Quantcast

Sealing crankcase halves.

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

PTAirco

Well-Known Member
Joined
Sep 20, 2003
Messages
3,563
Location
Corona CA
Everyone seems to have an opinion on this.

My Continental A65 manual does not specify any form of sealant, gasket or thread between the case halves. A film from the 1940s detailing the overhaul of one of these does show the application of some kind of unspecified sealant.

Is there a better, modern alternative to using something like "Perfect Seal" and silk thread? (Silk thread is $60 a roll!) Since the manual does not mention thread I'm afraid it might end up creating more bearing clearance. Supposedly it mashes down to nothing, but something has to be thicker than nothing, right?

I am leaning to some modern non-hardening sealant, but no idea which yet. And no, I did not at any stage think about squirting some RV stuff in there.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,666
Location
Memphis, TN
I found some silk thread from Joann’s and used the permatex equivalent of perfect seal from autozone.
 

PTAirco

Well-Known Member
Joined
Sep 20, 2003
Messages
3,563
Location
Corona CA
How do you know if it was the right diameter thread? If it's too thick , it will create a gap and the bearing clearances will be too great.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,666
Location
Memphis, TN
The thread only goes on the spine not main caps. Bearings are accounted for. I have to back up and say I was doing my Lycoming G not a Continental. Lycoming manual specs the size thread. It’s not going to be too thick, finding thick enough will be more of the challenge. I ran two parallel threads. I just did an AD on a couple of year old factory OH and they used double thread on it. The right rtv can be used but to get the ideal amount on biased to the outer edge. I have heard of PRC used too. Just getting it right so it does not squeeze excess in is the hardest part.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
8,396
Location
USA.
Fresno Airparts sells the thread. .20 cents a foot. I ordered 10' for my Cont -C-85. Big spender :)
Ordered the thread along with most everything for the major overhaul.
Used Aviation #3 Permatex to seal the case. Been using the #3 since I built my first aviation engine in the early 1970's.



https://www.fresnoairpartsco.com/product/641543-silk-thread-1-foot/

Added -- also used two parallel lines.
 
Last edited:

BoKu

Pundit
HBA Supporter
Joined
Aug 15, 2013
Messages
2,754
Location
Western US
How do you know if it was the right diameter thread? If it's too thick , it will create a gap and the bearing clearances will be too great.
I'd guess that elastic deflection of the case accommodates a fairly wide range of thread thicknesses without any significant disruption of the main or cam bearing clearances. Remember that the main bearings have their own studs and nuts, and their clamping forces will certainly overwhelm the thickness of the thread and sealant.

I'm inclined to believe that the silk thread is probably superfluous these days, and a modern sealing compound would work perfectly fine without it. That said, when I crack open my A65 next year I'll probably use the thread when I close it back up.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,666
Location
Memphis, TN
People have put thread on the caps thinking they are making it all even. What happens is the cap walks and makes a mess of the case.
 

proppastie

Well-Known Member
Log Member
Joined
Feb 19, 2012
Messages
4,703
Location
NJ
you put the thread around the outside edge flanges not the bearing journals. Lycoming calls out "oo" silk thread.

upload_2019-9-10_22-4-8.png
 

Daleandee

Well-Known Member
Joined
Sep 11, 2015
Messages
1,154
Location
SC
Used Aviation #3 Permatex to seal the case. Been using the #3 since I built my first aviation engine in the early 1970's.
I knew I was doing something right! This really is good stuff.

Dale
N319WF
 

PTAirco

Well-Known Member
Joined
Sep 20, 2003
Messages
3,563
Location
Corona CA
you put the thread around the outside edge flanges not the bearing journals. Lycoming calls out "oo" silk thread.

View attachment 88590
I realize that, but I you're still putting something between the case halves, separating them to some degree. Since the manual doesn't call for it, in case of the A65, I think I'll just go for the sealant. I keep seeing references to Loctite 515 or 518. ANyone have any experience with that?
 

rmeyers

Active Member
Joined
Dec 20, 2013
Messages
29
Location
Denver, CO
I realize that, but I you're still putting something between the case halves, separating them to some degree. Since the manual doesn't call for it, in case of the A65, I think I'll just go for the sealant. I keep seeing references to Loctite 515 or 518. ANyone have any experience with that?
HondaBond#4, YamaBond#3, SuzukiBond#?. They're all a gray butyl rubber compond. It's what all Jap motorcycle cases are sealed with. Harley too I think, at least I've seen a gray line at the parting line on some new Harleys.
Gooey, but not very thick. Most importantly of all, unlike RTV, any that squeezes out of the joint will flow like paint and 'harden' (it doesn't really get hard) on the inside or outside surface. It will not clump up and break free like RTV. Clean up with acetone or MEK. It adds no measurable thickness. Designed for surfaces that are machined and relatively flat/smooth.
Loctite makes something similar but at 10 times the cost. Don't know the Loctite number.
 

proppastie

Well-Known Member
Log Member
Joined
Feb 19, 2012
Messages
4,703
Location
NJ
I realize that, but I you're still putting something between the case halves, separating them to some degree. Since the manual doesn't call for it, in case of the A65, I think I'll just go for the sealant. I keep seeing references to Loctite 515 or 518. ANyone have any experience with that?
I did an A65 with thread and it did not leak......somewhere I saw that it was the way you were supposed to do it...It was the first aircraft engine I ever did. The thread smashes down to "very thin" I also use Permatex #2 or 3......
 

