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MG Midget and Sprite Technical - Con rod bolts and engine stuff

I am getting completed on my engine teardown .. Just have few details to get sorted to put it all back together..

Read somewhere that con rod bolts must be replaced on this engine ( 1500) as they are toqued to yield ( on plastic Limit)

If this is true - where to get some good news ones - as these are fairly key to stopping rods exit thorugh the side of the block later I have heard..

I have found that some of the tappets are pitted ( looks like casting faults almost also one cam peak is pitted.. Probably need renewing..

John Barber

Well that may be what's advised, if it is, I must have missed it.

I rebuilt a girlfriends Tiumph 1500TC saloon, (I think that's essentially the same engine), without replacing the rod bolts. I (and she) thrashed the crap out of that engine for over 6 years, and it didn't blow up.

I can't find reference in Haynes to replacing the bolts, but I suppose they could have omitted that in error.

Where did you read it?
Lawrence Slater

When I had my 1500 I used to replace big end shells every 25,000 as a routine service item. I don't recall ever fitting new bolts though! If I were building a race engine, then I probably would, although that road car had a "fast road" cam (whatever that was) and a skimmed and ported head. And I used to drive it.
Guy

25000? Wow, were they wearing that fast? You must have been brutal to that car Guy. :)
Lawrence Slater

It was purely a precautionary measure for the long stroke 1500 engine! I would much rather replace a set of shells in situ at £12, than delay and end up having to do an engine out strip down to get the crank reground. The shells were usually marked, but never worn that much. I think I did them 3 times in all and the journals were still perfect.
Guy

John,

If what you say is true then this is very interesting from an engineers point of view.

Can you tell me what the diameter of the bolts are, and what the reccomended torque is - I can hardly believe the torque would be high enough to yield the steel.

If you want to replace them - then look on the MGOC spares site... they must spec them.

Christian
C L Carter

Hi Christian,

According to my Haynes, there are 2 different types of rod bolt.

Colour dyed, 50lbft.
Phosphated, 46lbft.

Those processes, colour and phosphate treated, probably mean more to you than me. You could enlighten me if you felt like it. :)

But of course haynes might be wrong if it says something else somewhere else.
Lawrence Slater

I'm actually not sure Lawrence. Its possible Colour Dyed means tempering (reference to the colour of the steel during the process) - which is the treatment I would expect to see.
Phosphated presumbably referes to the inclusion of phosphate into the steel? But I would assume this would increase the hardness, not the ductility of the steel.

I have heard of torque to yield fastners, but this is in much more modern engines and I've only seen it on heads - never big ends.

Unless they are tiny thin bolts - I doubt you'll even think about hitting yield stresses at those torques.

C L Carter

Is torque to yield the same terminology as stretch bolts?
Guy

Yup. The bolts are torqued so that they stretch beyond their elastic limit and as such they permanently stretch. But as said, I've only ever head of head bolts being that type.
S Overy

Hmm,
I suspect that strictly speaking you would set them by torquing to a specified figure, and then tighten by a further specified angle. Once they start to stretch the normal torque figure will likely behave weirdly anyway.
I did big ends on a TVR and I am pretty sure that was the method.
Guy

I must confess I am sceptacle about it being true - but its on this Triumph engine site see link below..

http://www.totallytriumph.net/spitfire/engine_building.shtml#Places

Could be its wrong ?
John Barber

I think he's talking about building the engine to the limit of what you can get from it. For standard use, I reckon re-use the bolts, no problem.
Lawrence Slater

I have no experience with Triumph engines, but I would always use new rod bolts on an engine build on the basis that if they break it tends to be somewhat catastrophic...
James Bilsland

Yeah but you race James, and must put the engine to many factors of strain, that a road engine never gets.
Lawrence Slater

Bit off topic here John but,

Presumably you have your engine mounting on a stand of some sort..?

What bolts do you use to attach it to the stand - do you use the ones which hold the bellhousing of the gearbox to engine block - or do you need longer ones - or different ones?

C L Carter

No actually I haven't got a stand just a rather robust steel desk .. So I can't say from experience. I would suggest the widest pattern of bolting you can achieve the bell housing bolt holes should be okay I would think .. Someone else may have direct experience..
John Barber

This thread was discussed between 04/03/2012 and 07/03/2012

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