conversion - both good conversions in their own right.

If the idea of long stroke is for more torque, why not consider low-pressure turbo (5-7 psi). Does ''turbo' not get you nearer the bar in terms of bragging rights? Seems like less effort, anyhow.

A
Anthony Cutler

David - thnaks it's so nice to get some fresh input that we haven't read in Vizard. V interesting.

Howard - is you 1480 on a 74.7 bore?? If so what are experinces? Which make of piston does it run?

Anthony - cheers for head's up on starter/alternator. One presumes that if you could get the rod/stroke ratio back to standard, and the weight on par, then the 2nd order would be as standard? However getting the rod/stroke ratio back would be diffecult, as my calcs indicate the rod would need to be lengthend by about stroke increase * 1.6 = 5mm?
J E G Eastwood 1

(No, it's not Andrew (the 1480 was built just before he was born) it's Paul - but the registration page won't recognise the difference/let another name on the same email or some other gremlin.)

I built up a 1480cc A series for a friend in the early '80's using 75mm pistons (Toyota Corolla) and a 1098 crank. It was built as a torquey autotest engine, so the cam was very civilised (948), compression about 10.5:1 and the usual tricks with the head.

It performed well, vibration wasn't a problem, neither was the one-off copper sandwich head gasket ... though in those days asbestos fillers were still available. (With my B engine, when that filler went NLA it was a mission to find a substitute that worked)

To achieve a workable thickness between the bores (2.0mm +/-0.05)we offset no 1 fwd 1.5mm, no 2 fwd 0.7mm, 3 & 4 backwards by the same amounts.

This of course broke through the oilways at the rear of the block, addressed by drilling (before boring) and sleeving the pump to relief valve oilway, while for the pump to filter oilway the original was plugged off a short distance in from each end, a new one drilled parallel to it, then cross drilled to link up with the original at the ends. As the breakthrough was below ring contact, we just left that there rather than try & fully plug & machine.

We had ultasonic tested the wall thicknesses front & rear before starting & established that all was Ok apart from one each of the water pump and timing cover bolt holes. These we welded & redrilled prior to boring.

It all worked a treat, lovely engine to drive. Sadly the car went to a panelshop for repair and never surfaced from it - panelbeater took too long, stored the car outside & all got ruined. Someone bought it ostensibly for rebuild but in fact to break it up.

Autotest evolution 2 was going to be a supercharged engine (see Anthony's suggestion above!) but then the K series came along, and though always having scorend engine swaps the equation of more power, more reliability, less weight, less cost was pretty hard to argue with. The clincher was to use an MGF engine, at least that kept it in the family. And thus the Widget, which we have brought to the Uk a couple of times now, was born. And soon, brother of Widget

Andrew Walbran

Paul, many thanks, at last some one who has built one. Sounds like a great project, 75mm v impressive, many thanks for the detailed responce. I was once told you couldn't fit a 1098 Mini crank to a Mini 1275 block, and had always assumed this applied to the Midget engines, but it clearly doesn't, and as a bonus I have a 1098 Midget engine sitting idle.

Which block did you use?
Did you get any unnacceptable blow-by issues?
Did you ARC or MIG weld?
Did you make your own copper gasket or have one made?
I'm afraid you lost me on the filler comment can you expand on that?
Once you knew your wall thickness's from the ultra sonic measurements, what thickness did you deem acceptable to have remaining once the boring was complete.

Can you share email address. Mine should have come thru with this post.

Many thanks again, James E
J E G Eastwood 1

James it needs to be a crank from a 10CC prefix engine - they have larger bearings than the earlier ones (10CG etc)
David Smith

(Still the old fella ...)

James

It was a 1275 Marina block, didn't wan't to bastardise a good Midget 1275 on an experiment in case it didn't work (though having taken this precaution, of course it was OK)

Yes, you can fit the 2" MAIN 10cc crank (NOT 10GC which is 1.75" mains) to the 1275 block ... mains are the same size & spacing, but the crank web as a differnt thickness so the thrusts needed to be ground a few thou thinner. (Sorry, 25+ years ago can't remember the precise details, but it was easy enough if you had a magnetic table and a surface grinder.)

