51 replies to this topic

[ANON]

theres companys out there who will make a set of pistons for you... but unless your doing a run of them they certainly still wont be cheap, plus then finding correct rings would be a pain, would have to go to a ring manufacturer and give them dimensions and see if theres an OE ones to suit

you can get pistons made at a reasonable price. i spoke to derek chin a few years ago when audi ur quattro pistons went nla from ks. they put me onto someone (can't remember the name sorry) and they gave me a very good quote on pistons.

[Ethel]

Have you thought of contacting the piston manufacturers? Your, too small, pistons must have been bigger prior to machining, they might be persuaded to find some that you can machine yourself at your college (in the name of education). I think trimming down rings should be ok too as long as it's very little - round rings in a round cylinder will have equal distortion all the way round (save the bit nearest the gap) - they should stay round but will be tighter on the bores.

[chrisandsarah]

i once knew a guy who bought a morris minor that had a solid oak piston.

Is that an innuendo

[1977 Loud_Mini]

Thanks for all the comments everyone been very helpful.
I suppose i could try contacting piston making companies but i thought they were really expensive, more so that pistons for sale out there now?
The pistons are probably the normal size but it's the bore that has just seemed to have been bored to a size without considering the piston size. As for the rings, the end gaps were just huge at 20thou, 40 thou and then 80 thou for the oil scraper so thats my reason for going 74mm and then adjust accordingly.
Heres a video to show what i'm talking about:

If i was going to bore the block out then this would most probably break into the oil ways in which case would cost more to get another block bored out, so this block is practically useless so i thought why not try and make my own pistons, but is complicated getting everything just right etc etc.

Evan.

[roofless]

Hi all, as some may know i had problems with my 1380cc engine being that the pistons and rings were too small for the bore.
At college i was talking to my tutor and he suggested making some pistons on the CNC milling machine they have there along with a lathe. I've drawn up a design in software called SolidWorks which means the drawing can be transfered to AutoCAD and then converted into code (G-codes) for the CNC milling machine to recognise.
I've been researching the required material which is 2618 Aluminium so that the piston can cope with the stresses of heat and combustion etc, just don't know where to buy it if it can be bought anywhere.

I was thinking to get some 74mm piston rings and then file them down to the correct ring gap so that they will be sort of custom rings for my application.
If anyone has any other information it would be great to here from you.

Thanks, Evan.

Evan, Solidworks is a much superior cad package than Autocad..... why not create the G & M coding in SW ?? and yes I know where to get 2618 ally from if you really want to make 1 or some etc....

coding from autocad mechanical desktop is as good as, and possibly better than solid works. its only that solidworks includes this feature / facility in standard form I believe.....and in autocad its a bolt on only available to subscription customers.

used to be standard in autocad....then they realised they can get more money out of you

[icklemini]

so thats my reason for going 74mm and then adjust accordingly

It doesnt work like that, a 74 mm ring set fits in a 74 mm bore.. the installed radius of it is correct for the 74mm hole its running in and the piston its installed on.

Getting a 74mm ring in a 73.5 mm bore will result in the ring not seating in the bore correctly - or in the piston... you are effectivly 'pinching' it together..

As an example - get a circlip from say the end of a drive shaft and try making that a smaller diameter with out it going oval...

Edited by icklemini, 26 January 2011 - 08:24 PM.

[Southy]

Are you sure that it's just not the ring's which are worn as I seem to recall you didn't fit new ring's? How far up is the block from the 2.8937 correct size for the 1380?Have you had it professionally sized using a bore guage?

[1977 Loud_Mini]

Ok i understand about the rings, it's just i have heard of this being done and wrongly assumed it was something i could do.
The 1380 bore as listed on another topic about a week ago using pictures was 73.50mm on one bore, 73.52mm on another, 73.51mm and 73.52mm. Obviously this there could be mistakes made due to human error and the fact was using a vernier gauge measuring from only the top of the block. The pistons were not new but hardly used and apparently it had new rings which looked to be the case. The problem is the piston rock that i had which can be seen in the video in the link i posted and the ring end gaps which were just unreal.

[mini670]

Interesting thread.

I wonder if lost wax casting would work. Where i work one side of the business vac casts titanium and cobalt chrome then cnc machine parts, maybe a wax could be made....you got me thinking now!
I make bits from solid 412 stainless on true 5-axis. How I wish how to design on solid works or pro engineer

[mini93]

lost wax could work, but what you have to think about is the excess flash will require finishing plus, if theres a large volume of the metal cooling down there can be left with sinkage in the surface due to the contraction, not sure how much contraction would happen due to the materials, the amounts of material used and crown thickness, but it could be plausable

[MRA]

The difference in cross sectional areas would/could lead to high stress points, that once machined will allow the item to go out of shape....... it would be interesting to see the results though...

[mini670]

yea there is a hell of a lot of work there, for something that is just a test.
Had a quick read up on pistons, how about making some from titanium, apparently a Honda NR 750 has oval titanium pistons!
It would be a dream to have enough time (and £££) to play about with stuff like this.

[1977 Loud_Mini]

Just a little sort of update. After designing the piston on SolidWorks i have now converted to an STL file and had it printed out on a 3D printer and heres the result. Not too bad, just some little things need working out, the pistons i made just under 73.5mm to the correct tolerance fit in the bore, anyway it's about .5-1mm too small so will need adjusting. Are pistons definately 73.5mm when purchased or is this the bore size as my bores measured slightly bigger than that and i went on the bores being exactly 73.5mm to see what was going on.

Also you may notice i haven't got a dish in the top, more of a recess which is about 9cc. Does anyone know how to calculate the dish dimensions (e.g. diameter and the radius of dish) in most pistons as then i can have it dished rather than how it is now. Let me know what you think.

Evan.

[Ethel]

Nice work, but I think they might melt

Dimensions are bore size rather than piston size.

[1977 Loud_Mini]

Nice work, but I think they might melt

Dimensions are bore size rather than piston size.

Haha yeah i should think they would
I thought that was the case, well whoever the previous owner had bore the block must have got it a bit wrong. I made the piston around 3 thou smaller than 73.5mm which isn't right in my case. I think i will have to get the bore professionally measured rather than doing it myself with a vernier gauge and then judge the piston to bore clearance. Do you know if it's normally 3 thou clearance?

Edited by 1977 Loud_Mini, 03 February 2011 - 05:24 PM.