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How To Prepare Thermostat Mating Surface


Best Answer zero_wlv , 02 August 2020 - 02:05 PM

Thanks everyone for the advice,  it's sorted now (sort of,  for the time being anyway).

 

I spent a while sanding down the cylinder head surface with a block of 120 wet and dry.   And then with a whetstone.  It didn't seem to be taking much material off so the area around the third stud hole still looks very marginal,  but it might have helped.

 

I used Hylomar Blue on both mating surfaces and left it to evaporate for half an hour before assembling (unlike rushing it like I did before).   I also made sure there was plenty of sealant around the dodgy stud hole.

 

I used a thick cork gasket instead of the skinny blue paper ones I've always used before - might have helped.

 

I also used studs - I found the joint much easier to assemble neatly compared to the bolts I had before.

 

 

After the first 1 mile run there was a fair bit of leakage out of exactly the same place  :ohno:   - the top of the third bolt  -  which is why I'm still a bit dubious that the seal might be on a bit of a knifeedge and might suddenly fail in the future    (I was actually about to tear it apart and have yet another attempt!)

Getting the engine up to temperature must have sealed the joint though as it's been perfect since and has been on a 35 mile high speed run and stayed bone dry.

So fingers crossed,  it'll stay that way!

 

I'm sort of wishing I'd been even more persistent with the wet and dry but not sure it was making much difference anyway.

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#1 zero_wlv

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Posted 27 July 2020 - 02:30 PM

Hi everyone,

 

I've attached two pics.

The first one with the yellow arrow shows where my thermostat housing is leaking  -  coolant is bubbling through the top of the bolt where the arrow is.    The area around the gasket itself is fine.

The second photo shows the mating surface on the cylinder head  -  i.e. not very good.     I bought the engine newly reconditioned a couple of years ago but the build quality in some areas leaves something to be desired  (the supplier shall remain nameless).

The mating surface on the thermostat housing itself is fine so I haven't bothered attaching a pic of that.

 

The last couple of times I've had the thermostat housing off I've noticed how bad the surface of the head seems to be in this area,  but fingers crossed,  it sealed ok both times so I've just left it alone.

This time though I've been through three gaskets in one day and just can't stop coolant from finding it's way up that one bolt nearest the rad.

Look closely at the photo and see that the gap between the bolt hole and the channel is tiny  -  looking at it,  it's difficult to believe the gasket could be sealing properly in this area.

 

Am I right in thinking this is the likely cause of the leak?

It's never leaked before but I'm thinking if the material near that bolt hole was marginal to begin with it could have very recently chipped off.

 

How would I go about rectifying it?    Would a garage have the tools to be able to machine it down a bit to give a new flat surface,  with the head in situ   (I really don't want to be taking the head out)?

I could use no. 120 sandpaper at home but I'm guessing that's far too much to ask of sandpaper,  and it would be difficult to keep the surface even?

 

Quite why the surface is in this sort of condition I don't know  -  I've never used metal implements on it and only ever cleaned it with plastic scrapers.   I also never figured out how bits of engine paint ended up in the middle of it.

 

 

I have just realised I have some spare housing studs and nuts in a bag (brand new),  so I could try using those,  but I really can't see how different studs would help if there's a leak in the first place.

I also read somewhere that in some heads the holes are drilled straight through to the coolant channels,  and need a bit of thread sealant at the bottom,  but if this wasn't the case then the leak would have been happening right from the beginning.

 

--

 

On a side note,  what is the semi-circular indent for on the gaskets?   It doesn't seem to do anything.   I've been assuming the gaskets can go in any orientation.

Also,  I have gaskets from two different suppliers and on both of them the third hole is quite a way out.   Why is this?   They're both specified for 998cc heads.

 

Thanks in advance,

Liam

 

 

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#2 nitrodave

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Posted 27 July 2020 - 02:45 PM

You're best chance is a skim of gasket sealer on the block. It looks like it's corroded between the thermostat and bolt hole, so let the liquid gasket properly dry before running car.

 

I recently swapped thermostats and noticed the notch in the gasket and couldn't figure out what it was for either. I also noticed one hole was slightly off also. I just whacked it on, tightened it up and it's been fine.



#3 GraemeC

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Posted 27 July 2020 - 03:10 PM

The gaskets only fit one way (yes, just one out of the 6 possibilities!) - the holes aren't at regular intervals. From memory the indent goes as close to the front corner of the rocker cover as possible (although I always offer the gasket up to see which is the best orientation).

 

A cork gasket may be more effective as it would squash into that corrosion


Edited by GraemeC, 27 July 2020 - 03:10 PM.