Offset ground Marina crank to 1.625" bigend will gain a bit more stroke (about .015"). It is a good way to go too, but note:
1. Marina crank is inferior steel, so nitride it (then it works well)
2. Marina crank is not crossdrilled, so do that too. Failure to do both these will result in run bearings, been there, done that! Once both are done, it works fine.
3. Marina flyweel mount is different from 1275 Midget, so adaption needed there too. (On the other hand,so is the 1098 flywheel, even more so. As I recall we used a modified 1098 flyweel with this project, with a 1275 clutch grafted on. Clutch worked well despite the recess in the centre of the flywheel being slightly larger than the ID if the 1275 clutch linings .. this was one of this "oh @#**!!" moments when we thought this was the simplest way out, couldn't do much harm other than lose drive .. and it worked!)


Blow-by - not significant

Welding - sorry a bit of a black box matter this ... there was a brilliant specialist welding company here at the time, anything tricky point it at them. All very secretive but they always had impeccable results, so I left this to them. Subsequent to their retirement (with their secrets) we have subsequently arc welded blocks with apopriate rods with good results. The rod spec is the key, check this out first.

We had the copper/filler/copper gasket made by a gasket make who specialised in one-offs. (One of the advantages being a small country and this far removed from the seat of most action is that we have adapted to low volume /one-off production items very well, otherwise many companies would survive. hence all sorts of creativen specialist engineering services available).

Filler comment - this was in relation to the material in the middle of the traditional copper/filler/copper gasket (ie copper sheet each side of some other material, not solid copper as perhaps it may have seemed to you from my earlier post). Asbestos based products available at the time had far better heat resistance charactistics than their later substitutes, it turned out. on the other hand they had far better cancer-inducing properties too ...

Wall thickness - basically, we measured the end walls (no 1 towards front, no 4 towards rear) and them shuffled bore centres around to get them all as even as possible. We had no idea whether it would work or not, more that we had a target capacity (nearly 1500cc 'cos we'd heard others had done it) and tried to make that fit & see if it would work. It did!!!

My email is paulatmgpartsdotcodotnz, you are welcome to contact direct if you like. Only problem is that it was nearly 30 years ago I built the engine, kept many notes at the time but the rest is dependent on a dodgy memory!

Andrew Walbran

Andrew, David, Anthony and other contributers - many thanks for input, I've saved the whole thread as a Word doc to make sure I don't loose your input.

I have a 1330 midget block from my old sprinting standard engine, which was scored when I broke the crank at Goodwood last year, and apart from an overbore useage is scrap. I also have the old 1098 from my car, and as it was the last year of 1098 manufacture (1966) I am hoping it is the 2" main version.

Next key step is to get the ultrasonic testing done, and determine how much boring and offsetting can be accomodated. The project will be a slow burner, as I now have a 1380 mill, but I will check back in when it progresses.

Thanks again. James Eastwood
J E G Eastwood 1

The major problems with going to 74.7mm bore are (as has been pointed out):

1. Liner thickness (or thinness!) leading to flex and excessive ring 'blow-by'. Some blocks will take this bore, some won't. You'd need to use a good oil control ring and possibly the second compression ring as well. This means rings as fitted to Omega pistons.

2. Breaking into oilways. Specifically that of the oil pressure release valve. Not such a problem since these lie below the piston stroke so can be sleeved.


3. The major problem is with the TR6/GT6 piston. IIRC these are of the slotted skirt design (beneath the oil control ring) and are thus not ideal. Furthermore the ring design is poor for an application requiring good oil control. Hepolites are no longer available and all the aftermarket cast pistons (such as 'County') are not, IMO, up to the job. This means you are looking at a set of forged 'JE' pistons or 'Venolia' pistons from the USA which will set you back around £750 - £800.

4. Another problem with the Triumph piston is the relatively low deck height to the first ring land, which weakens the crown.

5. When fitted to A series rods (not a problem since the TR piston has the same gudgeon pin size as the A Series), the fact that the TR piston has a relatively short deck to gudgeon distance means that, even on a stock 81mm stroke crank the piston at TDC will be some way below the block deck so the block will need to be machined to suit.

6. TR pistons are flat face so, in order to get an acceptable Static CR, you will need some serious work to the cylinder head.
Deborah Evans

Deborah,

Many thanks more great input.

1) I'm hoping the ultrasonic block testing will tell me how to optimise the bore and offset, in fact the plan would be to have this measured first before settling on a exact bore size. I am also planning to offset 10thou away from the thrust side to maximise the robustnbess of the thrust side against the thrust load, to minimise thrust related distortion and blow by.

