RTV Silicone for head gasket

[Charles uk]

RTV both sides of the gasket, moderation is the word, the head gasket is thicker around the cylinder so all you need is enough to stop any water leaking out of the head, gasket, water jacket wall interface & bed in position around the cylinder wall.

Don't worry about the heat path as long as the water jacket is clean & you have a good flow, all should be well, 2-strokes work better when warm/hot, cooling the hot parts is how the fuel particles vaporise, essential for an efficient fuel burn & you've got a water cooled head.

[Stelios_Rjk]

Yeap just like my avatar. Thin smear always!

On the head side here is what I do. I fit the gasket on the head and apply some RTV around the ring and on the gasket. After doing that you should have a red ring all around the head ring. This red ring is about 1-2mm thick. That's all. When you tight the head that spreads and seals. It may not be the proper way but I have seen good results.

About my avatar. This is just a response to C-I avatar

I use Hylomar the last period. Saves time and procedure/curing. RTV (permatex) after being applied, the head bolts should be tightened only with as much torque as your fingers can apply. NO wrench. Then wait for one hour and then turn the bolts 1/4 to 1/2 of a turn. Then you have to wait for 24hours for the sealant to be fully cured. That's the directions on that product.

What I do. I tight the bolts with a wrench until I see RTV starting to go out of the edges or until I feel that RTV and metal parts are all in contact. But if you do this you have to stop early. Before all RTV come off its place. Then I wait for one hour and apply 18 foot pounds. That is almost equal to 1/4 to 1/2 turning the bolts.

The only advantage that I saw when using RTV is that it sealed better than hylomar a cylinder with scratches that grinding couldn't remove them. Hylomar lead to a head leaking gasket and after this trick above (with RTV) I had a non leaking head gasket. (Same gasket used, annealed).

Apart from this occasion I use Hylomar as it has almost no curing time.

Of course as Charles said, since we are talking about QBs it is OK to apply RTV on the gasket as water gets in the head. What I described is what I do on centuries. On those engine the water doesn't go in the head so thermal exchange is a matter.

[Collector Inspector]

I stand my ground however. One does as one does.

No more from me re RTV. I could be regarded as a Troll if I kept up aye?

Forgive an Old Man in that the avatar remains for a while...........replaced with others but..............might come back again. (A bit like an Australian Prime Minister)

Forewarned is "prepared" and Done and Dusted is what ever anyone else thinks.

I like your reply Charles......amusing actually.

B

[Asallante]

Thanks everyone, two camps on the RTV issue makes a nice balance. I got the head channels cleared without too much effort but the block is proving a bit trickier... and its worrying that its not flakes of loose material coming off but actually small chunks She must have been laid up with salt water in her...

[Stelios_Rjk]

That happens sometimes. Don't worry unless you see something really bad, small holes that let the light come in on the waterjacket or bore's walls getting too thin.
After removing all rust/deposits it may be beneficial a rust remover based on phosphoric acid.

[Asallante]

Thanks for the help everyone... finally got the curlew rebuilt last Saturday. Quick test in the wheelie bin showed not much sign of improvement on water flow though Decided next to check the water delivery tube so split the power head + exhaust from the leg + gearbox. This was much easier than I expected; two nuts undone and a couple of taps with a wooden drift to separate. I heard that the bottom of the delivery tube where it enters the gearbox housing is prone to seizure but mine was perfectly fine and slipped out dead easy. Next took the exhaust cover off... everything looked great inside, both exhaust and delivery tube all the way to the top. Next removed the exhaust from the block and that's where the problem was; entry hole was almost completely blocked. Return hole for the exhaust cooling was pretty much clear though. So cleaned it all out with some hoking/poking, water/air flushing then put it all back together again with some RTV on top of the existing gasket. Wheelie bin test was great - loads of water flow! Almost danced around the garden in delight Following day, took it out for a few laps of the local harbour and all went great. Soooo happy!

[Asallante]

Spoke too soon... took the Curlew out for a longer test run at the weekend on open water. Ran great for about 30mins or so but then progressively began to pump less and less water. Tried back-flushing with hose when I got her home but no joy. Took the exhaust off again to check the water delivery pipe and port but all looked fine. Back flushing with the exhaust off showed much more flow from the exhaust coolant port than the entry port. So I expect something has become dislodged around the cylinder water jacket... Going to have to split it at the crankcase end this time I think so that means removal of the flywheel for the QB series. Anything I should be careful about when doing this? Also was wondering if I need to pull the head off again, can I reuse the gasket? Any help, much appreciated.

