Jump to content

New piston rings


Mike Watters
 Share

Recommended Posts

I decided to give my YS70FZ a overhaul, after the engine started to surge in flight at full throttle. From experience with these engines, it's most likely a leak somewhere. 

I stripped the engine and noticed that the top of the piston, brand new (three flight old OS F plug) were jet black. There was a lot of carbon build up on the exhaust valve, which I know is common. 

I cleaned the engine and found both inlet and exhaust valves leaked. I lapped the inside cylinder valve seat and both valves were replaced with new brand new valves. No leaking. 

I had a new piston ring too and that was replaced, so pretty much a newer engine. 

 

The cylinder head incorporates the liner, which looked nice and shiny, almost polished with no score marks. 

I've been told that the new piston ring will not bed in well because it's lost most of the honing marks from manufacturer, and a new head will be required. 

 

Has anyone else had problems bedding in new rings? The engines back assembled, compression feels very good, but I've not run it yet. 

 

Cheers Mike. 

Link to comment
Share on other sites

Hi Mike, I have no direct experience of the YS engines. However, I did recently fit a new ring to an OS 91FX (2 stroke). I retained the original liner, which was very smooth inside. I believe that OS applied a very effective hardening process to these liners, so they do not "wear in" very much at all. This makes sense, because even though the old ring was worn out, there was no wear ridge inside the liner at TDC. I had no problems with the rebuilt engine. The ring seal is good. It starts first or second flick, no electric starter required. The idle is stable and throttles very smoothly. It remains one of my best behaved engines.

 

What is there to lose? Just go for it and see what happens. Most likely you will save yourself the cost of a new head. At worst, you might have to later on buy a head and another ring I suppose, but hopefully not. Good luck!

Link to comment
Share on other sites

Sounds like a load of rubbish to me Mike. I replace rings in old engines all the time and the compression comes up every time. 

 

I recommend you use a small ish prop (bottom of the recommended range, maybe an inch smaller) and give it some high rpm with light load. Tune it for max performance, but just be gentle with the throttle for a bit. Short bursts (10 seconds or so) at full power until the ring comes in. 

 

On the carbon thing, what fuel are you using? It may have castor hiding out in there causing the carbon so a new fuel may be in order. I know YS like their nitro and for some reason recommend gallons of oil too. I assume its to help prevent leaks in their fancy fuel system. Still, 24% oil in a 4 stroke glow seems like insanity to me!

Edited by Jon - Laser Engines
Link to comment
Share on other sites

Does the 70 FZ have the sealing ring on the crankweb? If it does that might be worth examining as I think but can't remember for sure if that ring runs direct in aluminium which might explain the black stuff.

IIRC high oil content is to insanely assist with cooling the engine as most of these engines used to end up in competition aerobatic ships where constant speed in manoeuvres is helpful toward scores hence v. high throttle settings in the vertical which may explain it away as these tend to be frequent and of relatively long duration.

Much of the blowby oil is recycled through the engine in any event so gives it a double whammy as Maggie used to say.

I have to say that I never was a competitive flyer,yspistonporting.thumb.jpg.13b28cff609d2e5d1a80f1dc593e2c17.jpg using the ys just using my YS like I used my lasers, OS etc. TBH I used westons liquid gold 20% nitro in all my 4 strokes, I was happy with it.

BTW YS recommended modifying the piston by drilling 3 x 1/16" diameter holes equally from the crown to the root of the ring groove to assist in squeezing the ring against the cylinder walls. I've managed to get a pic of it transferred here. I'm pretty sure this was a factory recommendation but I can't get to the file to verify, I did it on my 110 and my 140 came with it though I recollect more holes in the 140 (I thought 4 but could be wrong as it was 10 years or more ago).

Link to comment
Share on other sites

I've run YS engines on Southern Model craft fuel with lower oil content (17 %) synthetic  and 20% nitro. No problem in years of running. I still have some YS recommended Bekra fuel that contains 18%  oil that they also running very well on although very smokey ? The carbon on the valves is likely to be castor in the fuel.

Fit new ring making sure the gap is OK and check for any wear ridge at top of liner (   very unlikely ) . Run normally and bring up to gull temp then allow to cool. Repeat a dozen times or so using a smallish prop as Jonh advises then fly it. Just don't go mad with throttle for a couple of flights and it will be fine

In my youth we bought castor oil from the chemist in small bottles and mixed it in our fuel on our motorbikes to get the smell.  Ahhhhh.....castor.   fast run past your mates onChelsea Bridge to give the smell! Great until you decoked the engine ( remember maintenance on 50s and 60s british bikes was intense :-). ) The castor would set like glass on the piston and exhaust valves.

Link to comment
Share on other sites

4 minutes ago, Olaf Pedersen said:

Does the 70 FZ have the sealing ring on the crankweb? If it does that might be worth examining as I think but can't remember for sure if that ring runs direct in aluminium which might explain the black stuff.

 

I don't think the 70 has the crank Web ring. Only come across it in some of the 140 models , some of those don't have it. Don't know about the latest models though.

Link to comment
Share on other sites

Thanks for everyone's replys.

 

Since new, the engine has only run on Model Technics Yamada 20. Interesting to hear what fuel others are running their YS engines on. 

 

The 70 does have a crankshaft ring. It also has four cut outs slots machined into the top edge of the piston at the factory, like in Olaf drawing. 

 

I thought it was a bit extreme having to replace the whole head everytime you replaced a piston ring. I'll give the engine a run as you recommended. 

Link to comment
Share on other sites

11 hours ago, Mike Watters said:

A great fuel, used it years ago, but not seen it stocked anywhere in the UK. 

It is available on the Model Techniks site but not stocked by shops probably  due to the new regs re Nitro methane . 30% nitro content fuel and above need an explosives licence due to nere-do-wells activity . It is available but will cost you about £40 a year for the license ! plus the cost of the fuel !!! Liquid gold it really is ?

 

Somthing else Ive learnt re the YS 70 having a crank ring seal.  Never stripped a 70 down having mainly used the 63, 91 and 140 's

Edited by Engine Doctor
Link to comment
Share on other sites

Well I ran the YS70 up on a Graupner 11x7 (normally I use a apc 12x8). Short runs, full throttle 10 seconds, no more than 10k rpm to get the temperature up, stop and let it cool. 

After running 6oz of fuel, the compression is really strong. Tick over nice and reliable, like it's a new motor. 

So it goes to show, like Jon said you can bed in a new piston ring to a partially worn liner. 

  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...