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300Tdi broke con rod - why?


JimAttrill
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Hi all, my boss's 300Tdi broke a con rod the other day. On taking it apart it is hard to tell if the rod broke just above the bolts or whether a bolt broke. Another rod had a crack in the same place above the bolts but that could be caused by the stress of the engine stopping suddenly.

His engine was overhauled by a decent workshop about 5 or 6 years ago.

When I undid my big-ends on my engine I changed the con rod nuts (as per the manual) but the manual says nothing about the bolts.

Should the nuts and the bolts be changed?

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Jim,

I would be replacing both the bolts as well as the nuts, same as head bolts the savings by re-using the old bigend bolts and main bearing bolts, flywheel bolts etc. simply isnt justified by the cost of them against the cost of destroying an engine if one fails at high revolutions.

I used to have a worthy contact at Dehaviland who used to Xray all my engine bolts for me for free but like me he grew old and retired so now if I want to go into that realm of engineering I have to pay for it but all aircraft bolts are inspected in this way, you really dont want a big end failure at 6000 feet :blush:

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I am not keen to replace bolts nowadays, even with new OEM or LR parts. As an example, we found that the rear main oil seals (supposed to be Dowty) from Bearmach were faulty, the holes were not in the right places. So we bought (at great expense) a 'Genuine LR part'. It had exactly the same fault and had obviously been made in the same factory, most likely somwhere near Bangalore :blink:

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Well his engine had about 150k kms and mine has 311k kms and counting....

Difficult to know,all components have a life,my 300TDI is on 193,000m and goes VERY well,uses no oil/water etc.My RRC with 200TDI engine which I bought back for its engine,(To go in my SII) has 232,000m on it,has always used a little drop of oil but no water etc.Both could blow up tomorrow,both have worked very hard for me,who knows what would be the first part to fail and when ?

All I can say is that for all the engines and vehicles I have been paid to look after/repair/rebuild for the last 30 years the 200 and 300 have been amongst the best,along with David Brown, Lister and Kubota diesels.Vauxhall/GM and Stihl petrol enginesThere have been many other makes on many different types of vehicles and machines,not many have proved the long term worth of the good ones I have listed.

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Jim, The problem with most torqued bolts is that they are designed to s-t-r-e-t-c-h and in so doing are under constant restrain, in some instances, something has to give and this can be with catastrophic results. The second and subsequent "stretch" weakens the bolt even more and increases the chance of possible failure. I wouldnt risk it.

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Jim, The problem with most torqued bolts is that they are designed to s-t-r-e-t-c-h and in so doing are under constant restrain, in some instances, something has to give and this can be with catastrophic results. The second and subsequent "stretch" weakens the bolt even more and increases the chance of possible failure. I wouldnt risk it.

These aren't officially or technically 'stretch' bolts, rather like the 300tdi head bolts that the manual says can be used 5 times (including or excluding the first assembly?)

TD5 head bolts are waisted stretch bolts and you can feel they have hit their 'elastic limit' at the last few degrees of turn. That is why you mustn't reuse TD5 head bolts, even though they cost a lot of money. Anyone who has done destructive testing of steel will know all about elastic limits. I did it in training (ahem) all of 46 years ago now!

Stretch bolts or not, they will stretch slightly and I will replace them with OEM bolts when I 'do' my engine. I have had all the parts for about 4 years now - I used to take parts in lieu of wages to avoid the income tax etc :blink: I never bought con-rod bolts (or head bolts) at the time but will buy the con-rod bolts. The head bolts I will reuse as per the book.

I still think a bolt broke rather than the rod itself. I have seen a few bent rods caused by hydraulicking (how do you spell that word :wacko: ) but never a broken rod.

I also agree that the tdi engines are very good motors so long as you don't rev them too much (as Disco drivers tend to do). I cruise my Defender at 60-65 mph (for you imperial types) and no faster.

IIRC we used to measure the length of con-rod bolts for the venerable Gypsy Major engines - over a certain length and they were replaced.

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From my experience conrods "go" when off power, seldom do they have a moment when under power, rather the instant power is taken off (during a manual gear change with high revs) the suden change from compression to no compression is the time they will decide to remove the starter motor off the side of the block resulting in the driver informing the chief steward that he had to pull out due to an electrical failure :hysterical:

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These aren't officially or technically 'stretch' bolts, rather like the 300tdi head bolts that the manual says can be used 5 times

The fact that they can only be used 5 times indicates that they weaken each time - otherwise they would have an infinite re-use life.

It stands to reason that the engine will have been designed to have a reasonable life with those bolts, but each time you re-use them, you will shorten its residual life. I would bet that 5 re-uses gives the engine just enough residual life to be outside the warranty on the repair!

I would put it down to experience - and replace all the bolts!

Si

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Maybe X-eng could make some decent con-rod bolts? - I don't really trust LR parts much any more.

Re the 5 times re-usable - maybe they asked the tea lady for a number between 1 and 10 :hysterical:

She is the same one who 'designed' the TD5 chain-driven oil pump (unique?)

Brian and I have a theory that the engineers designed a bit of the TD5 engine each. But the project leader forgot to get someone to design an oil pump. When they started it on the test bed it seized as there was no pump.

So they got the tea-lady to design one as an add-on :o

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