Piston/Sleeve/Ring material combinations

[Johann]

I am busy preparing to build a 4 stroke single cylinder Webster. I do however have a few questions on the material to use for different components.

The plan says:

1.Use 12L14 steel OR cast iron for the sleeve.

2.Use 6061-T6 Aluminium forthe piston and conrod.

3.The material specifiedfor the crankshaft is CRS (I am not familiar with this abreviation)

4.Valves are to be made from steel OR stainless steel.

I have available:

1.Cast Iron HT250

2.High Carbon Steel BS970 655M13(EN36B)(Case hardening)

My questions are:

1.What is the theory behind the different sleeve/piston/rings material combinations? What go with what and why are there different combinations (I have seen different options in different literature) Perhaps somebody has a link to a site to explain this in detail?

2.Will the HT250 cast ironbe OK for the sleeve? I would prefer cast iron for the sleeve for the ease of machining the fins. (If there are no obvious disadvantages above steel)

3.If you recommend castiron for the rings, will the HT250 do? I would like to try my hand on the manufacturing of rings, as I have a very good tutorial on it.

4.I plan to cast the aluminium stock from old truck pistons, will that be OK for the piston?

5.Will the EN36B be OK forthe valves and sleeve and should I go the steel sleeve route?

6.Why aluminium for the conrod? I am worried about stress singularties on

the conrod, as the tinyradii is difficult to be exacton (for me)

I realise that some ofthese questions might be irrelevant to a basic engine like the Webster, but todig deep into the basics makes it just so much more interesting to me.

Any help and/or pointers will be highly appreciated.

Kind regards

Johann

 

[Swifty]

Hi Johann, the letters CRS stand for cold rolled steel, also know as bright mild steel. This is different from hot rolled, as CRS will have no scale and be a better size tolerance. The larger sizes in mild steel are usually hot rolled and very rusty and covered in scale. 12L14 is just mild steel with the addition of lead to make it easier to machine, but not the best to weld in my experience.

Paul.

 

[Entropy455]

Cold-rolled mild steel is typically 10 to 15 percent stronger than its hot-rolled counterpart. The cold-rolling process work-hardens the steel, increasing its yield strength. Note that the tensile strength remains relatively unchanged by cold-rolling.

12L14 free-machining steel is stronger than 1018 cold-rolled steel, both in yield and in tensile strength. Granted, it’s not significantly stronger, but it’s stronger nonetheless. Thus a general rule of thumb - if you aren’t going to weld the part, make it out of 12L14. If you are going to weld it, use 1018 cold-rolled or A36 hot rolled.

My personal opinion on aluminum – it has no business being used to construct connecting rods, or any other machine part that is subjected to high cyclical fatigue. Aluminum work-hardens and fatigues with use, and will ultimately fail when loaded with enough cycles. For this reason, steel is a better choice 99.95% of the time.

Use caution when changing rod material. If your engine was designed for aluminum rods, you will make an out-of-balance condition when you switch over to steel. You will likely have to make adjustments to your crankshaft (or flywheel) counterweights - for the heavier rod.

High carbon steel is generally not a desirable material for hobby use. You can’t readily machine it unless it’s annealed. You need a heat-treating oven to properly harden and temper the metal. You need an electromagnetic induction machine to case-harden ($$$). Welding is also difficult. You “should” only weld high carbon steel in the annealed state, using low-hydrogen rod. After welding, the part "should" be fully annealed and then re-hardened. In my opinion, unless you intend to case-harden, or harden and temper your parts, I would avoid using high-carbon steels for hobbyist applications.

 

[kvom]

7075 aluminum is a good choice for rods, as it is very strong. It's used in most of the rotary airplane engine models for its combination of strength and lower weight.

 

[metalmudler]

Hi,

"4.I plan to cast the aluminium stock from old truck pistons, will that be OK for the piston?"

From my casting experience i would imagine the old truck pistons would be of a suitable material for casting the piston.

 

[Johann]

Hi Guys,

Firstly, thank you to all for the valuable information in your replies.

The local supplier could not source 7075 Aluminium. I did however get 6082, which from what I read, is a suitable and better replacement for the 6061. Apparently it is the strongest in the 6000 series aluminium. I am therefore going to use it for the piston & rod. This is my first very basic engine, but should I end up with something like the multi cylinder aero engines eventually, I will definately search for 7075.

I plan to cast the base and crank supports myself, (from the truck pistons) as it is not that critical. Whilst busy I will also try a cylindrical casting, to experiment with it as a piston material. As I am not an experienced 'caster', in fact this will be the first time, the possibility of a porous product wories me.

The sleeve was first on the manufacturing list, and I did the first run in bright mild steel. The finish was not up to my liking, and therefore I did it again, but this time in stainless - now I am more than happy for a first try.. Is there any reason why a stainless sleeve is not recommended ?

The head material is also a grey area for me. MUST it be Aluminium ? I have a stainless block large enough, but will it be suitable ?

Thanks

Johann

 

[Johann]

Just one remark on Entropy455's reply. I agree on the preference of a steel conrod, but this specific engine design does not even have a counter balance on the crank, and I would rather stick to the lighter material for now, for this engine. At a later stage, with more advanced projects, I will look into it. With ths first one I will be happy yust to get it running !