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MG MGA - Piston damper length


I've got the old dampers in my carbs--length .387". I want to cut them down to .308 per the factory CSM. What's the best way to do this--grind, file, sand, cut? The CSM says to mill it, but I don't have a milling machine. It seems that the biggest problem is how to hold the piston without scarring/scuffing it up, unless I use my fingers, which I don't fancy.

Any suggestions?

Thanks!
JM Morris

JM,
If you are just reducing the length by.079", a lathe would be the obvious answer. How critical is the length? If you need to be within .001" then a lathe or milling machine is just about your only option. Do you know anyone with a lathe? This would be about a 10 minute job if you had to pay to have it done. You could use a grinding wheel but you would have to be very careful and measure often. What does shortening the piston do for you? Have a good day!

John
John Progess

There is a factory service memorandum for the mga twin cam on shortening the SU damper piston (attached).
I do not know if there is also one for the pushrod engine.

Mick


M F Anderson


Thanks, John and Mick. Shortening the dampers apparently greatly improves the idle and running characteristics, according to the csm on Barney's site. I'll have to check again to see if it was for twin cam only. I don't recall that--don't think so.

Thanks!
JM Morris


Wow! Steve, you are right! That csm is for twin cams only! You have saved me a lot of grief! Thank you so much!!

--Jack
JM Morris

Jack

Assuming from your other thread that you are talking about the MGB's HS4 carbs, why were/are you trying to apply the MGA H6 measurements?

Steve
Steve Gyles


Steve--see my reply on the float level setting thread.

Thanks for keeping me out of trouble!

--Jack
JM Morris

I can tell you that a lot of standard H4 cars had the "O" marking that means shorter damper pistons - far more than could be accounted for by random TC parts floating about.

"greatly improves the idle and running characteristics" is a stretch, at least for most cars. It does change throttle response characteristics a bit, and an unmatched pair will give odd results. There are corresponding different length dampers for HS carbs too.

More to the point is having the vented dampers on unvented chambers, and unvented dampers on vented chambers.

FRM
Fletcher R Millmore

I am about to machine the pistons (as above) but not sure if I should. I use the original 1600 carbs, inlet, etc. on the 1800 head and the block is bored out to 1900.
Is there an advantage, is this okay, or is the modification only for the Twin Cam?
Barry
Barry Gannon

Barry -
As I said above, there are far too many (original that I worked on when new/unmolested) cars with "O" marked dampers for this to be a TC only thing. Frankly I seriously doubt that there will be any difference at all, except for possibly quicker initial throttle response. But it could just as well be worse! Even the factory was fumbling in those days, as evidenced by the "float bowl spacers" Barney refers to, now sold as "speed secrets".
In theory and my experience, the dampers do absolutely nothing in steady state operation. It's possible that certain engines with lumpy cams could have pressure pulses that cause the pistons to "bounce", in which event the short dampers might matter. That might be what was happening with the TC, but the float bowl vibration was likely the real problem here too.
On emission control engines with lean setups, the longer undamped travel can give a little "bog". Most earlier and/or modified engines will be set up rich enough - usually too rich - and won't have it, so the response is likely to improve a bit. I had a late (converted from ZS to twin SU) B with two different damper strokes, and experimented with this. Mismatched ones gave hesitation on accel, and on this particular lean set engine it preferred two short stroke (long piston) dampers. As stated, there are two different stroke dampers for HS cars, and the original and "O" marked ones for H series. And I've seen apparently factory short piston/long stroke dampers with spacer washers to restrict the free movement to the earlier spec. That's your fallback if the mod doesn't work.

FRM
Fletcher R Millmore

Thanks FRM for your analysis. I think I might leave well alone as the motor sounds and responds just great at present and the little b%@@#*s are impossible to hold without damaging!!
I will be interested in other opinions!
Barry.
Barry Gannon

This thread was discussed between 20/04/2010 and 24/04/2010

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