Conti Prestina Espresso Machine Restoration 101 (Completed and Indexed) - Page 26

#251: Post by **jonny** » June 20th, 2012, 1:09 pm

drgary wrote:...I wonder whether the choice of gasket material can also make the seal more forgiving if the material fills in surface imperfections.

Absolutely. Teflon will be much more picky about a flat surface than a fiber gasket or rubber gasket. The fiber expands to make a tight seal and rubber is just squishy as everyone would guess. Teflon just doesn't conform much. I'm using teflon on my Princess, so I'll see how that ends up. Hopefully good!

June 20th, 2012, 3:35 pm

Thanks, Jonny:

My Isomac Amica has a Teflon boiler gasket. It's quite small compared to the boiler gaskets we're talking about. It developed a leak and I had to tighten it down hard to make a good seal, but that worked. I could see it deforming a bit as I tightened it. I don't know if you'll be able to achieve that level on a large commercial machine boiler without risking stripping the bolts. Are you sure you want to use Teflon, and if so why?

#253: Post by **jedovaty** » June 20th, 2012, 4:22 pm

Why would one use teflon vs. fiber vs. silicone? I think the prior application here was for other little gaskets, and I'm more curious about the application towards the boiler gasket.

Researching the internet provides usual responses to be "because X is better", which doesn't really clarify things as you can imagine!

My guesses: teflon lasts longest, silicone good for irregular surfaces, and fiber is cheap?

Somewhat going off topic - how hot does the temp get inside these smaller commercial boilers when at 1-1.5 bar? 250F? 300F?

June 20th, 2012, 4:39 pm

jedovaty wrote:Somewhat going off topic - how hot does the temp get inside these smaller commercial boilers when at 1-1.5 bar? 250F? 300F?

... but interesting. I think I've seen references to some way of correlating pressure with temperature.* Maybe someone can point to that. Also I think I saw on a thread about Bosco commercial levers that someone was running a commercial boiler at about 1 bar to achieve the desired group temperature for espresso. That would be very similar to desired boiler pressure in a home lever like an Elektra Microcasa a Leva or a Pavoni.

* Okay. I found Dan Kehn's pointer to this site: Saturated Steam Table.

#255: Post by **jonny** » June 20th, 2012, 6:23 pm

jedovaty wrote:Why would one use teflon vs. fiber vs. silicone? I think the prior application here was for other little gaskets, and I'm more curious about the application towards the boiler gasket.

Researching the internet provides usual responses to be "because X is better", which doesn't really clarify things as you can imagine!

My guesses: teflon lasts longest, silicone good for irregular surfaces, and fiber is cheap?

Somewhat going off topic - how hot does the temp get inside these smaller commercial boilers when at 1-1.5 bar? 250F? 300F?

I chose teflon because it is cleaner (as far as removal goes). I want to be able to easily open the boiler every couple years for maintenance. I will try it out and if it can't hold a seal at reasonable bolt pressures, I'll abandon it. It has 9 bolts to hold it down, so I'm hoping it will work. I will be extremely cautious when pressure testing, mind you.

Boiler size has nothing to do with temperature so any boiler running around 1-1.5 bar will be around 250-260 degrees fahrenheit. (don't forget to add 1 bar for atmosphere when using that converter)

#256: Post by **RayJohns** » June 20th, 2012, 9:57 pm

drgary wrote:Ray,

You and Paul Pratt both use this step. Yet when I took apart the boiler I noticed casting irregularities that hadn't been smoothed out and the seal had held. It's a large flange for a boiler not under a lot of pressure. I'll think about your suggestion and check with the guy who fixes these and similar all the time. I wonder whether the choice of gasket material can also make the seal more forgiving if the material fills in surface imperfections.

It can, but don't expect *too much* from a gasket if the surfaces are not flat.

What you really want are perfectly machined, flat surfaces, a good gasket - then just use even torque so you don't bow or deform anything during installation. I don't think the pressures are going to be enough to cause any major problems, but try to get it as clean as possible.

Ray

June 21st, 2012, 12:57 am

Here's a close-up of the roughest part of the boiler flange after I wire brushed it. You'll notice a casting dip in the middle. All of this existed from its manufacturing date and the fiber gaskets didn't leak. This is very solid cast bronze or brass so I don't think it's warped or very capable of bending. The main thing is some surface imperfections. My best guess is this will hold up well if adequately and evenly bolted together with a fiber gasket. I will check with my repair tech mentor of course.

Ray, if I were to use emery cloth to polish that down, do I just bare-hand it? How many hours could that take for a boiler about a foot across and two surfaces meeting each other? I certainly don't need to glitz it up any more than necessary if it was already holding a seal.

#258: Post by **RayJohns** » June 21st, 2012, 1:44 am

drgary wrote: Ray, if I were to use emery cloth to polish that down, do I just bare-hand it? How many hours could that take for a boiler about a foot across and two surfaces meeting each other? I certainly don't need to glitz it up any more than necessary if it was already holding a seal.

I would wet the emery cloth and stick it down to a sheet of glass or something - or maybe a granite counter top. Something very flat. Then lap the entire flange against that.

Ideally this should be done on a milling machine or a cylinder head/black surfacing machine. Unless there are critical height tolerances, I think I'd look up my local machine shop and see if they can't surface both flanges and maybe knock down .020 or .040" on each side to give you a perfect mating surface to work with.

Ray

July 10th, 2012, 10:31 pm

Prestina Gaskets!

These arrived today along with the group to boiler studs and some o-rings. I sourced an extra gasket since one was available and I really whiffed on my first try at cutting one by hand. The saga continues ...

August 25th, 2012, 9:04 pm

Somewhere In Here is a Conti Prestina

I've finally cleared the calendar and am starting the rebuild. I took the parts out of storage and assembled them on my bench. But it's been so long! I know that there's a Prestina in here somewhere.

Where oh where do I start? The last thing I took apart was the boiler and everything that attached to the faceplate. I think I'll start there. So here's the faceplate in the bench vise.

What goes on there again? Here I start to gather those parts together. Most of them are cleaned up, some a bit grungy. I'm so excited to start assembling this thing I'll do the final cleaning as I go. I could get lost making all the parts meticulously perfect and lose the vision of the whole machine. That's my goal and will keep me motivated.