I should have noted right away that GB is correct. It takes a considerable amount of pressure to reinsert the piston into the group. If this is done via the Mousetail's method, there is some risk of bending or shearing the bolts that hold the group to EITHER the boiler (Export) or the body (LUSSO). I KNOW, BECAUSE I'VE DONE IT.

I tried upward pressure supported by the base of the Export, and I bent the aluminum base where the bolts attach to the body.

The best (and only mechanically sound) way to reinsert the piston is apply upward pressure on the piston SUPPORTED BY THE TOP OF THE GROUP. THUS Mousetail's device should be modified by having a brace mounted across the top of the group to support the bottom brace.

Thanks for the warning!

peacecup wrote:I tried upward pressure supported by the base of the Export, and I bent the aluminum base where the bolts attach to the body.

I haven't tried it, but for the record, the Lusso's front face, frame, and base is heavy gauge steel, not aluminum. The group is secured with four bolts.

HB wrote:Hmm-m.

I think I'm with Dan on this one. While I can't argue with Peacecup's experience, I don't understand how the Export can withstand the compression force resulting from depressing the lever, but not with the extension force of jacking up the piston from the base.

Unless I'm making a fundamental error - or, possibly, misunderstanding Newton's 3rd law - the downforce between the group and the base while pulling a shot is the force necessary to compress the spring, since the fulcrum of the lever pushes down on the group, and "for a force there is always an equal and opposite reaction". I'm surprised that a fraction of this force in the opposite direction, resulting from my device compressing the spring only to its rest position, causes a problem. Is there something about the design of the group/boiler or boiler/base fixings on the Export that makes them weaker in extension than compression?

It is, of course, important to minimise the risk of damage to the base by spreading the load as widely as possible across it. That's the reason for using such a hefty piece of timber and cutting it carefully to fit into the drip-tray space; and you need to be sure you are pushing only the piston and not any part of the head.

My method certainly works on the Lusso. The last seal change was the easiest I've done and involved a considerable reduction in the use of Band-aids and expletives.

This simple diagram below shows where the difference in force is found between your method (A) and the c-clamp method (B). A simple modification of your device to allow the top of the group to receive the force would fix the problem. But there is certainly some force being absorbed by the group-body connection and by the body-base connection in figure A. It is whatever downward force the piston spring is applying as it is pushed up. If you were to take two photos of the group-body connection just before the procedure, and just before the piston is re-attached to the lever I'll wager you could see some of the flexing. On the Export it was obvious.

PC

Understood, but you don't address my point. If you were to make a diagram 'C' representing a normal operation of the lever, the stresses at the points indicated in diagram A would be the same but in the opposite direction. Clamping to the top of the group does bypass these stresses, but in my experience is extremely difficult because of the tiny area available to seat the top of the clamp.

Yes, the lever would assert some force, but I think the fulcrum, and the length of the lever, reduce some of this, don't they?

In any event, the boiler-base connenction on the Export is a steel bolt through an aluminum flange, and it did not take kindly to the amount of force I tried to use to re-insert the piston by applying force between the base and the piston. The flange bent, and rather than bend it back and risk breaking it I left it as is. Its only a mm or two, but its noticeable if you look for it. Sometime I hope to replace the base and case with some custom-made stainless pieces.

PC

A question: is the force required to begin compressing a spring (preload) greater, lesser, or the same as the force required to compress further an already partially compressed spring?

I quoted Newton's law and I think you've just introduced Hooke's.

http://en.wikipedia.org/wiki/Hooke%27s_Law

I'm aware that I'm beginning to sound like a physics geek.

Over my head, or a least within the allotted time.

One thing you're failing to account for is that much of the force of the lever is being applied to the top of the group at the fulcrum. This, in addition to the roller cam, absorb the force.

The simple question is does the roller cam on the lever fulcrum reduce the force that would be exerted (in reverse) in the figure A above? I suppose it does, otherwise the machine would tip over. In reverse, if you tried to insert the piston by pushing it upwards without applying counter-force (i.e. holding the machine) the machine would tip over. Yet it does not do so when you pull the lever down. So.. that force is going somewhere.

So, as the spring is being compressed, more and more force is required to keep compressing it. The force required to compress an already preloaded spring is greater than the force required to preload it.

That's why brew pressure tapers off with a spring lever: the more compressed the spring, the greater the decompression (restorative) force. As it decompresses, the decompressing force decreases.

If the chassis/bolts are strong enough to support the forces necessary to compress the preloaded spring during the shot, it will be strong enough to handle the relatively lower forces required to preload the spring.... provided the chassis is equally strong in both directions, as Bob mentioned earlier.