Bore to piston clerance

[Drei]

Hi, first of all Hpy Xmas ...

I aleady read a few articls but i wanted to be sure about it; i want to know what clerances should i leave between a 25mm cast iron bore and a aluminum piston. Also the clerance between the piston rings groove from the rings?
The engine that im talking on is the Lynx NE15, Ive already built on but now I am building another one with my one design but with the same specs.
i have already machiend the bore from cast iron and lapped it with a piece of round wood which i machiened with some clerance. i used fine corse carburandum then finsihed with autosol. i ran the lathe at about 1000 rev which i think it was too high.i got a good finish but didnt acheive that much cross hatch pattern. i also was wondering for a parallel bore but i achieved a 0.02 greater clerance on the upper part of the bore; which case its better that would have been on the lower part of the bore. i think that im am going to lap again the bore to make it parallel or its better that i have a taperd bore gat the top?? After the i will machien the piston accordingly.

 

[Ken I]

Here I am replying from my knowledge of full sized engines - not models.

The tapered bore is not desireable in that it will fatigue your rings by constantly flexing them.

You need a good cross hatch pattern to "hold" the lube - use a much slower speed and faster longitudanal movement when honing.

As far as clearance goes I would suggest you measure an automotive application and scale down.

Note: Engine pistons are all out of shape cold - only the ring grooves are round. the rest of the piston is oval and tapered (larger at the skirt) this is to allow for differential expansions within the piston when running (ie it is meant to be round when hot).

Some piston manufacturers even machine the wrist pin hole out of round.

I would guess for a model engine use the largest scaled clearance since you are not going to be making oval tapered pistons.

For Your Information only.

Ken

 

[coopertje]

Hi Drei,

I have build a lister engine with a casting Iron cylinder and aluminum piston. The fitting I made between them was as tight a possible. The test I make is the following. When the upper side of the cylinder is free the piston can move up and down freely. When the upper side of the cylinder is covered by hand, the piston sucks a vacuum.
I was afraid that this tight tolerance would lead to a blocked situation when running, but it works perfect. I even doubt if you need piston rings with this tolerance. For my next engine I used the same, made the grooves in the piston for the rings, but did not make the rings yet. I will try to run it without first, if not working I can make the rings later.

I use a mixture of oil and steel polisher to "sand" the piston and cylinder. Put the cylinder in the lathe (do not tighten to much, its will deform!) and then run it on slow speed sliding the piston (on a mandrel) in and out.

Another tip is to use a reamer to make the cylinder bore. If you let it swim in lubrication oil the result is amazing. And very accurate in dimension and parallelism!

Hope this helps a little and good luck with your engine!

Regards Jeroen

 

[gbritnell]

Hi Drei,
Having worked on cars and motorcycles for years I have found that there are different clearances relative to a particular engine and also the purpose of that engine. On some small Honda motorcycles with around 2.50 inch bore they have .0015 clearance on a new bore. Others are not usually that tight but close.
For model work on bores up to 1.00 I never go over .0015 for piston to cylinder clearance and I have never had a piston stick.
With an aluminum piston it expands quicker than the iron cylinder but as they both come up to temperature they both expand fairly closely.
As has been mentioned, the bore needs to be parallel and also round for a good seal.
gbritnell

 

[Drei]

I have made some reserch about them clearnce; i was going to machien the piston at about 0.05 to 0.08mm less than the bore after finishing the bore as parallel as possible. This will give me a slide a slide through the bore and reduce friction but achieve the seal that i want with the rings. i am going to leave the piston finish "as rough as possible"; i will sharpen the tool like for cutting a thread. This form will give ma small circular lines on the piston which will hold lubrication in them and also reduce the surface are of the piston thus less friction.
i will machien the rings the same diameter of thye bore then open them in a jig 1mm bigger and heat them and leave them cool alne. They will hold this diameter and have the necessey tension on the bore walls to achieve sealing. i will machien the groovs 0.05 bigger than the rings to be able to insert them easily and would be enough for the gasses to pass under the rings to push the ring on the bore walls.
Thats my plan, has anyone have a better idae please let me know..

Thanks

 

[Ken I]

It all depends on how hot your engine is going to run - admittedly that's a big question mark.

Aluminium expands at 0.0000230 / C°
Cast Iron expands at 0.0000104 / C°

So assuming your cylinder reaches 100 C° then it is going to be 25.026 in diameter
If your piston reaches 300 C° then you are going to need a 24.8545 cold size for it to be exactly the same size as the cylinder 25.026 when it is at 300 C° - you then need a couple of microns for the oil film.

I don't have a clue as to the temperatures achieved in model I.C. engines but are probably a lot lower than in automotive.

Suggestions anyone ??

Ken

 

[Drei]

Yes Ken thats a good question,
I wanted to leave the clerance between the cylinder because my previous engine i made the cylinder almost a tight fit in which case its wrong while in that moment i tought i was doing a good thing bnecause i was sure that this will give me a good compression. But i learnd that the rings have to give you compression not the tight pistion. Tought i will have to be extra catious to machien a good and effective set of rings!

Drei

 

[Drei]

Any more informations are welcomed
How much clerances do you leave when machining your pistons and sleves guyes....???

Drei

 

[Jasonb]

I'm just doing a 1" bore hit and miss and the drawings call for a 0.995" Piston (0.13mm), This does not get as hot as the type of engine you are thinking of, I've seen an Nemett running a red hot exhaust pipe!!

[ame]http://www.youtube.com/watch?v=wTq0ogyTFEc[/ame]

Rings are machined tight to the 1.000" bore and spread at 3.0mm which is similar to what pat was saying in the rig gap thread, then gapped at 0.003 in the bore.

Jason

 

[mu38&Bg#]

You'll want a little more clearance above the top ring as that is the hottest part of the piston. This is a link to a site I follow. The fit in this engine is very tight. I wrote to him with my clearance calculations for the materials he used, but he said he found this worked. Much less than the 0.13mm I thought necessary for a 2024 piston and steel sleeve.

http://homepage2.nifty.com/modelicengine/k080901.htm

 

[Drei]

Thanks Jason,
I also notised that you leave the glow plug initer pluged while running just like me. Whwn in my case i remove it the engine slows and stops. I dont know whats your case but even the the guide book by Malcolm Stride the one incorporated with thye prints says that maybe the engine requires a hotter glow plug. And well done taht s very nice looking and sounds great

Drei

 

[Maryak]

Piston size = 0.98 x Bore diameter.

Hope this helps

Best Regards
Bob

 

[Jasonb]

Drei, unfortunatly thats not my engine I just remembered seing it a couple of years ago.

Hudmut who occasionally post here has built the same engine, these are his pics

http://s249.photobucket.com/albums/gg203/hudmut/4 Stroke Engine/

Jason