3) This is very useful input, I'm beginning to investigate current production pistons. I had a look at our ford engines, but the Sigma is available in a 71.9mm and a 76mm bore neither of whichis suitable, (which is a shame as I would have got a very favourable employee discount). The K-Series is availabe in a 75mm bore, but a think the crown has a 4 valve type 'shape'. But I believe the modern piston options would be much lighter, less expensive and have more technical ring pack solutions. Most also come with a teflon coating and high-spot design to minimise friction.

5) Current assumption is to use 1098 83.7mm crank, once I've assessed the possible bore size, I will have to come up with a piston solution with fits the deck height, bore, dish and cost compromise.

6) Whilst we discuss the diffeculties of getting the Triumph piston to the top of the deck, we also discuss getting sufficient combustion volume to enable a healthy CR. The obvious question is why not leave the crown slightly below the deck to adjust the CR. Well (before every one shouts at me) that would leave you with an ineffective squish area, a corresponding loss of trubulance and hence the combustion proccess would not be so efficient, and you would need to run a bit more advance, but just how much that would affect the Wide Open Throttle (WOT) pereforance I realy don't know. But I would say it should be avoided if possible, the combustion process is v important.

James
J E G Eastwood 1

I'd agree that leaving the pistons down the bores would cause all sorts of combustion problems. I'd bet it would cause emissions problems as well. You might be able to cure this with a fully mapped 3D ignition system (eg EDIS + Megajolt), but I'd rather go down the route of decking the block to get the correct piston height and doing sufficient head work. Obviously the Dynamic CR will depend upon the cam duration and LCA.

Certainly this is what we have recently done with a short stroke big bore 1000cc A Series race motor running flat top pistons.

Having said that, decking the block will alter your rocker geometry which will need to be examined. I've just done a Midget 1500 for a client and in order to get a 60 thou overbore I had to deck the block by 30 thou to get rid of the bore recesses (so I could get the bore size I wanted and use the earlier head gasket). In order to get a static CR of 10.5 : 1 the head was machined by 55 thou. As a result we were into shorter push rods as well as some shimming of the rocker pedestals to get the geometry back.



With regards to the 75mm K Series pistons. These have a domed top with 4 valve cut outs. You could mill them to be flat topped but I would be wary of this since doing so would put the first ring land perilously close to the crown which would weaken it.


Another option would be an early Ford piston. Many moons ago when I was racing Spitfires we used Ford Fiesta pistons (in those days you couldn't get forged Triumph pistons at any reasonable cost and the stock cast ones were not up to racing). I can't remember whether the Ford pistons we used were standard or oversize (although gut feeling leans towards the latter), I do remember that it meant we ran an engine with a 45thou overbore (we had to stay within a 60 thou overbore owing to the race regs). Bearing in mind that the Spitfire 1300 nominal bore size was 2.9" (same as the Midget 1500), then a 45 thou oversize would be 74.8mm so this may be worth looking into.
Deborah Evans

When checking the valve geometry what specifically should be checked.

I've just built a 1380 A series. The block was decked 25 thou to match the pistons and head skimmed. Its now fully assembled and there doesn't appear to be any issue with the geometry. Its got 1.5 roller rockers and A+ push rods.

The pushrods arent fouling their holes or the rockers etc during all movement. Are there some specific checksI should be doing ?



Dean Smith ('73 RWA)

if i recal correctly the center of the rochershaft should line up horizontaly with the tip of the valve
Onno Könemann

Dean,

You need to check that the rocker tips are wiping across the centre of the valve stems. You can do this by setting up the rocker clearances and then smearing some engineer's blue across the valve stem. Turn the motor over and remove the rocker assembly.

If the 'witness' marks in the blue are at the centre of the valve stem then you have the correct rocker offset.

If the marks are towards the manifold side then the push rods need to be shortened (and possibly the pedestals), otherwise you get excessive thrust loads on the valve guide which will cause premature failure.

If the marks are towards the rocker side there is less of a problem but shimming the pedestals may help.

The problem with overly short push rods is that, in order to get the correct valve clearances the adjuster must be wound out further. This will decrease the effective rocker ratio (because the adjustment is on the push rod side of the rocker), and hence, valve lift.

Overall its a case of marrying up push rod length and pedestal shimming until you get the correct geometry and rocker ratio.
Deborah Evans

This thread was discussed between 18/03/2009 and 03/04/2009

MG Midget and Sprite Technical index

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