[david doyle]

Lets hope it is not a gob of sealant

Was thinking on the "inside/outside" problem and got wondering would filling the cooling galleries with melted wax letting it harden and dressing flat before applying the sealant/head eliminate the sealant in the galleries issue? The engine could be run in a wheelie bin filled with hot tap water and the the wax would melt (somewhere between 45 and 65 d F) and be gone well before it caused any trouble? (this is all speculation)

[Horsley-Anarak]

removal of the flywheel for the QB series. Anything I should be careful about when doing this?
if I need to pull the head off again, can I reuse the gasket?

My curlew does the same as yours, runs fine then flow decreases. I have not yet bothered to find the problem, put at back of shed in disgace .

I suspect a hot spot in the cylinder that creates a little steam, thus restricting the flow to the cylinder head outlet, and pushing more water down the exhaust.

It is pumping well

It could be a simple case of removing some more rust, or reducing the size of the inner exhaust water outlet.

I did also wonder if the gasket between the inner exhaust and the cylinder is leaking a little.

I normally reuse the heas gasket, if it is not damaged, I anneal it first.

Removing the flywheel on CDi motors can prove difficult, on Charles L's recommendation I made up a puller.

You will need to drill and tap the flywheel first.

I can't see any problem with a little sealant in the water galleries.

H-A

[Stelios_Rjk]

Great idea for that puller!

[Asallante]

Great idea to test bottom end pumping there! Made me smile.

So decided to tackle the crankcase removal last night. My flywheel came off with just two sharp taps of the hammer. I can see how the puller could have to be used when the wheel's tight and probably a must for more lightweight flywheels though. All cylinder block fastening nuts came off easy and the split was smooth. Thats where the problems started... Was expecting the piston to slip out relatively easily at this point but it would hardly budge. Before taking the flywheel off I had set it to the bottom of the stroke as per the manual so it was moving up/down the bore just fine then... At this point I noticed that the piston skirt had a slight flat spot at its lowest point (with engine standing vertical) and a fine crack running up to the bottom wrist pin moulding.... Not good. I managed to wriggle the piston out of the cylinder a little bit but in the end had to remove the head and tap it out with a wooden drift - piston came apart along the crack in the process. With the piston out I could see there was some corrosion in the bore where the flat spot was so it guess that's what caused the piston damage.

Has anyone else come across this problem or might have an idea how the corrosion could come about?

Also wondering if there is any way I can check the crank to ensure I have not damaged anything when tapping the piston out?

Many thanks, as always!

[Horsley-Anarak]

That was bad luck, post some pictures so we can see the damage.

Not sure who has spare curlew pistons, try John he had Curlew bits.

How bad is the bore?

The crank can be checked in a lathe with a dial gauge, or your local motorbike rebuild shop should be able to look at it for you.

H-A

[Asallante]

Didn't have much time to inspect the bore thoroughly but it looked alright apart from the little bit of corrosion at the base where the piston flat spot & crack was.

I have other Curlew parts on order from Sheridan (helping while John is away?) so have asked if they have any pistons. Costs are starting to add up though

Will post pictures of the damage tonight hopefully for all to enjoy

[Stelios_Rjk]

Hi,

I am sorry about those finds, John is here and can supply parts, you can try.

Here in Greece my friend and also member of this forum (Manos) has a piston of a curlew but it's not in the best condition. The rings are stuck. If you can't find a piston you could try this piston but it would require a lot labor to remove the stuck rings.

[Asallante]

Attaching a few snaps of the damage.

The bore looks fine to my untrained eye apart from the spot of corrosion at the base. What's the best way to clean that up?

There is a lot of debris in and around the crankcase. Would flushing with petrol & airlining be ok. If possible I don't want to split it in half.

With everything off the cylinder I can now see what needs to be done to clear the water jacket properly. Looking in from the head the water delivery hole sits underneath the exhaust port so its going to be very tricky to clear out. Everything really depends on getting this cleared though.

Any other things I should check or be wary about?

Also thinking I might need a new piston... "CU1296 - piston c/with gudgeon pin & circlips £47.54"... ouch!