Today I had a little time to move forward cleaning and fitting the steam/water taps and heating element to the boiler. McDave at Allann Brothers advised me on sealing these boiler attachments. The logic is that o-rings attach between the end of the part and the boiler plate and a hex nut seals that part from the other side. The heating element o-rings attach inside the boiler and the tap o-rings outside. These photos show the parts being fitted in place. I'll do some additional clean-up and sealing of these parts and application of Dow 111 lubricant but this is the logic of how these will go together. Some of you may find this part dull but I'm showing close-ups for anyone who needs to rebuild a Prestina and figure out what goes where. It's one thing to look at a parts diagram and another to see that something that looks like a washer is actually an o-ring.

This close-up doesn't show the thinness of the o-ring. He calls this HNBR 12.5mm. A workaround they also have in stock is 12mm (inner diameter) or part #15-2820.



Here they are in place.



The washers on the other side fasten without a washer, although McDave said a washer would be nice too. He advises re-sealing the element end with high temperature silicone gasket material. Doug advised against that but it wouldn't be exposed to boiler water (I hope)! Doug is sending me some glyptal that I'll apply to seal it since that's on the way.



The o-ring that slips over the tap is a 16mm NBR 70, part #15-2610. I'll lube this after cleaning the scale off the thread and lubing it too with Dow 111 or food safe anti-seize compound.



Here are the valve pistons cleaned and reassembled with new o-rings and bib washers.



This is one inserted into the handle fitting to create the valve body. The ends of the c-clip are seen inside the slot. Getting this in was tricky and required making sure the washer below it was flush with the left side of the gap so the c-clip could catch the groove in the piston head and hold it in place. I used a stubby, flat screwdriver to push it in.

My admonition to not use silicone sealant has nothing to do with food safety, but that it is not the right product for the job. Look at the specs for specific application...glyptal is specifically a high temp flowable insulating electrical sealant. Silicone does not have the flowable nature of glyptal. You can use silicone but you will basically just be smearing it around and not necessarily be sealing the small openings.

Same goes with the heating element seal. The schematic shows a flat seal...likely alimentary but can be Teflon. O rings can be used here but it has to be very specific. Look at your picture....when tightened, if one of the points of the hex pushes the ring out it will not seal. When you use a ring in such an application make sure it is completely contained under the hex and simply cannot pop out in some way (a thinner ring would be better here).

Now, I don't want to sound like a sabeletodo, but are those o ring names indicative of rubber compositions? HNBR and NBR sound a lot like designations for BUNA rubber. The lowest temp rating available. The material should be at least EPDM, HSEPDM, Viton is best (silicone has a higher temp rating but is often too soft for some applications). I have never seen BUNA used on an espresso machine unless it was sourced at the Home Despot by someone who did not know any better.

Thanks, Doug. This reminds me of the old Hill Street Blues adage, "Let's be careful out there."

As I dismantled mine, I found a number of melted grommets/o-rings that had to be picked off (and they were installed "recently" as in a few years ago); I'm hoping to be able to source food-safe high-temp ones, I believe they are typically red.

Using o-rings to seal the heating element would be OK if it were designed for that, e.g. early Rancilio Silvia's and other Rancilio products. That machine is not and I would guarantee you would have a easy leak path. A flat sealing washer such as you showed on the parts diagram is the correct part.

Maybe you can find a suitable one here: http://www.mcmaster.com/#sealing-washers/=jd53b0 . I would clean up those sealing surfaces with a light duty wire brush wheel or a 3M nylon mesh disk such as this:
http://www.amazon.com/3M-Automotive-MMM ... s_indust_2

I'm going to cut some out of Durometer. Just ordered leather punches for that size. Will be glad to send a set to you, Jano.

erics wrote:Using o-rings to seal the heating element would be OK if it were designed for that, e.g. early Rancilio Silvia's and other Rancilio products. That machine is not and I would guarantee you would have a easy leak path. A flat sealing washer such as you showed on the parts diagram is the correct part.

I would clean up those sealing surfaces with a light duty wire brush wheel ....

Thanks Eric.

For surface cleaning I'm using a rotary tool with a wire brush on the end. Those parts look grungier than they feel. At this point what you see is discoloration except for the threads that need some cleanup. I'll try cutting those small gaskets with leather punches. If the Durometer I've got is too thick I'll do this with a 0.07" Viton sheet. Also I'm guessing an o-ring can be securely fastened under a hex nut if there's a metal washer between the two.

McDave warns me the steam and water taps tend to turn. I'll try a 0.125" thick Durometer gasket on the same side as the fastening nut. That should hold those in place better than an o-ring.

Tried that and the Durometer's too thick and soft so it squishes like this. I post these failed attempts so others don't repeat my mistakes!



So I've just ordered a sheet of Viton half that thickness. The punches I used are these:



The punches worked for the inside diameter. Unfortunately the largest punch isn't quite big enough, so I had to hand cut the outer edge. For this application, though, that should be close enough:



I followed Doug's suggestion and painted the contact flange with Glyptal to fill in the gaps. After this photo I dealt with the bubble some of you might see and made sure the gaps are all filled. That precision work was done with a Q-tip.

what is that red sheet material ?

'90a durometer' is a measure of hardness for plastic or rubber parts,
like skateboard wheels or polyurethane automobile suspension bushings...

drgary wrote:Thanks Eric.

For surface cleaning I'm using a rotary tool with a wire brush on the end. Those parts look grungier than they feel. At this point what you see is discoloration except for the threads that need some cleanup. I'll try cutting those small gaskets with leather punches. If the Durometer I've got is too thick I'll do this with a 0.07" Viton sheet. Also I'm guessing an o-ring can be securely fastened under a hex nut if there's a metal washer between the two.

McDave warns me the steam and water taps tend to turn. I'll try a 0.125" thick Durometer gasket on the same side as the fastening nut. That should hold those in place better than an o-ring.

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Wouldn't it be better to use copper crush washers instead of rubber at the boiler-plate connections instead of o-rings? Assuming one could find lead-free copper crush washers.