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[The Sheriff]

.....and, no doubt, I will be mocked. But I can take it, so.......

Just been watching an old Wheeler Dealers, the DB7 one. Exhaust manifolds are cracked and, as this happens with so many cars, my question is: Why are they made of cast iron? Why not steel or alloy or anything that won't crack?

[RobG]

Longevity and cheap to produce

[The Sheriff]

Longevity and cheap to produce

Hmmm......longevity.....they keep fick breaking 

[RobG]

Longevity and cheap to produce

Hmmm......longevity.....they keep fick breaking 

Taking into account the numerous heat cycles that they endure they last quite well.

[The Sheriff]

Longevity and cheap to produce

Hmmm......longevity.....they keep fick breaking 

Taking into account the numerous heat cycles that they endure they last quite well.

Yeah....well....I could have had a look at 1aauto.com I suppose.

[cem_devecioglu]

there are alternative steel mixtures like 310 which has extreme strength but prices are definitely incomparable..  and its very hard to machine it..

[Vamps]

.....and, no doubt, I will be mocked. But I can take it, so.......

Just been watching an old Wheeler Dealers, the DB7 one. Exhaust manifolds are cracked and, as this happens with so many cars, my question is: Why are they made of cast iron? Why not steel or alloy or anything that won't crack?

See, no one mocked you Steve............

[Broomies Mate]

Mock Mock Mock Mock Mock. 


It's a cast part, which means it's cheap to reproduce in large numbers.  It is subject to extreme temperatures from an automotive POV.  Heat cycles mean it has to be durable.  Unless you use an extremely expensive material, which would mean using an extremely expensive manufacturing procedure, you wont beat cast iron.

[Broomies Mate]

I hear Daewoo use Vac-Formed Plastic for their exhaust manifolds? 

[Lizzie_Zoom]

Not a daft question indeed!

I have always doubted the worth of using cast iron in machines, but it appears it is great for engine parts, be it on the internal combustion engines (once commonly with the main cylinder block) and on steam engines.  My suspicion comes from railway history when most early bridges were made of cast iron.  As engines became heavier they found the cast iron would become brittle, cack, and under bridges often gave way as trains crossed them. After some engines disappeared into black holes where bridges had been a few seconds before, all load bearing bridges were replaced with steel ones once that technology became available.

So cast iron parts for our cars seem to be not ideal, but as others have stated, is a cheap second best solution.

Remember those cast iron cylinder blocks of yesterday cracking all so readily? Thank God for the alloy alternative of today! 

[Andy B]

..... they found the cast iron would become brittle, ......

Cast iron IS brittle ....... it doesn't become brittle 

[Broomies Mate]

Remember those cast iron cylinder blocks of yesterday cracking all so readily? Thank God for the alloy alternative of today! 

I'm not sure the Alloy alternative is used for longevity Lizzie.

I think the Alloy alternative is used as a weight saving feature to provide a more 50/50 weight distribution.  'All Alloy' engines have been proven to suffer problems earlier in their life than the Iron blocks they replaced.

Manufacturing practices have come a long way and will continue to progress, though. 

[Lizzie_Zoom]

Remember those cast iron cylinder blocks of yesterday cracking all so readily? Thank God for the alloy alternative of today! 

I'm not sure the Alloy alternative is used for longevity Lizzie.

I think the Alloy alternative is used as a weight saving feature to provide a more 50/50 weight distribution. 'All Alloy' engines have been proven to suffer problems earlier in their life than the Iron blocks they replaced.

Manufacturing practices have come a long way and will continue to progress, though. 

That is interesting BM.

I remember when cracked cylinder blocks were a regular feature in ending a car's life when under 80-100K miles in the 50s/60/70s.  Yet look at our Omega's, and so many more cars that get to 200K+ without that ending.  In fact, for me at least, I hear little about cracked cylinder blocks, or indeed other old flaws (like broken rockers, push rods, crankshafts, big ends, etc) that used to end a cars life on many a lonely road.  I personally drove a 1955 Humber Hawke in 1970 that ended it's life on the outskirts of Bromley after a rocker broke and did other damage.

[Nick W]

Lizzie, may I recommend a book called Structures or why things don't fall down by JE Gordon? This clearly explains why cast iron was an unsuitable material to build bridges with, among many other things. A nice clear explanation of tensile strength and the difference between stress and strain are well worth the cost.

[The Sheriff]

Lizzie, may I recommend a book called Structures or why things don't fall down by JE Gordon? This clearly explains why cast iron was an unsuitable material to build bridges with, among many other things. A nice clear explanation of tensile strength and the difference between stress and strain are well worth the cost.

She'll be all over that, Nick