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MG Midget and Sprite Technical - To strap or not to strap

So the new block is cracktested and approved so now it is time to make the plans.
And it has to be more powerfull then my current 1380.

I know we had this discussion before but I am opening the snake pit again.

Should i strap my center main bearing cap?
I am leaning towards no (though i did it on my 1380)

Why you ask?

If you fit center main strap i believe you have to do it right.
Which means lineboring the main caps to ensure roundness a tricky and expensive job.

But wil it not go pear shaped without one @ 7000 rpm?

I think not and some big names ( Keith Calver) with me.
If you have a very wel balanced crank ( i plan to wedge and balance it) the stress should not be so big on the center mains.
CK even states that a non straped center is stronger!!

So any other views on it?
Onno Könemann

I am sure that there are lots of opinions, but I will go with experienced engine builders like KC and Hap Waldrop at ACME speed shop in the US, that unless you are seriously racing the motor you are better off without the center cap.



Norm
Norm Kerr

On a road going engine that will not see sustained use of 7000 RPM a 4 bolt centre main / main strap is not necessary.

However you MUST dynamically balance the crank/pulley/flywheel/clutch assembly (the A Series crank is poor in this respect). Additionally you MUST use a damper on the front pulley.
Deborah Evans

Onno, my hillclimb engine (though only 1310cc) has survived un-strapped for 30 or 40 events and 5k miles. Its in a fairly high state of tune and regularly sees 7.5k. Bottom end is unbalanced and standard apart from being converted to Metro Turbo mains which are supposed to be stronger. Having said all that I'm always waiting for it to go bang!
John Payne

7000rpm is about the max I rev to (regularly but not constant)
So that strap won't be needed.

The new engine build is to rectify some problems encountered with my 1380 build.
-could rev smoother (vibration not power)
-cracked block resulting in a water leak (temporary fixed)

so the new crank wil be wedged ad indeed dynamicly balanced with flywheel clutch and damper.

Rods and pistons wil be balanced and matched.

The only thing I am unsure about is if i wil go with pres fit or fully floating pistons.
Since a friend had his pres fit pin come out and ruin the bore!
Though I have heard the circlips can fail as well but teflon butons could do the trick.
Onno Könemann

The real question might be do I need to take out an insurance policy or not?

Is the crank damper going to be a standard crank damper, albeit a balanced one? Or will be the larger diameter Cooper S or MED?

On my road going engine I regularly used sustained 7.5K because the car had a 4.55 diff which meant 110mph. Sometimes it was the Lewes bypass and sometimes a stretch of motorway. It was at one time every weekend.

It's ok saying you won't rev it past 7K and someone else saying they've used 7.5K without problems.

BUT WHAT IF?

You accidentally rev it to more than 7K or more than 7.5K?

Once by accident I revved my roading going Sprite to 8.2K! That's 8,200 rpm on a Stack tacho with a tell tale (that I played back several times because I couldn't beleive my eyes) and was 100% accurate.

The engine survived and had several more years thrashing after that but never went about 7.5K or maybe 7.6K or 7.7K that one time.

Was it more likely that a piston might have failed than the centre main at 8.2K? I don't know.

I like insurance against a big risk. It's also the case that not all centre main strapping is done the same way. On my old engine there is still some metal each side of the strap but I've seen some done differently.

The other question is whether or not it's better to forget about strapping the centre main and fit a 4 bolt main instead. Does KC say a regular 2 bolt main is better than a 4 bolt main?
Daniel Thirteen-Twelve

KC mentiones the 4 bolt as being posibly a weaker solution since there is an other hole drilled in the web.

found the article
http://www.calverst.com/articles/Engine - Main cap issues.htm

The damper is going to be a std one but may have a trigger wheel fitted to keep an ECU route posible.

Sure mis shifts can happen but they are incidents and only for short periods.
Onno Könemann

Onno my view is very simple

Whatever helps is a good thing and a centre strap ill not do harm and could be good.

The same as balancing not totally necessary (re JP) but does not do any harm. If it is a big deal financially then forget it but if it comes at a small cost then why not?
Robert (Bob) Midget Turbo

You can get a MED damper with a trigger wheel.

With a 4 bolt main you have to consider not so much there is another pair of holes drilled in the block but rather than you are putting metal into those holes - bolts or studs. The 4 bolt main is also wider than a 2 bolt main so spreads the load better across the centre web of the block.
Daniel Thirteen-Twelve

hi Onno

Good idea balancing but be careful about wedging, do this wrong and you can set up vibration frequency problems. The factory discovered the hard way what worked and didnt work. I know this has caused quite a few cranks to fail in race B engines.

You already know my opinion on centre strapping from a previous thread covering the subject where I brought up Keith Calvers opinion. We strap race engines only. I think I would always put my pennies into high quality pistons with A series engine builds.

Peter
P Burgess

Peter

I recon a reputable UK midget engine builder can do a good wedging job.

