VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics

  #1  
Old 07-17-2019, 09:17 PM
1flyingyogi 1flyingyogi is offline
 
Join Date: Dec 2015
Location: Long Beach, CA
Posts: 319
Default How to tell if I have a "decked" case?

Sorry if I'm using the wrong words, but if I heard correctly, Lycon said that my case was probably "decked" - meaning that someone in the past milled down the area of the case where the base of the cylinder sits with the purpose of increasing compression.

I had my cylinders ported and flow balanced and then installed high compression pistons (9.5:1 from Combustion Tech). When we put the cylinders back on (even before installing the pushrods), the prop would NOT turn all the way around. It gets stuck when any of the pistons moves to the top of the cylinder. (like the cylinder height is too short to allow the piston top to move any further)

I called Lycon and sent them some photos and took some approximate measurements and they think my case has been decked so these high compression pistons, although only .06" taller than the stock pistons, will top out because they're too tall for the cylinder. And the reason why is because the cylinder sits on a shorter base because it was milled down (decked).

So my question is, how do I verify? Is there some published dimension somewhere that I can look at and compare with mine? Would it be the distance from one side to the other where the cylinder base sits? (sorry I don't know what that's called).

Look at the photo attached and you can see a "lip" at the base of cylinder #3. It is raised a little bit. According to Lycon, that's where the face height originally was and it was milled down from there to effectively increase compression. Does that sound right to you guys?

At first we thought maybe we were sent the wrong pistons or that something was wrong with the cylinders or maybe they might have switched up my cylinders and sent the wrong ones. But I verified with Combustion Tech that it's the right pistons and Lycon said they definitely did not send me the wrong cylinders. So this is the only explanation. Or is it??

https://drive.google.com/open?id=106...imnD-FYW98tNyO
__________________
Brian RV4
0-320 AFP fuel injection, EFII Electronic Ignition, Sterba wood prop
Dues Happily Paid
Reply With Quote
  #2  
Old 07-17-2019, 10:04 PM
rocketbob's Avatar
rocketbob rocketbob is offline
 
Join Date: Jun 2006
Location: 8I3
Posts: 3,279
Default

This was the way to increase compressions back when there weren't anything other than stock pistons available.

I'm guessing if you went with stock pistons you'd be ok.

Deck height is something the case overhaulers would know and you'd have to measure the case halves on a surface plate.
__________________
Bob Japundza CFI A&PIA
N55BC RV-6 borrowed, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
Reply With Quote
  #3  
Old 07-18-2019, 10:21 AM
Jim Ball Jim Ball is offline
 
Join Date: Sep 2016
Location: Mobile, Alabama
Posts: 14
Default Deck Height minimum 4.775"

I have the data from when ECi was a repair station in San Antonio that overhauled crankcases. The minimum approved deck height to certify Lycoming 320, 360, 540, and 720 crankcases was 4.775 there. That is measured from the parting surface of the crankcase halves to the cylinder deck.
__________________
James Ball (J.B.)
Continental Aerospace Technologies
2039 South Broad St.
Mobile, AL 36695
Phone 251 436 8122
jball@continental.aero
Reply With Quote
  #4  
Old 07-18-2019, 11:07 AM
Zuldarin's Avatar
Zuldarin Zuldarin is offline
 
Join Date: Mar 2013
Location: Snoqualmie, wa
Posts: 339
Default

I know this is a long shot but are you sure its the piston hitting the top? The rings on these engines do have an upside and a downside. If you install the rings (any 1) up side down the piston will not go all the way to the bottom of the tapered barrel. You should have noticed this when installing/gaping the rings but just a thought.
__________________
Darin
Snoqualmie WA
RV-9A Flying as of 03/2015
RV-10 Building - Finishing Kit
www.DarinAnderson.com
2019 donation sent!
Reply With Quote
  #5  
Old 07-18-2019, 12:06 PM
Ralph Inkster Ralph Inkster is offline
 
Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 522
Default

Quote:
Originally Posted by Jim Ball View Post
I have the data from when ECi was a repair station in San Antonio that overhauled crankcases. The minimum approved deck height to certify Lycoming 320, 360, 540, and 720 crankcases was 4.775 there. That is measured from the parting surface of the crankcase halves to the cylinder deck.
Jim- the 4.775 confuses me a bit as I just measured 3 engines (cases assembled, flange to flange) and come up with
360 wide deck = 9" across
540 wide deck angle valve = 9 1/16"
320 narrow deck = 9 1/16"
granted the measurement method was crude but result is a lot less than
4.775 X 2. Is there a raised surface between the two mating case halves that would account for the 0.275 x 2 dimension difference?
__________________
Ralph
Maintain lots, upgraded - repaired - & modified more, rebuild a few, & built some too.
Reply With Quote
  #6  
Old 07-18-2019, 12:40 PM
1flyingyogi 1flyingyogi is offline
 
Join Date: Dec 2015
Location: Long Beach, CA
Posts: 319
Default

Thanks for the replies. I'll measure and see what I get.

