# Sealing crankcase halves.

Discussion in 'Firewall Forward / Props / Fuel system' started by PTAirco, Sep 11, 2019.

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1. Sep 11, 2019

### PTAirco

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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. 2. Sep 11, 2019 ### TFF ### TFF #### Well-Known Member Joined: Apr 28, 2010 Messages: 11,425 Likes Received: 3,170 Location: Memphis, TN I found some silk thread from Joann’s and used the permatex equivalent of perfect seal from autozone. 3. Sep 11, 2019 ### PTAirco ### PTAirco #### Well-Known Member Joined: Sep 21, 2003 Messages: 3,447 Likes Received: 980 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. 4. Sep 11, 2019 ### TFF ### TFF #### Well-Known Member Joined: Apr 28, 2010 Messages: 11,425 Likes Received: 3,170 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. 5. Sep 11, 2019 ### Pops ### Pops #### Well-Known Member Joined: Jan 1, 2013 Messages: 6,920 Likes Received: 5,835 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: Sep 11, 2019 proppastie, BoKu and Daleandee like this. 6. Sep 11, 2019 ### BoKu ### BoKu #### Well-Known MemberHBA Supporter Joined: Aug 16, 2013 Messages: 2,248 Likes Received: 2,800 Location: Western US 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. 7. Sep 11, 2019 ### TFF ### TFF #### Well-Known Member Joined: Apr 28, 2010 Messages: 11,425 Likes Received: 3,170 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. 8. Sep 11, 2019 ### proppastie ### proppastie #### Well-Known Member Joined: Feb 19, 2012 Messages: 3,655 Likes Received: 1,054 Location: NJ you put the thread around the outside edge flanges not the bearing journals. Lycoming calls out "oo" silk thread. 9. Sep 11, 2019 ### Daleandee ### Daleandee #### Well-Known Member Joined: Sep 11, 2015 Messages: 852 Likes Received: 514 Location: SC I knew I was doing something right! This really is good stuff. Dale N319WF BoKu likes this. 10. Sep 11, 2019 ### PTAirco ### PTAirco #### Well-Known Member Joined: Sep 21, 2003 Messages: 3,447 Likes Received: 980 Location: Corona CA 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? 11. Sep 11, 2019 ### rmeyers ### rmeyers #### Active Member Joined: Dec 20, 2013 Messages: 25 Likes Received: 22 Location: Denver, CO 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. 12. Sep 11, 2019 ### proppastie ### proppastie #### Well-Known Member Joined: Feb 19, 2012 Messages: 3,655 Likes Received: 1,054 Location: NJ 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...... akwrencher likes this. 13. Sep 11, 2019 ### Turd Ferguson ### Turd Ferguson #### Well-Known Member Joined: Mar 14, 2008 Messages: 4,662 Likes Received: 1,627 Location: Upper midwest in a house 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. 14. Sep 11, 2019 ### Dan Thomas ### Dan Thomas #### Well-Known Member Joined: Sep 18, 2008 Messages: 4,785 Likes Received: 2,016 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. BoKu likes this. 15. Sep 12, 2019 at 1:06 AM ### TFF ### TFF #### Well-Known Member Joined: Apr 28, 2010 Messages: 11,425 Likes Received: 3,170 Location: Memphis, TN Hi Tack is the same chemistry as Perfect Seal. One red, one yellow. 16. Sep 12, 2019 at 6:34 AM ### Victor Bravo ### Victor Bravo #### Well-Known Member Joined: Jul 30, 2014 Messages: 5,872 Likes Received: 4,701 Location: KWHP, Los Angeles CA, 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 proppastie and akwrencher like this. 17. Sep 12, 2019 at 2:08 PM ### Pops ### Pops #### Well-Known Member Joined: Jan 1, 2013 Messages: 6,920 Likes Received: 5,835 Location: USA.$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.

18. Sep 12, 2019 at 2:59 PM

### TFF

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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.

19. Sep 13, 2019 at 2:24 AM

### Marc W

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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: Sep 13, 2019 at 4:59 PM
20. Sep 13, 2019 at 2:59 PM

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