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6315 Views 13 Replies 10 Participants Last post by  Mick Barratt
Hi Folks,

I've found hylomar (the old standard blue one) to be so-so on gaskets, was wandering around the internet and found this by chance.

It appears ot indicate that it's not great as a gasket dressing but then says it's good for head gaskets ...?

I'm guessing water cooled (hence coller) head gaskets only?

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I have used Blue Hylomar on head gaskets since 1979. Never had a head gasket leak during this time. I use it on all paper gaskets and they never leak either. Petrol unions,oil drain plug, in fact, any surface that has to keep oil petrol or water in the right place. I have used copper permatex on some outer covers but it is a silicone type. It will seal if the two surfaces have no gasket.
Having used blue Hylomar this long and getting no oil or petrol leaks, it is my preferred product. Back in the 60s, we were using red and golden hermetite. Often, this was to seal parts we had wrecked by prising things with chisels before we could buy some proper tools. This was the reason old British bikes got that bad oil leak reputation. Us young ones using bicycle tools and old WW2 hammers.
i still have half an old metal tube of red hermetite.

dunno why
I once tried blue Hylomar on the crankcase webbing on my twin. Never again. Problem was hylomar doesn't set so every time the motor got warm the cases would expand and push a small bead of hylomar out, this never got sucked back in when the cases cooled - I also assumed that there was a similar bead on the inside, which worried me. You could wash the bead off or it would wear off on a ride. Eventually after say 5000km my web join started leaking. I had to do a strip and reassembly using a different sealant.
I lost all interest in the product after that. It could be good in other areas, but I have those covered with other trusted products.
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.
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I sealed my crankcase halves with Hylomar blue in 2012 and still no leaks. Originally, this joint was sealed with a clear varnish looking liquid from the factory.
It was the thing to use when I served my time as an apprentice. All the Old Boys swore by the stuff, I always ended up swearing at it! Now I use a Cycle product similar to three bond. We use it at work for cooling systems, sump pans, engine covers ect. Works really well and parts are easily separated it required. I use it on the engine covers on both my bikes, if I have to remove them I apply some paint thinner to the joint with a brush and then score lightly with a razor blade.
ive used hylomar a lot. not so much anymore because it squeezes out into the interior as little blobs like silicone and possibly fouls the oilways. great for primary and gearbox covers though

i use wellseal now. its the stock product originally used and is both a good sealant and doesn;'t harden into anything that can block the oil passages. spead it on with a little brush, let it evaporate, put the piecs together, done.

wellseal is hard to find and expensive. but i love it.

I eliminate gaskets as much as possible and use Loctite 518. A few years ago used Hylomar M on the engine cases of my T140.....Applied it very sparingly to both halves. It doesn't leak but gets very slightly damp at the joint. I spend a bit of time removing old sealer and repeated wiping with alcohol before applying sealer. I also use RTV sometimes, again, very little is needed..
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The more I hear about Loctite # 518 it sounds like stuff I had in 1980 that at least ‘ seemed ‘ to harden under pressure. At least it cured hard stayed in place 40 years. Then it came right apart ( I was worried ) and remnants chipped, peeled or scraped off quite easily.
I may try some of that to put them back together or else some of the heavier duty RTV silicone.
I used RTV on transaxle for a front wheel drive car a few years back and it held auto trans fluid without a leak.
Hi Detroit23, I've used Loctie 518 on case halfs, timing cover, both inner & outer trans covers. On pressure relief valve copper or alloy rings. The copper rings on primary cover.

Use thin coat like with an acid brush on surface only. Bone dry surface is best. Does not leak. Separates well later.

Not so good on paper gaskets or o-rings.

Some sealants work super good, but heaven help to to separate later. Some silicones will stick so strong you'll nearly or will break/distort case.
I am also a fan of Loctite 518, though it is very messy and very red! The excess on the outside of joints does not harden, and will wipe away using solvent. Also any excess inside will eventually wash away in oil, it does not form globules like silicone that will get circulating and block holes.
not many actual paper gasket out there these days -- but what i did and still do if i have to use one is impregnate the paper with silicon gasket stuff -- rub it between thumb and first finger with a small amount of the goo -- just enough to soak into the paper - avoid build up - let it dry completely before use
I actually do the same thing with most gaskets using Loctite 518. Trying to wipe jointing compound around holes and studs of castings is not especially easy
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