And yes, we made sure the rings are installed properly and carefully checked the gap for each one.

I can see the piston top and feel it with the tip of my pinky through the spark plug hole. This is the absolute limit of how far up it can go. I also checked by taking the cylinder off and pushing the piston as far up into the barrel as I could and it's in this same position. This photo is of it on the engine and with the prop at the "stuck" position.

https://drive.google.com/open?id=10A...5-Hxrz5owx0dNR
__________________
Brian RV4
0-320 AFP fuel injection, EFII Electronic Ignition, Sterba wood prop
Dues Happily Paid
Reply With Quote
  #7  
Old 07-18-2019, 12:51 PM
Ralph Inkster Ralph Inkster is offline
 
Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 522
Default

maybe it's a case of milling a taper around the top edge of the piston for more clearance.
I remember something about folks swapping C-85 pistons into a O-200 for increased CR and having to taper the edges on some engines, or similar.

If it turns out your case halves have been shaved, using the new pistons & tapering the edges may get it to rotate acceptably but than you may end up with a higher unknown CR that may not be a good thing.
__________________
Ralph
Maintain lots, upgraded - repaired - & modified more, rebuild a few, & built some too.

Last edited by Ralph Inkster : 07-18-2019 at 12:55 PM.
Reply With Quote
  #8  
Old 07-18-2019, 01:26 PM
1flyingyogi 1flyingyogi is offline
 
Join Date: Dec 2015
Location: Long Beach, CA
Posts: 319
Default

Quote:
Originally Posted by Ralph Inkster View Post
maybe it's a case of milling a taper around the top edge of the piston for more clearance.
I remember something about folks swapping C-85 pistons into a O-200 for increased CR and having to taper the edges on some engines, or similar.

If it turns out your case halves have been shaved, using the new pistons & tapering the edges may get it to rotate acceptably but than you may end up with a higher unknown CR that may not be a good thing.
If in fact my case was decked, I already have high compression, even with the stock 8.5:1 pistons. I don't want to go higher.

Lycon told me to get exact measurements, and they can calculate what CR I have now with the stock pistons. They said just by the fact that the 9.5:1 are topping out, they think I'm at least 10:1 or 10.5:1 with my stock pistons (8.5:1).

This would explain a lot. I always thought I was "lucky" to have an unusually strong engine. My engine turns a prop that's bigger and has more pitch than what's recommended for a 160hp engine. And I'm quite happy with my speed and climb. I get about 205mph WOT level and climb at least 2200fpm.

However, I was greedy and wanted even more power so I went through all this which ended up being a huge hassle and very costly in time and money.
__________________
Brian RV4
0-320 AFP fuel injection, EFII Electronic Ignition, Sterba wood prop
Dues Happily Paid
Reply With Quote
  #9  
Old 07-19-2019, 07:36 AM
1flyingyogi 1flyingyogi is offline
 
Join Date: Dec 2015
Location: Long Beach, CA
Posts: 319
Default

Quote:
Originally Posted by Ralph Inkster View Post
Jim- the 4.775 confuses me a bit as I just measured 3 engines (cases assembled, flange to flange) and come up with
360 wide deck = 9" across
540 wide deck angle valve = 9 1/16"
320 narrow deck = 9 1/16"
granted the measurement method was crude but result is a lot less than
4.775 X 2. Is there a raised surface between the two mating case halves that would account for the 0.275 x 2 dimension difference?
Ralph, which model 320 was it that you measured? It's a pretty big difference between your number and Jim's. This would more than account for a difference in piston heights of different compression pistons. For example, a 9.5:1 piston is only .06" taller than an 8.5:1 piston.

Isn't there a manual or some place where we can look this up and know for sure what that number is supposed to be?
__________________
Brian RV4
0-320 AFP fuel injection, EFII Electronic Ignition, Sterba wood prop
Dues Happily Paid
Reply With Quote
  #10  
Old 07-19-2019, 08:18 AM
jacoby jacoby is offline
 
Join Date: Jul 2018
Location: WNC
Posts: 164
Default

Quote:
Originally Posted by 1flyingyogi View Post
Ralph, which model 320 was it that you measured? It's a pretty big difference between your number and Jim's. This would more than account for a difference in piston heights of different compression pistons. For example, a 9.5:1 piston is only .06" taller than an 8.5:1 piston.

Isn't there a manual or some place where we can look this up and know for sure what that number is supposed to be?
You would think so but it's not listed in the Lycoming table of limits.

As a data point, these guys list 4.475" min on their 8130s and it appears it's supposed to be nominally 4.5" for these crankcases.
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 09:43 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.