Hi, My first experience with Hylomar was on Mercedes alloy V-8 front engine covers. We had timing chain wear issues & converted single row to double row chain. Factory had Mercedes branded Hylomar. I believe it was Hylomar M.
This was metal to metal surface & worked quite well. Imperative surface was oil free. Had to let solvent flash off at least 10 minutes. Coolant passages had o-rings. Was very successful. Allows easy removal of cover later.
For oil pans with out gaskets we used Mercedes equivalent of Loctite 574. This is hardening sealant. Removal of cover later is very difficult. I'd seal Triumph cases forever, but you might break case separating them.
In last several years silicon sealant is used at factory & dealership. Very hard to split cases. On tin oil pans, often you destroy pan at removal. You can easily break thin castings. Plus it gets into oil ways. Finally factory started a 1 week instructional class specifically to teach proper use of silicon without plugging oil way or damaging surfaces when scraping off old silicon.
Hylomar can migrate, that's for sure, I've observed it on occasion. I would not use it on case halves. It would probably be fine for trans covers. Since Hylomar doesn't harden it call allow gaskets to migrate. I saw this at work on the weak asbestos free water pump gaskets. Asbestos is bad for lungs, but good for gaskets & brakes. Another subject.
I've found Hylomar works quite well to stop/reduce seepage on my frame sump plate gaskets. I also works well to stop seeping on my valve cover gaskets. It can work quite well on sealing rings copper, alloy, fiber. It also can seal seeping threads such as the primary chain plug. It helps reduce seepage on primary gasket. It works much better on clean dry, oil free surfaces. It's most important to let the solvent flash off. At least 10 min. Longer in cold garage in the 40s f.
Gaskets under high clamping force such as cyl base & rocker box I don't like Hylomar as it can allow gasket to migrate. I use Loctite 574 for these. Soaks into gasket. Hardens from lack of air. Locks gasket in place. I've used this for 35 years at work to same good results. I use it on Cov-seal rocker gaskets. Will stop seeps where dry assembly allowed slight seeps. It lasts for at least 35 years. The gasket will tear apart on removal & you'll have to scrape off old gasket from both surfaces. But it doesn't leak. Leaking base nuts? Clean & paint 574 on threads, washers. Let cure 24 hrs. No more leak.
For case halves, timing cover, trans covers without gasket I like Loctite 518. Again must be applied to clean dry surface. It says can be slightly oily, but not best results then. I have never seen 518 migrate out of joint as Hylomar can. It mostly hardens, while Hylomar never hardens. Yet 518 will allow case splitting without undue force. Was designed for piston air craft motor cases which need to be split for inspection/overhaul, yet not leak.
There is no one best sealant. For best results you need to use whatever sealant is best for each joint.
Also... Did you know Hylomar Universal Blue has a 2 year shelf life? It is explained in Hylomar web site. How to read code. 2 years from date of manufacture. Look at crimp bottom of tube. First 2 digits year, next 2 month. Others are batch. Tube I got recently had 1904 March 2019. So already getting there. I don't see date code on box.
I can speak from bitter personal experience using expired sealant can give very poor results. Complete sealant failure like it never cures or hardens. Soon oil pushes sealant out. This may be part of the Hylomar failure on the motor cases?? I don't know. Aged silicon is really bad. Might seem to cure, then just leaks. Often never cures ever.
Hylomar makes many sealant products. So you must know what Hylomar product you actually have.
As far as I can tell, Hylomar blue is now called/sold as Hylomar Universal Blue. Please correct me if I'm wrong.
Don
