Compression and piston to bore clearance

[V22]

I have been building a Simple 2 Stroke (Ridders) and I am wondering if there are any rules of thumb in terms of compression. I am not exactly sure how to measure the running clearances of the piston to bore (feeler gauge seems a bit crude). Telescopic gauge is not repeatable (I do not have enough practice). I am somewhere between 0.04 to 0.06mm piston to bore. It falls outside of the 0.03mm recommended. That said, when I pull the piston via the rod I can feel decent suction.The piston is sticky somewhat in the bore, but will fall through the bore under its own weight.

Any rules of thumb/simple tests in piston to bore clearance?

 

[johanvanzanten]

Rule of thumb: With a clean and dry cylinde ( no oil ) When you shut one side of the cylinder the piston should stay in place. When you open the cylinder the piston should fall out inmediately by it's one weight.

 

[V22]

Rule of thumb: With a clean and dry cylinde ( no oil ) When you shut one side of the cylinder the piston should stay in place. When you open the cylinder the piston should fall out inmediately by it's one weight.

Thank you. This is exactly the type of information I was looking for. The unfortunate thing is that my piston fails this test. It slips out of the cylinder in 5 seconds under its own weight. Looks like I need to make another piston.

 

[gus]

Rule of thumb: With a clean and dry cylinde ( no oil ) When you shut one side of the cylinder the piston should stay in place. When you open the cylinder the piston should fall out inmediately by it's one weight.

Hi Johan,

Thanks for litmus test on 2 stroke compression. My first and last two
(2 stroke) engines did not past this unforgiving fit test.
Plan to redo these engines. I have been shying away from two stroke engines.
My three 4 stroke engines were successfully built to print and did start and ran well. Plan to spin the Nemett-Lynx Engine tomorrow.Will clean up and display on my PC Desk.

Thanks again---------Maestro.

 

[tornitore45]

Before remaking the piston give it a try. 5 seconds is not too bad. It may reduce compression on a cold start but after oil and heat should still run.
You can always make a new piston.

I like Johan true rule of thumb; usually you use your thumb to close the cylinder.

 

[tornitore45]

Gus, what makes you shy away from 2 strokes?
Although I never built one, I would guess they are less demanding than a 4 stroke.

 

[AussieJimG]

Two strokes seem to be harder than four strokes to get running. Perhaps it is because the port timing is built into the cylinder or because it is symmetrical about TDC.

I am not sure which of Jan's Two Strokes V22 is building. I built the Debbie and spent about 5 weeks trying to get it running. Eventually, the problem turned out to be compression (http://start-model-engineering.co.uk/tag/two-stroke/). Jan is able to build engines with very little friction and which are thus able to run with very little power. My engines need more power and that means more compression in some cases.

I have just finished Jan's Puppy (the smaller version of Debbie) which pops but won't run. I am still playing with the ignition timing as the compression seems to be ok - at least it has bounce at both ends of the cylinder. But if this doesn't work, I will increase the compression by the simple expedient of making a new head with a longer spigot; at least two stroke heads are easy.

On both the Debbie and Puppy, I lapped the cast iron bore to ensure that it was parallel and then lapped the cast iron piston using wet & dry paper and WD40 backed by a piece of aluminium until it just fitted the bore with a bit of a push. Then finished by lapping the piston and bore together with brasso.

Good luck with it V22. If you want to chat, send me a PM.

Jim

BTW here is a photo of my version of the Puppy. I just want to brag a bit.

 

[Brian Rupnow]

AussieJimg--That is a very nice puppy indeed. I find that 4 stroke engines are extremely forgiving. Once you get the hang of getting the valves to seal, its almost impossible to NOT have them run. I haven't built a 2 stroke, but I have it in mind that they are going to be a much more difficult engine to get running.---Brian

 

[V22]

Thanks Jim for the tips. I am a bit concerned it is harder to get 2 strokes running. Never mind, I am almost finished with the machining.

I managed an accurate measurement with the telescoping gauges and micrometer. Looks like I am 0.06mm piston to bore. Not to worry I will make another piston to 0.03mm clearance. I will also make a higher compression version in case it does not run.

I had grand dreams of reworking the piston and head to a more traditional 2 stroke format (bowled, hat on piston) but will not get ahead of myself. Get it running then fiddle..

 

[V22]

I made another piston for the simple 2 stroke (Debbie). This one came out at 0.014mm clearance to bore. It passes the hand test. It will only very slowly (60 seconds+) creep down the bore with my hand over the end. Removing my hand and it drops through. That said it can be a slightly sticky in parts. The compression on the up stroke is significantly better in that the air squeezes past my hand making a noise. With the other piston the air would not escape past my hand. It would escape past the piston down the bore.

Is 0.014mm asking for it to seize or is that good clearance? Jan recommends <=0.03mm. What is the minimum recommended clearance given the piston (and bore) will expand under heat?

 

[AussieJimG]

I made a new head for the Puppy with the spigot length increased from 1mm to 1.5mm and set the ignition to fire 1mm from TDC. And it RAN. Only briefly, but it did run. So once again, compression seems to be the critical thing in two strokes.

And I can't resist showing you: [ame]https://www.youtube.com/watch?v=fdx0j9AlN6Y[/ame]

Jim

 

[Theclockworks]

I made another piston for the simple 2 stroke (Debbie). This one came out at 0.014mm clearance to bore. It passes the hand test. It will only very slowly (60 seconds+) creep down the bore with my hand over the end. Removing my hand and it drops through. That said it can be a slightly sticky in parts. The compression on the up stroke is significantly better in that the air squeezes past my hand making a noise. With the other piston the air would not escape past my hand. It would escape past the piston down the bore.

Is 0.014mm asking for it to seize or is that good clearance? Jan recommends <=0.03mm. What is the minimum recommended clearance given the piston (and bore) will expand under heat?

I would give it the lollipop test suck it and see in other words see if it runs if not start again with the piston. One other thing about smelly 2 strokes you all so have crankcase compression that can lead to poor starting and performance if you don't have a go seal crank to crankcases.

 

[AussieJimG]

If the piston is cast iron in a cast iron bore, there will be no difficulty. If the piston is aluminium, I would give it a go and see; I suspect that it needs to wear in a bit anyway.

Jim

 

[tornitore45]

Aluminum piston in steel or cast iron sleeve is not a good fit, aluminum coefficient of expansion is 2 to 3 times that of various ferrous alloy.
Too tight it will seize
Too loose is difficult to start

Cast iron rings on undersized aluminum piston works well.