Just wondering if anyone has tried anodizing the SU carb piston to accommodate a better tolerance on warn out pistons with a specific micrometer thickness. Haven’t read anything was just brainstorming on ways to make it work. I imagine you could do this to the dashpot as well- may hold up longer without polish?
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In my view the piston dashpot is best left alone and cleaned on a regular basis with non abrasive and the shaft lightly oiled when assembled.
The test is in the manual where without spring you drop the dashpot off the piston (onto a cushion) and count the seconds. if it passes this test and does not bind on the shaft, it is good.
The tuning process does account for airleak past the piston and as long as the piston to dashpot clearance is the same along its run, it will tune up as perfect as possible
I suspect one can spend a lot of money trying to tighten up tolerances with little benefit given this is 80 plus year old technology!
Dennis
60 OTS
I can’t imagine that it wouldn’t work as anodising will easily give a minimum of 2 thou (I looked it up on the Internet so it must be true).
The problem in my mind would be accurately polishing the coated surface back down while maintaining it perfectly round and flat.
I reckon it’s worth giving a go as proof of concept. I doubt that anodising a single piston would cost much in comparison to buying a new matched piston and bell and it might just work.
Interesting idea. You might be able to use a fine valve grinding compound and “lap them in” to finish tolerance.
The DPO of my car CHROMED the bells. Chromed! Inside and out! They looked very pretty, but for decades, I struggled with the pistons sticking. I tried to have the chrome turned down in the two worst bells, but chrome is tough to machine. The chrome chipped off down the original surface, which had been worked before plating to make up for the extra thickness. That left me with two very loose domes and one sticky one. I finally gave up, and tracked down a replacement set. If the dome don’t fit, you gotta quit. I’m thinking of turning the old domes into desk clocks and offering them on E-Bay.
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Ah… musta been a devotee of the ol’ hot rodder’s adage: if it don’t go, chrome it!
The wonderful design of the Stromberg carbs prevent such idiocy Mike…
Ha! Very interesting! I’m rebuilding my carbs. now and was trying to think of methods to fix this issue as I struggle to get the pits out of the dash pots to polish the outside…wondering if it’s worth all the labor if I can’t tune the carbs right.
I’ve taken proper precautions not to damage the piston when rebuilding. I didn’t blast the internals but one of them settles quicker when doing the drop test - about 12 seconds instead of the 20 seconds (HD8). I’m not sure how much it really matters at the end of the day. I’m currently hoping I can tweak this with varying grades of oil viscosity? Anyone have luck with this? Really just hoping to get the car together this century and carbs running at 90% will be better than no carbs running. Maybe I can experiment with anodizing if they really can’t be tuned.
You can’t fix it with oil viscosity. The just affects the speed at which it moves to a new state so in effect controls what is called the “accelerator pump” on other types of carbs.
Why not post photos of the pits ? Are they inside or outside? If inside they are a problem.
The piston drop times should be almost identical. Are you doing it with the dashpot oil cap tightened down? And the piston inside shaft lightly oiled? You should be.
Dennis
Hi,
Here is an album that kinda shows what I did toward the end. The pits are only on the outside of the dash pots. The interior aluminum does look discolored from corrosion but isn’t pitted or changing the way it works- apart from etching mildly with some aluminum cleaner I haven’t done much. I lube the center shaft and had the caps tighted with window seal weatherstrip stuffed in the holes of the piston.
The dashpot damper lids should not have breathing holes in them. Do they?
Are all the springs the same length when put side by side?
Do all the pistons slide completely freely in and out of the dashpots when you slide them by hand and not plug the holes?
Could it be that someone has mismatched the pistons to dashpots?
It may be if you switch them around and retest, they all have the same drop time?
Dennis
I think I did try swapping them with that assumption too but I’ll try all the things you listed again to see if I get the same results. Thanks!
Perhaps it’s the way you’re doing your test but either drop time seems excessive. I do mine with damper removed, vent holes in the bottom of the piston plugged and they clock in at 6-7 seconds. Damper on, very lightly oiled but with tubes dry it’s another second or two. If you’re getting 20 seconds it might be that neither your damper caps nor suction chambers are vented when one or the other, or optionally both, need to be.
Awaiting one of the site’s SU experts to chime in.
Typically the E Type domes are internally vented and don’t use the caps with holes. The pistons shouldn’t touch the inside of the bells, so wear is impossible. If they are touching and wearing there is some other problem.
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This JCNA article provides some insight.
https://www.jcna.com/library/tech/tech0006.html
Either suction chamber or damper cap must be vented, but not both.
Edit: side by side pics of vented and non-vented bells:
A“ You might be able to use a fine valve grinding compound and “lap them in” to finish tolerance.”
Cast aluminum alloy parts can be somewhat porous and can retain fine particles of abrasive. These then wear out other parts rubbing against them. Carborundum paste vs anodized piston would only have one outcome - assuming it didn’t find its way into the inlets. Not an issue to be over-played, but abrasives and inlet tracts or engine internals do not play nicely together. Even valve grinding clean-up has me using fresh paper towels for every wipe - so to speak…
Yep, understood. Anyone unwilling to be surgically fastidious, ought not be playing with expensive toys to begin with! I think the whole idea is a long shot. But while we are all long shooting…
To deal with pits and other marks on the outside of the vacuum chamber you will need to sand them first. Mine were pretty rough, and I wet sanded with 220 grit in the roughest areas, then blended with 320 all over, then 400, 600, 800, 1000, before polishing with SimiChrome. Took all day for the set of 3, and my hands ended up pretty smooth, too!
Probably pretty blackened to. I dread polishing day.
I typically wear my industrial blue nitrile gloves when doing any icky work. Makes me look very professional 
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