Rear crank flange coolant leak.

[climb4hope]

A few months ago I bought 2001 Allroad with blown engine.
Finished rebuild a few weeks ago. This weekend put everything back in the car. After filling the coolant got a paddle of it coming down between the engine and transmission. I could not see any leaks from above and therefore had to pull the engine back.
While building the engine I decided to go cheap with the rear main seal and do not replace the entire flange like Audi recommends. I replaced only a seal and cleaned up the flange from the old gasket. Installed new gasket and this is it. It turns out that the leak was from the top and middle left bolts on the flange. I was surprised because the leak was major, not a tiny bit of dripping. And visually I couldn't tell that something was wrong. Removed the flange and it turns out the flange was not perfectly flat. It had 0.5mm gap right near the middle bolt where the coolant groove in the block ends.

Lesson learnt: always check the flange for flatness if reusing the old one.

Question: would you use gasket sealant with the new flange (think layer on both sides of the gasket) or just the gasket itself?

[climb4hope]

Here's the location where the coolant leaked:

https://s23.postimg.org/lks99k0tz/coolant_leak.jpg (https://postimg.org/image/lks99k0tz/)

[spacecadet]

just the gasket no sealant

[Grainreaper]

At least it was difficult to get to right? I've found over the years as a mechanic that things are more likely to go wrong if it's difficult to get to the problem spot. Long ago I was doing a clutch job on a Mitsubishi 3000GT VR4, got it all together, fluids filled in the trans, transfer case etc, tried to start it and just heard a spinning starter.... A side glance to the bench revealed the ring gear was still on the old flywheel, the new one didn't come with a gear on it and I'd neglected to swap it over in my rush to get the job done... I just closed my toolbox and headed home for the day, tackled it the next morning.

[glennda5id]

Did you follow the proper torquing sequence bolts?

Generally you should not use RTV sealant with a gasket. Although you can use Loctite or Permatex HiTack gasket sealant which I really like with paper and rubber gaskets. It is a thin layer that you spray on and let dry on the gasket then you install the gasket.

[climb4hope]

just the gasket no sealant

So, would put a sealant only on metal to metal surface and not metal to gasket? Any specific reason besides the precaution you have to take to make sure that sealant is not going to be squeezed in and possibly get some loose pieces inside?

[climb4hope]

Did you follow the proper torquing sequence bolts?

Generally you should not use RTV sealant with a gasket. Although you can use Loctite or Permatex HiTack gasket sealant which I really like with paper and rubber gaskets. It is a thin layer that you spray on and let dry on the gasket then you install the gasket.

The reason I used the sealant is that a reused metal surface is never perfect as it comes out of the factory. I did the same on the water pump a few times and it worked just fine. If the flange is new, probably I can use the sealant only on its bottom where it contacts lower oil pan metal-to-metal. I haven't tried spray gasket sealants yet. What I do with RTV is simply take it on my finger and smooth it evenly over the gasket surface to create a thin layer, then put the gasket on the flange with the sealant on and put another thin layer on the other side before installing it on the engine.

The sequence of the bolts was right, but honestly I overtorqued them a fem Nm, not intentionally but at least evenly. I was surprised though that in the area of the leak the sealant didn't stick to the block surface well. Maybe I did a poor job with degreasing its surface perfectly. Should get a new flange in a few days.

Thanks for the advise.

[climb4hope]

At least it was difficult to get to right? I've found over the years as a mechanic that things are more likely to go wrong if it's difficult to get to the problem spot. Long ago I was doing a clutch job on a Mitsubishi 3000GT VR4, got it all together, fluids filled in the trans, transfer case etc, tried to start it and just heard a spinning starter.... A side glance to the bench revealed the ring gear was still on the old flywheel, the new one didn't come with a gear on it and I'd neglected to swap it over in my rush to get the job done... I just closed my toolbox and headed home for the day, tackled it the next morning.

Was not on the first place, because I had the engine out anyways for the rebuild. However it was so unfortunate to put everything back together and then realize that I have to pull it out again. At least pulling it out on the fresh memory of all bolts, connectors and clips took only 2.5 hours the next day :) Now have to repeat the process of putting everything back together once I get new flange and put it on.