Steel Main cap material

[Exminiman]

Hi

Been lurking for a while, this is first real post, so please be gentle

I am building an Inline A series engine (1275) and for one reason and another will need to get some steel main caps made.

Would be interested hear your thoughts as to which steel type, initially I thought E19 because of its machinability and strength, but I am also thinking that the expansion properties of the steel in relation to the cast iron block may also be a factor?

Cheers

Marc

[Geoff2]

Steel & cast iron have virtually the same expansion rate, relative #s are 11.5 v 12. Only when you get to stainless steel do you get a major change, about 16.

[Exminiman]

Thanks for the reply Geoff, any opinions on using EN16 steel ?

[frnkeore]

I'm not familiar with UK standards for steel so, I don't know what EN16 is but, by US standards, just 1018 mild steel will be fine and stronger than the block.

[Exminiman]

I am wondering if EN19 may be over speced, believe its equivelant of US Spec 4140
https://www.smithmetal.com/pdf/engineering/en19.pdf

[Dan Timberlake]

What do the stock caps do wrong?

If breakage is a problem, do you have any pictures of typical broken caps ?

'Most all steel will have the same stiffness. The important material property is E, or modulus of elasticity.

E of all steels are ~ 30,000,000 psi.
E of Cast iron varies considerably. From 10 - 20,0000,000 psi
Even ductile iron is about 24,000,000 psi.

Early on, whether the goal is greater stiffness or longer fatigue life the geometry of the cap is likely to be more important than the material.
I'm guessing Nice and thick in the vertical direction, and a nice shape with gentle radiuses is going to be hard to beat.
https://static.summitracing.com/global/ ... ?rep=False

[Ericnova]

I am wondering if EN19 may be over speced, believe its equivelant of US Spec 4140
https://www.smithmetal.com/pdf/engineering/en19.pdf

Way overspec'd....EN3B would be plenty, EN8 if you want to go overkill.

You certainly don't need a hardenable tool steel alloy for main caps. Block will fail long before mild steel gives you any problems.

I'd recommend plain old 1018 hot rolled steel here in the US, hot roll is more dimensionally stable when machined, Cold rolled can move around a bit as those cold pressed stresses are relieved when you machine it.

[Nefario]

You may also want some "plain ol' " steel like CR1020 to make the line boring cutting properties more like the cast iron block. Virtually any steel design will be stronger than the original. Use Loctited studs to take advantage of the stronger cap.

[4sfed]

The difference in expansion coefficient is not a problem. I have used steel main caps on aluminum blocks with good success. The combination can also be found on some Cosworth race engines.

[Kevin Johnson]

The difference in expansion coefficient is not a problem. I have used steel main caps on aluminum blocks with good success. The combination can also be found on some Cosworth race engines.

Suzuki G13B : steel on alloy block (versus G13 with alloy caps on alloy block)

Nissan SR series (steel caps on alloy blocks with alloy bearing beam)

Many examples of alloy parts (bedplates) internally reinforced with ferrous parts.

Yes, many, many examples.

Vega: alloy block and steel head. Ooops.

(Samantha loved her Vega even though she was the only person who knew its inner starting Chi.)

[Schurkey]

Vega: alloy block and steel head. Ooops.

Was the Vega engine problem the iron head, or the open-deck block?

We put aluminum heads on iron blocks and think nothing of the material mis-match due to coefficient of expansion. There've been some head gasket issues, largely solved with multi-layer steel gaskets that "slide" easier.

Iron head on an aluminum block should be LESS problem than aluminum head on iron block, since the head runs hotter than the block, the expansion differences would be lessened.

I blame the open deck of the Vega block more than I blame the iron head. But that's me, and it's not like I've conducted actual research.

[Exminiman]

What do the stock caps do wrong?

If breakage is a problem, do you have any pictures of typical broken caps ?

'Most all steel will have the same stiffness. The important material property is E, or modulus of elasticity.

E of all steels are ~ 30,000,000 psi.
E of Cast iron varies considerably. From 10 - 20,0000,000 psi
Even ductile iron is about 24,000,000 psi.

Early on, whether the goal is greater stiffness or longer fatigue life the geometry of the cap is likely to be more important than the material.
I'm guessing Nice and thick in the vertical direction, and a nice shape with gentle radiuses is going to be hard to beat.
https://static.summitracing.com/global/ ... ?rep=False

Stock centre cap is suffering from some porosity, which I am concerned will start a stress crack
The crank is Forged EN40B steel which is quite stiff, I am concerned it will aggravate any shortcomings on main caps...
Maybe I am over thinking it.....

D22517A0-448A-42B0-9496-7958FCE68A77.jpeg

[Kevin Johnson]

...
Stock centre cap is suffering from some porosity, which I am concerned will start a stress crack
The crank is Forged EN40B steel which is quite stiff, I am concerned it will aggravate any shortcomings on main caps...
Maybe I am over thinking it.....
...

I do not think that you are over thinking it. You have identified a weak link. Trust your judgment.

For those who do not know, this is a three-main bearing crank.

~~~~~~~~~~~~~~~~~~

Any pics of the new crank? Does it use dummy mains? Just curious as I made many parts for these engines.

[modok]

The crank is Forged EN40B steel which is quite stiff, I am concerned it will aggravate any shortcomings on main caps...
Maybe I am over thinking it.....

A stiffer crank would give the main caps an EASIER life.
What type of steel the crank is made from makes no difference in how stiff it is.

[Kevin Johnson]

Strong consideration should be given to the (long???) cyclic stress history of the aluminium part: a part with already noted defects in a critical area and comprised of a material with finite fatigue life.

It is not so much that the stress will be lessened but that is will be stressed differently and this could accelerate failure.