Aluminium grade for pistons?

[Richard P]

Hi, I am in the process of making a Westbury Kiwi Mark ii and anticipate that I may need to make a second piston from barstock aluminium. I have been running in the engine with the piston made from the casting from Hemingway kits and have seen some significant rubbing and pick up.

Is there any particular grade of Aluminium I should be sourcing for a 1 inch diameter x 1 inch stroke engine? I am a UK based, in case that makes any difference on grade names.

Many thanks

richard

 

[stevehuckss396]

Not sure what it is over there but I typically use 7075 for pistons in the states. From what I have read it is much stronger and more stable than 6061.

 

[Charles Lamont]

AIUI the most common grade of wrought alloy in UK is HE30 (or 6082). It should be strong enough and hard enough and should machine well.
I am no expert, but would have thought 7075 rather OTT for most models.

https://www.1stchoicemetals.co.uk/aluminium/grades-guide/

 

[Richard P]

Thanks for the information, very helpful. I'll investigate availability and cost of 6082 and also whether I can source some reasonably priced offcuts of 7075.

Thanks again

 

[xpylonracer]

Sometimes if the piston diameter is too large, so does not have enough room for expansion when running, there will be evidence of the tightness on the piston surface as you describe.
Best clean the damaged areas and run again to see if it appears again, after a few run and strip to clean operations no more will appear.

xpylonracer

 

[methuselah1]

I have no idea what grade of aluminium is used in the casting, but I'm guessing it's nothing fantastic. I didn't like the way it was machining, so I ended up making mine from bar stock. The two common ally grades (UK) are HE15 and HE30. The latter is good stuff, and perfectly adequate, but it loses strength rapidly with temperatures over 200C. That's not a concern with the Kiwi. HE15 has a dollop of copper in it, and temperature resistance is far better - ditto for corrosion. It's what the common man calls "dural" or "duralumin" and it's twice the price, too!
I got lucky, and bought a couple of bar ends from a friendly automatic lathe workshop, so HE15 is what I use exclusively - it might be a complete overkill, but hey, I have it, so I'll use it, and most especially for conrods; it's worth keeping a small stock to hand for those, if not for pistons.

Andrew UK

 

[Richard P]

I have no idea what grade of aluminium is used in the casting, but I'm guessing it's nothing fantastic. I didn't like the way it was machining, so I ended up making mine from bar stock. The two common ally grades (UK) are HE15 and HE30. The latter is good stuff, and perfectly adequate, but it loses strength rapidly with temperatures over 200C. That's not a concern with the Kiwi. HE15 has a dollop of copper in it, and temperature resistance is far better - ditto for corrosion. It's what the common man calls "dural" or "duralumin" and it's twice the price, too!
I got lucky, and bought a couple of bar ends from a friendly automatic lathe workshop, so HE15 is what I use exclusively - it might be a complete overkill, but hey, I have it, so I'll use it, and most especially for conrods; it's worth keeping a small stock to hand for those, if not for pistons.

Andrew UK

Great, thanks again Andrew!

 

[Badhippie]

I have personally had the pleasure of line boring EMD engines up to 253 inches in length
For almost 12 years when I managed a locomotive engine rebuild shop and machine shop. Great times when you have a natural sag or bow that you must contend with along with the serrations on the main caps and the A-frames. Or ones that have busted the crank or through rods and pistons out the side. Or broke the A-frames off and have to be welded back up. Had a lot of good times doing depending on the work scope from $200,000 - $1.7 million dollar engine rebuilds the higher price are the tier 4 rebuilds. The line bore when completed
would be within .0005 - .002 that’s not to bad over approx 253 inches in length
Thanks
Tom

 

[Mechanicboy]

Hi, I am in the process of making a Westbury Kiwi Mark ii and anticipate that I may need to make a second piston from barstock aluminium. I have been running in the engine with the piston made from the casting from Hemingway kits and have seen some significant rubbing and pick up.

Is there any particular grade of Aluminium I should be sourcing for a 1 inch diameter x 1 inch stroke engine? I am a UK based, in case that makes any difference on grade names.

Many thanks

richard

Hi Richard..

When I got aluminum that was best suited for pistons, I melted old pistons and poured them into steel pipes. After the aluminum was cooled, I could turn / process into a finished piston. Pay attention to heat expansion: The upper part of the piston should be slightly smaller than the middle of the piston and down to the piston skirt (a bit tapered from middle to top of piston). I calculated this based on the coefficient of thermal expansion for aluminum and steel so that the piston does not get stuck in the cylinder when the engine is hot. Set a temperature range to 200-250 degrees Celsious / 392-482 degrees Fahrenheit as a starting point when calculating to a desired diameter of the piston that is smaller than the cylinder bore itself when the engine is cold and the piston is near same diameter as cylinder bore when the engine is hot.