#4 zero_wlv

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Posted 27 July 2020 - 03:55 PM

Thanks for the suggestions,   I'm gonna try using a cork gasket with Hylomar on both sides.   I might use thread sealant near the bottom of the bolts just in case one of the holes does go straight through.

Just a few questions:

 

1.      I've been using electrical contact cleaner to clean the surface with.   Is this ok?   Or would WD40 do?

2.      As for "let the liquid gasket properly dry before running car"   do you mean I should assemble everything and bolt down and then leave for an hour or so before running the car?

3.      Haynes says 11Nm torque,    my torque wrench doesn't go that low so I'm guessing this is say a quarter of a turn tighter than finger tight?

4.      Is there any advantage to studs over bolts?   I have both in the garage so can choose either.    

 

 

Apologies for the string of questions but I got to my wits end with it last night,  so hopefully will get everything right next time!

I've just spent weeks doing tonnes of electrical work and tidying up other nagging issues,   this is the final thing left that is stopping me from actually driving the car!

 

Thanks,

Liam



#5 Ethel

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Posted 27 July 2020 - 04:16 PM

Is the bolt hole through to the water jacket? Put sealant on the threads.

 

You shouldn't have a problem getting a gasket to seal, but you could dry fit without a gasket and look for gaps with a feeler blade thinner than the gasket.



#6 nicklouse

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Posted 27 July 2020 - 04:29 PM

personally i would bin the bolts and fit studs and nuts,



#7 GraemeC

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Posted 27 July 2020 - 05:03 PM

Nip the bolts/nuts util you can just see the cork gasket start to compress, making sure it is even all the way round.

 

If you use bolts, ensure there is sufficient thread that they don't go tight into the head before the housing is pulled down.

Personally I prefer bolts here - each to their own.


Edited by GraemeC, 27 July 2020 - 05:09 PM.


#8 Moke Spider

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Posted 27 July 2020 - 07:05 PM

Maybe try a better sealant here like 3bond ?  It's a little thicker and better than most. I've found the cork ones may work for a while, but as the squash up, they don't seem to bounce back and so in time leak again, though you might have better success here.

 

I too prefer bolts here and I lightly grease them every time I fit them. My thinking is at least with bolts, when you do get corrosion between the bolt and the housing, at least by turning the bolt you have some chance of loosening up that grip the corrosion has on the shank of the bolt when removing it. I'd say 1 in 3 that's had studs I've ended up breaking the housing trying to get it off.



#9 Revd

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Posted 27 July 2020 - 08:26 PM

For troublesome joints I would choose studs over bolts, non-locking nuts and plain washers. Check both mating faces with a straight edge, the housing could be bowed, in which case replacement is the easiest option. It might even be bowing when tightened -If the head is not true then you are more limited with what you do in situ. However with a scraper remove all traces of the old gasket, degrease and sparingly apply some sealant such as Wellseal to both sides of the new gasket. KAD sell a thick gasket and stainless stud kit. DSN and MED offer a billet thermostat housing. when it comes tightening 11nm isn't a lot but go beyond this and you need to have a good feel for when enough is enough. 



#10 imack

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Posted 27 July 2020 - 08:56 PM

Personally I like the thick blue paper gaskets from minispares. As mentioned above, I find the cork gaskets just keep on compressing and oozing out the sides of the housing.

#11 zero_wlv

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Posted 27 July 2020 - 09:28 PM

Thanks everyone,  I'll have another go at it on Wednesday when my cork gaskets arrive   (I'd usually avoid cork gaskets as imack says,  but the three I tried on Sunday were all the blue ones from Minispares so might as well try something different).

 

Out of curiosity,  can anyone explain why my head looks different to most A-series heads I see photos of?     Most heads look like the blue one below,   whereas mine (the red one) has a huge continuous machined region extending from the thermostat area.    What does this tell me about its origins?   Or is it simply that it's been heavily machined and reworked when it was reconditioned?    The supplier told me it was a 12G940 if that makes any difference:

 

 

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#12 mini13

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Posted 27 July 2020 - 09:37 PM

a buff over the area with somthing flat and some wet & dry wouldnt hurt, also the thermostat housings I give a quick lick too, a breezeblock garage wall is supprisingly good for this.



#13 Moke Spider

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Posted 27 July 2020 - 11:26 PM

Your 12G940 Head is just an older type of casting. Nothing in the world wrong with them.



#14 Ethel

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Posted 28 July 2020 - 08:04 AM

Anorak mode: the raised detail first appeared on Metro Turbos, the casting was redesigned to improve cooling. It was later adopted for all 1275s, but the difference isn't really that great over earlier heads.



#15 Dusky

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Posted 28 July 2020 - 10:37 AM

I Only use loctite blue sealant these days to fit thermostat housings, dont use a gasket anymore. I'd go over that with an oilstone first though




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