Piston Fit

i have a small, 3/4” by 3/4” two cylinder engine I’ve built. It slobbers oil from the exhaust. It actually run pretty well inspite of that.

Currently the piston clearance is .004”. It’s an aluminum piston in a cast iron liner. I wonder what is the tightest fitting piston that will work without seizing when the engine heats up? Comments please. Done all I can regarding rings with 4 rings and oil drain holes in lower ring groove. Had very high compression with domed pistons but flat topping them and lowering compression didn’t help the oiling problem and dosent idle quite as well.

The engine is Bob Shores Silver Bullet but modified to 180 crank, hemi heads with Harley knuckle head type rocker arms.

Thanks, John

I should think you could quite happily halve that or even a bit more depending on how hot you let the engine run.

Could also swap out one of the rings for a Viton O ring or even a Quad ring.

Oil slobber is most likely to be caused by the rings not sealing to the bores. Prolonged running at high speed off load will glaze the bores and produce oil slobber.

Check the fit of the rings to the grooves and hone the bore to remove any glaze.

Then when the engine is running, load it. Also, in early life do not use high detergency oil, since this can prevent the engine from running in, (i.e. bedding the rings to the bore. )

Howard

Thanks for the reply’s.

The engine runs about 170 degrees, it has a cooling system. Running no hotter than this I think I can go with a tighter piston to cylinder fit. Believe me I’ve tried about everything re. the rings. They appear to be seating ok. I have dippers on the rods. I run a low oil level and installed a windage tray. Not comfortable with such a low oil level. Others who have built the engine have had similar problems cured with low oil level.

I guess I’ll just make new pistons with only a couple of thou clearance and see what happens. I can make pistons pretty quick.

Will o rings work on a relatively high speed engine? It idles around 1,200 rpm and probably goes up to maybe 5,000 rpm.

John

Is it really oil or is it possibly unburnt, contaminated diesel?

Neil

US, I guess? That is over 5 thous per inch of bore.! Most large engines would be built to far less than that. Not a chance that there will be zero clearance at 80 degrees. That is only a temperature rise of about 60 degrees.

If I rebuilt a car engine with 15 thous. piston to bore clearance, I would fit the next rebore size of piston! (+10 thous).

Have a look at this engine, good for 8,000rpm on Viton rings. Also worth looking at the video on the page before where it was running without baffles and a lot of oil was getting thrown up into the bore and out the exhaust. We also discuss baffles and O rings throughout the thread.

This is a question I have tried to find a definitive answer for, without much luck. I have made quite a few engines over the years, mainly compression ignition, where, for success, the piston/cylinder fit has to be perfect, with no bypass leak. It took me several goes before I perfected the technique to achieve this and every engine C/ I or Glow that I have made, I lap to this standard. To me ,an engine is judged by its compression in the first instance. None of these have had rings. A couple of years ago I made the 15 cc Chenery V-Twin engine that has two rings per piston. No information on piston fit/clearance was in the notes and I found that I could not persuade myself to make the pistons other than the perfect fit.that I always developed The pistons were Aluminium in Cast iron liners.. This engine has had a total to date of about 5 hours run time. Because it is a thirsty beast,a run does not go much longer than 5 minutes. I always add 5% more Castor oil to a commercial fuel. I have not had any over heating problems with the piston /cylinder fit and the compression is still as good as it was originally.

I have since made the Jones 10cc Glow engine that has one ring on an Ali piston in a Cast iron liner. Once again I made the piston to have my standard fit and have had no problems with overheating. Test runs are all bench mounted . My days of flying are behind me.. Whether my experience is due to the extra Castor, I have no idea.

For my next project, I am looking at the Westbury designed Kiwi Mark 11.15 cc four stroke. His notes /drawings give a clearance of 0.005" above the ring and 0.003 "below to the skirt., for the aluminium piston with two rings.

I have no problem understanding the requirement of rings for full size multi cylinder engines, but I am unsure of where the point of need for them on a model engine exists. As an instance, I have two Glow engines of about 8cc that do not have rings, so why would the 7cc size on the Vee-Twin need two rings? I would be very interested in information regarding piston clearance based on real practice that any one can point me to.

Keith

Piston Rings are intended to minimise leakage. the actual volume of leakage will depend on the pressure difference, and the time for which it exists.

Thus, a car engine with low compression, due to worn bores and rings will be difficult to start, but will run fairly well at high speeds. Similarly, a small compression ignition engine may run beautifully at 12,000 rpm, but be be right pig to start, for just that reason.

Engines of around 1 litre per cylinder work with piston bore clearances of the order of 0.001" But the pistons are not round, they will be tapered and oval to accommodate the temperature gradients within the piston, especially when run at full load at rated speed. And every configuration and marque will be different

So one cannot completely read across from full size practice into engines with much smaller bores!

Which is why clearances are often quoted as " a thou per inch" or something similar.So here we are dealing with very small dimensions, hence the use of honing in many cases.

With regard to oil slobber, the scraping feature of piston rings can be induced in various ways. By slanting the ring groove, (unusual ) by tapering the face of the ring, or by chamfering or stepping the inner diameter of the ring. These latter persuade the ring to twist, to present a sharp edge to the bore, and so scrape the oil.

As Dr Dykes said, many years ago, a ring can be upward scraping, or downward scraping; so care is needed to fit the ring the right way up! And we are not going into wall pressures or surface finish / honing patterns on bores!

Howard

I have added a picture of the Silver Bullet. I ran it in for a couple hours and the oiling problem is improved.

Video test

Edited By John Vietti on 30/11/2019 21:43:21

What a lovely little engine. Top work.