Glow Engines Discuss RC glow engines

Super Tigre S 75 head shims.

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Old 02-08-2019, 10:31 AM
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oskartek
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Question Super Tigre S 75 head shims.

While rebuilding an Italian Super Tigre S 75 I found no head shim and a very baked but not damaged piston and pin. This led me to believe this particular engine was for European use but run for a couple of seasons on 15% nitro and 20% synthetic/castor blend.
So here's my question, what shim thickness would one suggest I be starting with using 15% Nitro at 1250 ft altitude?
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Old 02-08-2019, 01:56 PM
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Start with .1mm and have a second .1mm shim handy. You’re gonna have to do some testing and experimenting.
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Old 02-08-2019, 02:59 PM
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I may have a couple of those copper like gaskets, I'll check in the AM.
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Old 02-08-2019, 09:39 PM
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here, I hope this link helps, you may be able to get shims for your 75 there too.

http://www.mecoa.com/faq/squish/squish.htm

Jim
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Old 02-09-2019, 01:45 AM
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ST heads are hard to measure squish angles and stuff - and really - pretty far beyond the capability of the average modeler. Not ripping on that post, but moreso just pointing out that ST heads are not typical - because the piston comes flush to the top of the liner. Combustion doesn’t take place inside the liner as is typical; combustion takes place above the liner within the head. Thus the different head design.

Every ST engine I own has a copper head shim that is 0.1mm thick. If the OPs engine doesn’t have it, someone removed it. That shim is necessary for running up to 5% nitromethane fuel. Some engines tolerate more, but the old Italian engines generally protest at over 5%. Going to .2mm worth of shine should get you to a safe spot to run 10-15% nitromethane. Run the engine on a test stand FIRST before running the engine in a plane. Warm the engine thoroughly and set it for the flight mixture setting you normally use and listen for a cracking sound. Like a frying egg. If you hear that, shut it down and add another 0.1mm shim and test again. You shouldn’t need more than 0.3mm. 0.4mm is about .015”, which is a lot... don’t exceed this.

Last edited by 1QwkSport2.5r; 02-09-2019 at 09:36 PM. Reason: apparently the swear filter filtered a non swear word.
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Old 02-09-2019, 08:16 PM
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Thanx to all of you, I now have ideas andaccess to a mill and lathe. I'll purchase an assortment of shim stock and go from there.
Some background, I purchased this engine new in the box about ten years ago and was told the original owner bought it in Italy and brought it to the US. I should have suspected it was for methanol only fuel. I flew it at 5800 ft. alt for two seasons on the 15% nitro fuel. I think that if it had been run at lower alt there could have significant damage to the piston, rod, and pin.
Once again, Thank you all for your advice.
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Old 02-09-2019, 09:36 PM
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1QwkSport2.5r, can't he get his compression ratio by CC'ing the head, I did it on my Chevy 350 boat engine I rebuilt back in the early 2000's. it was pretty easy, all I did was leveled the head, and took a thick, clear piece of plastic, drilled a hole in it and placed it over the combustion chamber, and used a syringe to fill the head, and I kept track on the CC's as I filled the head. there is a calculation to convert the CC's to the ratio

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Old 02-09-2019, 09:42 PM
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High nitro is fine at high altitudes without much if any modification.at typical low altitudes, you’ll have detonation. My S90K is from a time when the Italians had them setup a bit overcompressed for typical US fuels. I tried 15% nitro in my .90 and within seconds had it shut down and 5% put in the tank after emptying the 15%. What a difference that made!

Most Italian Tigres will not tolerate high nitro and low altitudes together for long. Some will, most wont.
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Old 02-09-2019, 09:48 PM
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Originally Posted by the Wasp View Post
1QwkSport2.5r, can't he get his compression ratio by CC'ing the head, I did it on my Chevy 350 boat engine I rebuilt back in the early 2000's. it was pretty easy, all I did was leveled the head, and took a thick, clear piece of plastic, drilled a hole in it and placed it over the combustion chamber, and used a syringe to fill the head, and I kept track on the CC's as I filled the head. there is a calculation to convert the CC's to the ratio

Jim
He would need to determine combustion space in the cylinder and the head and follow the equation to determine the CR. The CR isnít the entire story though - heat range of glow plug has a LOT to do with it. I believe the CR equation is head volume + swept volume / head volume.
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Old 02-14-2019, 03:07 PM
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According to this test ( ST S.75 (2) ) the S75 has no shims and a .005" squish clearance with a compression of 10.8:1 (geometric) which is reasonably high but well below some of my other engines. When calculating compression, head volume must include the squish volume. Mike Billinton, who does some very good tests, actually forgets to include squish volume, even though he measures it, so his comp ratios end up being too high.

Last edited by downunder; 02-14-2019 at 03:10 PM.
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Old 02-14-2019, 03:47 PM
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I don’t understand why the S75 wouldn’t have a shim when nearly all modern Tigre’s have a shim. My smallest Tigre is an S25 and the biggest being an S90K and everything I have in between has a shim. Also, .005” squish clearance is pretty darn tight! I would consider .012” the minimum for most sport engines and .015” for high revving engines. Not trying to knock you, Brian. Just surprised by what is said about the engine in your post and the engine review.
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