 

[ytrose2]

I would think that melting down scrap pistons should do the trick. Take a page out of the story of "The World's Fastest Indian."

 

[Mechanicboy]

I would think that melting down scrap pistons should do the trick. Take a page out of the story of "The World's Fastest Indian."

Here is my history in RC universe.. Convertering the OS 61 to pure diesel engine - RCU Forums

 

[Drawfiler]

The favoured piston alloy is LM13, it is better diecast, my method similar to mechanicboy, you make up a steel pipe with a bung in the one end, this can be shaped to suit the inside of the piston, you also make plug to fit the other end.
melt the old pistons to 750* and heat the mould, pour the metal and put the top bung in and hit with a hammer to compress. If you slightly over fill the mould and stir the melt in the mould, the dross will rise to the top and be machined off later.
The chilling of the metal by the mould improves the metallurg.

 

[paulc]

To be frank, any aluminium material that turns well will work just fine for shop pistons. I made my pistons from 6061-T6 bar and they have given me no troubles and I have run this engine hard, the engine is 25 years old.
I can recall there was a cylinder bore to piston OD ratio I used for getting the ball park on the piston to cylinder clearance. My pistons have more clearance at the crown where there is more heat and expansion and reduced clearance down at the skirt where there is less heat.

The piston design you see pictured is the same design I once scaled up to make custom car pistons! These were performance pistons of around 90 mm in diameter and again 6061-T6. Far as I know they are still running today.

the typical cast aluminum piston would be considered fragile compared to strength of a billet machined piston. Sort the clearances out and you're gold.

 

[Richard P]

To be frank, any aluminium material that turns well will work just fine for shop pistons. I made my pistons from 6061-T6 bar and they have given me no troubles and I have run this engine hard, the engine is 25 years old.
I can recall there was a cylinder bore to piston OD ratio I used for getting the ball park on the piston to cylinder clearance. My pistons have more clearance at the crown where there is more heat and expansion and reduced clearance down at the skirt where there is less heat.

The piston design you see pictured is the same design I once scaled up to make custom car pistons! These were performance pistons of around 90 mm in diameter and again 6061-T6. Far as I know they are still running today.

the typical cast aluminum piston would be considered fragile compared to strength of a billet machined piston. Sort the clearances out and you're gold.

Very helpful, thanks!

 

[Badhippie]

Lmao sorry people I put this in the wrong chat
Thanks your local idiot
Tom

 

[josodl1953]

Some 30 years ago I was struggling with replacement parts for my Picco 91 marine racing engine. Worn out piston pin circlip grooves caused the circlips going astray causing havoc in the engine and there were no replacing ABC cilinder sets available. I tried to make pistons myself but with this type of ringless pistons with a tight fit in the chromed cylinder, expansion coefficient was critical and required a special type of aluminium. Luckily, I found at the scrapyard of a marine diesel engineering company a big piston which provided material for piston making in the last 30-odd years. Harvesting material was a matter of drilling a lot of holes in a circle.

Now all there is left is som kind of carcass. If I want to make more pistons I will have to cut it apart and melt it in a steel mold just like Burt Munro with his fastest Indian....

Greetings from Holland,
Jos

 

[Bentwings]

Very helpful, thanks!

I pretty much gree with young choice of material would be 7075 purely because it machines so Nicely were I to be the model engine designer I would look toward piston ring mfgrs and us a standard ring that I could purchase Making cast iron rings and installing these home made things can be an exercise in frustration . Looking back on our race car days what we did with pistons and rings just to get one more round in or run the final round would cause a lot of head shaking . LOL. We put broken rings back in after sanding burned and melted aluminum off . If the piston filled the hole it was run . There might be a bit of oil smoke but racers didn’t deal with emmisions . Getting down the track and not leaving a trail of oil was most important . A completed round was cash in pocket. A smoky runner up was better than did not finish or also ran. Early pistons were made of some eeldsblr aluminum so I sometimes milled out damaged or burned areas d TIG. Welded them then re machined them .

Todays pistons do not weld very well or at all. Use them and toss bad ones in recycle bin it’s just a couple hundred bucks . Not worth the risk of greater engine damage . In old days you could buy a block for $20 and a couple hours you could have a race ready block. Now it’s 3-4 grand for a block 5 grand or more for a crank and a good 5-6 grand for new rods.