Turd Ferguson

Well-Known Member
Joined
Mar 13, 2008
Messages
5,292
Location
Upper midwest in a house
Everyone seems to have an opinion on this.
Yes they do.

My Continental A65 manual does not specify any form of sealant, gasket or thread between the case halves.
The Continental Maintenance Manual "STANDARD PRACTICE FOR SPARK IGNITED ENGINES" has guidance for threading engine cases. The publication number is M-O.
The section on crankcase threading specifies "a continuous, single piece of grade “D” silk thread" P/N 641543. I have seen many people run a double strand but that is not authorized according to the maintenance manual and I verified that with a TCM Tech Rep. Single strand only. Sealant type is also specified.

Interestingly, the manual gives a pictorial diagram on how to "thread" the cases on the C75, C85, C90 and O-200 but nothing specific on the A-65. I asked a TCM tech rep about it at an IA conference why the manual does not specifically include the A-65 and he would only say "the practice applies to all four cylinder engines." (Also interesting is they DO specify "threading" the through bolt bosses at the bearings). The "unoffical" word I got was follow the diagram for the C-75. Continental apparently prefers not to provide any support the -65's anymore.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,473
Yes they do.



The Continental Maintenance Manual "STANDARD PRACTICE FOR SPARK IGNITED ENGINES" has guidance for threading engine cases. The publication number is M-O.
The section on crankcase threading specifies "a continuous, single piece of grade “D” silk thread" P/N 641543. I have seen many people run a double strand but that is not authorized according to the maintenance manual and I verified that with a TCM Tech Rep. Single strand only. Sealant type is also specified.

Interestingly, the manual gives a pictorial diagram on how to "thread" the cases on the C75, C85, C90 and O-200 but nothing specific on the A-65. I asked a TCM tech rep about it at an IA conference why the manual does not specifically include the A-65 and he would only say "the practice applies to all four cylinder engines." (Also interesting is they DO specify "threading" the through bolt bosses at the bearings). The "unoffical" word I got was follow the diagram for the C-75. Continental apparently prefers not to provide any support the -65's anymore.
I've used the thread on my A-65 and others and it works well. I would avoid using it around the bearing boss bolts.

The case will flex quite a bit, allowing the bearing bosses to seat and the thread will crush so much you can't measure any gap between the case halves at the flanges. Worrying about the thread opening the bosses is unfounded.

Sealants: Never use RTV or any other rubbery stuff. It extrudes into the case, breaks off, finds its way past the coarse pickup screen and can damage the pump. I, like Pops, have used the Permatex Aviation Form-A-Gasket for a long time, and it works but can harden and crack with heat and time. I've seen old Continentals leaking along that seam due to aged sealant. Permatex Hi-Tack is a more modern sealant I've used sometimes, too. The thing to remember is that the thread, saturated with sealant, is the seal, and any more sealant that that is wasted and can get into stuff it has no business being in. One of those situations where if a little is good, more is NOT better.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
8,396
Location
USA.
The best and most experienced engine builder I know (former QC inspector at Lycoming) uses a case sealer called Hylomar. He says it is absolutely unequivocably 1000% certain the best you can use.

Read the description and raison d'etre here:

https://www.newmantools.com/chemicals/hylomar.htm
$72 for 3.5 oz tube, not cheap, but looks like it would be perfect for the valve cover pans where you open up every 25 hrs.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,666
Location
Memphis, TN
Hylomar is good stuff. Auto store version is about $10. It was invented for Rolls Royce.

A lot of this is using stuff that has a lot of the characteristics of each other. They are thin, sticky, and don’t harden all the way or at least for years. Looking for perfect, there is no perfect. The sealants used here have to fill pores plus a couple of microns. Thread is a dam as much as a spacer to allow just little more sealant but not so much to change mechanical properties of the case.

There is a plane on my ramp that has orange rtv on the case halves. At least your not going to be that guy.
 

Marc W

Well-Known Member
Joined
Mar 31, 2017
Messages
534
Location
Colorado
I have used Hylomar for probably 20 years on auto engines. I largely replaced Permatex #2 with it and I have been a Permatex disciple since I was growing up on a farm in the '50's and '60's. Hylomar is pretty good stuff but it isn't the endall of engine sealants. One place I have found where it comes up short is on large pipe threads. I believe that is because pipe threads often don't match well. They just aren't cut that precisely. Hylomar seems to work fine on 1/8" NPT and 1/4" NPT but for anything bigger I use Permatex. I believe this statement is key to its use: "This makes Universal Blue a particularly effective sealant on close fitting large flanges". The important words are underlined.

I tried Loctite 518 on the cylinder spigots on my VW when I did a valve job. So far, after 40 hours, they are one of the few joints on that engine that are not weeping oil.
 
Last edited:

Toobuilder

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 19, 2010
Messages
4,904
Location
Mojave, Ca
I used fuel tank sealant with less catalyst than specified. My local engine shop calls it "brain matter" (the resulting color is gray) and swears by it. One of the first aircraft engines I've seen that doesn't leak.

The better option in my mind is to have a O ring grove machined in the case such as done by Ly-Con.
 
Top