I agree about the pistons but am stil indoubt if i should go full monty an get the forged omega's (460,- at MED) or just go for the cast ones (295,-)
problem is i can not find the weight for the cast ones.
Med states the forged ones as 288gr (shal be a +020 one) but states no weight for the cast ones.....

Daniel
Trigger wheels has an a-series kit for something like 40 quid.
I'd rather spend that 100 pound difference on pistons
Onno Könemann

Hi Onno

Have you looked at the Mini Spares Mega range of pistons, maybe not as light as Omega but same ring pack for friction and have served us well in Race A series and all our road A series engines for at least 9 years....I cannot remember when they started making them but that is when we started using them. i think they are around the £200 mark but don't take that as Gospel. They are similar to the Powermax pistons of yesteryear.

Will you reduce the splash oil feed to the cam followers if you wedge the crank? I know the lobe drilled cams would ease this.

I wonder how many centre bearing caps fail? I was assembling a Race A series rebuild yesterday and was admiring the smooth radius transition on the caps around the bolts on the front and rear caps. The engine was strapped when I first built it 3 years ago.

I also note King Bearings now manufacture race quality bearings for the 1275s. For years I have had to use standard materials, since Vandervell ones disappeared. Tried to order 10s in the usual material only to be told out of stock...oh dear I say, have engine to build...no worries they say, we have race heavy duty ones :)

Peter
P Burgess

Peter, aren't the race bearings tri-metal with a softer top layer to embed debriss as to prevent scoring of the journals? And that can be used on a race engine because they will be changed ever so often?
i.e. not really compatible with a road going engine wich should make more miles than revs?
Alex G Matla

Onno
If you go the floating piston pins and are worried about circlip failure why not use spiral locks. We've been using them for years in some very high revving engines and never had a failure On our big HP v8's we use 2 on each end ( 4 per piston ) just depends on how wide your circlip groove is though One on each end would be more than OK for what you're doing

There are several manufacturers of them especially in the USA

---A sample-------------------Roll down through this they are after the circlips---

http://www.embeeperformance.com/pdf/CP_Accessories.pdf

Willy
William Revit

Your local motorbike workshop would probably have them

Willy
William Revit

Onno

I run forged Omega pistons, centre main strap and steel crank and rods.

I have interference fit gudgeon pins (wrist pins) but intend to go fully floating at the next rebuild just to prevent that very problem of moving pins...

That said it is a race engine with the rev limiter set to 8200rpm and I hit the limit not infrequently.

On a road engine with a 7000rpm limit - even one which has been built for power - I do not think you will need a centre strap or forged pistons. Whether you decide to go for them for that added sense of security is up to you...

Regards

JB
James Bilsland

hi Alex

As far as I am aware the bearings are better in all situations just cost about twice the amount of the more basic ones. All bearings have a soft layer to 'catch' a little debris. The posh ones tend to have a copper middle layer. EG LC....lead copper as opposed to SA which are steel backed which I presume means it has the soft bearing material straight onto the steel.

Peter
P Burgess

Onno,

Welcome to the Hole...I think your in the same place I was several years ago. When my crankshaft broke in the number 4 main...I HAD to find the strongest crank and eliminate any and all possiailits that would cause the crank to bust ever agian...at the time I didnt even know i was doing this until trevor made note of it to me.

I think with your block cracking in your 1380 has placed you in the same state of mind, finding all possiable ways to never see this agian...Im just guessing at this... still watch for it.

I wish I could offer some pro or con advice as to my own experiance... All I can offer is do what you feel is nessacary regardless what others say or you may never trust your engine... and in the end thats huge when you give some serious thought to it

In the end, I dont really know if there is a true right or wrong, everyone has an opinion, KC dosnt like straping but vizard does. in the end its a polling question of people with alot of experiance your really asking

Prop

Well Prop appears to have cured his crank breaking problem that is for sure!
Robert (Bob) Midget Turbo

Prop
My block cracking has nothing to do with the fact that it is an 1380!

It has given me the chance to get a "do over" without the added stress of the midget not running.

The 1380 works fine with the addition of a botle of bars leaks every 5/6 months and a bit more oil adding than I want.
But it still stands up to a regulair thrashing ;)

So i have found a new block (by accident) and have the luxury to be able to build it on the bench.
And i would be an enormous stupid ass if i did not use the experiance from my first build.

So experiances
-Crack test
-do not trust original (or old) balancing
-don't go to an extreme overbore if it is not needed
-a general spend your money wisely (may have something to do with the recent purchase of my D midget)
Onno Könemann

well the crank isnt broken YET... But not for a lack of trying...LOL

I feel for ya Onno... that just sucks, when I figured out I was going to have to pull my engne apart last summer... that was a depressing time, I was looking for septic tanks to clean so I wouldnt have to deal with it... it really knocked the wind out of my sails

Im glad your alot more motivated ... thats 8/10ths the struggle

But what great luck that the bar leaks is working for you... that stuff works good when it works, I know a guy thats been using it for about 5 years with a cracked head in his ford ranger truck
Prop

This thread was discussed between 12/01/2011 and 15/01/2011

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