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MG Midget and Sprite General - upgrading armstrongs

Hi there

I have been reading a bit about the armstrongs.... I am not in for a FL upgrade (yet), so I want to improve the armstrongs first. As far as I know there are two options:

- thicker oil
- modifying the valve (spring / washers).

What to do?

PS Is it indeed correct that these units lose oil and have to be refilled once in a while?

a.o. arnold

No they should not leak oil.
I guess if the oil is 20 or 30 years old it might need replacing.
If you want to tinker it's a case of trial and error with washers, springs etc.
IMO it is far better to use the experience of those that have done the research and testing already, and simply buy rebuilt 30% uprated ones from somewhere like Peter May or Classic Car Clinic.
David Smith

in answer to your questions, here are links to a couple of articles about adjusting dampers:

http://mgaguru.com/mgtech/suspensn/fs119.htm

http://mgaguru.com/mgtech/suspensn/fs122.htm

the first article shows how they work, and how adjusting the washers will only make a few % difference. In order to get, say, 30% difference in damping rate it is necessary to change springs.

The second article shows modified dampers with the ability to adjust for racing. These are made by Peter Caldwell at World Wide Imports.

Talking with him about Armstrongs, he said that putting thicker oil into a damper in an attempt to raise damping rate can cause them to fail internally.



hope this helps,
Norm
Norm Kerr

Norm is exactly right. Don't use any oil thicker than a SAE 20 weight hydraulic oil in the shocks. The damping rate is internally controlled by the spring loading and size of a valve assembly. You can get a stiffer valve assembly that will help a lot, but heavy oil may cause the shock to almost lock hydraulically and cause the rocker piston assembly to strip the splines on the shaft resulting in a not functioning shock.
As for the shocks leaking oil, they should leak a small amount. It's the design of the shock to use some of the internal oil to lubricate the bearing surface between the shock body and the shaft, but it shouldn't be damp at the seal area. The original seal is only a type of packing material and not a modern lip type seal, so some leakage is expected. Peter's rebuilds use a special sleeve on the shaft and a true lip type seal and delrin bushings on the shaft so should not leak and are guaranteed for life.
I installed a set on my car last winter, best upgrade I've done so far. Selected the uprated valves be installed and the suspension in front is firm but not harsh.
As for the valves, Moss sells an uprated valve, but Peter said the heaviest valve Armstrong used was the same as used on the TR6 rear shocks, so if you can find a set of those you can swap the valves and have the heavy duty valves in your Midget shocks.

B Young

>>> and cause the rocker piston assembly to strip the splines on the shaft resulting in a not functioning shock. <<<

And keep in mind that those splines are what hold the lever arm in the damper body in the first place. I've seen photos of damaged shockers where the lever pin has pulled out of the body. In fact, I think somebody posted such a pic on this BBS not long back.

-:G:-
Gryf Ketcherside

Gryf,

I don't think that's quite true regarding what holds the arm in. Hopefully Peter Caldwell will confirm but my recollection is that there is a gap in the centre of the spline on the arm shaft and the centre of the rocker link, which operates the 2 conrod and piston assemblies, is staked into that gap to retain the shaft. I'm not saying that excessive load can't cause spline wear and subsequent wear allowing the assembly to come loose.
David Billington

David, you are correct, but when the splines strip and the rocker is allowed to rotate on the shaft that staking is also striped so the shaft is no longer retained in the housing. Peter said it's most usually a problem where the shaft starts to come out of the housing on rear shocks since there isn't anything connected to them that would place any restriction on that movement. On a Spridget front the restriction in movement fore and aft is controlled by the lower trunion and upper trunion, so very seldom does he see a front shock come apart. What he does more often see as a result of too heavy an oil being used is damage to the pistons themselves. In some shocks there is a small ball check valve incorporated into one of the pistons, he's see those broken out and driven into the sides of the piston bore because of heavy oil.
We just had an impromptu tech session from Peter at our Midget 50 while he was changing the front shock on a car at the event. Even had his collection of broken shocks and cut aways on hand to show us the various types of damage and how the shock worked.
B Young

Bill,

The collection of broken shocks would be interesting to see.

I'll have to admit to filling some on an A40 Farina I had with EP90, perceived wisdom at the time was that was OK late 1980s in the UK, and that sorted the shocks right out. Maybe the shocks had some other issue but when I bought the car it had no discernible front damping even though the dampers were full of oil. While poor front dampers, read play, on my frogeye and poor cheap replacements led me to design what became the FL telescopic units the dampers on that A40 seemed perfect, no play and they didn't leak but also didn't damp worth a damn until I added the heavier oil so I can see that in good condition they can work well. Must have bee some issue with them but we'll never know as I sold it long ago.
David Billington

Thx guys. If I may summarize: the dampers lose oil, I never looked at them, so taking them out and fill them with penrite shokck oil is not a bad idea to start with....?

I thinks that changing the springs etc is gonna be a wilde guess for me if it works out
a.o. arnold

Topping them off can't hurt anything as long as you don't over fill them. Leave a bit of room for expansion as they heat up in use. You should feel some resistance to lever movement, but you can't actually judge the condition of the shock by that amount of resistance as it's just oil passing through a bypass valve and orifice that is not calibrated, so may differ from shock to shock even though both work well on the car. What you shouldn't feel is an area of movement, usually at the center of travel where there's no resistance at all then resistance comes in. That's a sign usually of a loose rocker on the shaft as I remember from what Peter said.
Here's a photo Gryf took back in 2008 of Peter's tech session at Spridget 50 with his collection of old and new shocks on the table. We got to see what made the Armstrong shock work and many examples of damaged shocks.

B Young

Would love to see that on youtube in a John twist like video.....
Onno Könemann

Bill,
'they should leak a small amount. .... but it shouldn't be damp at the seal area.'
For a layman like myself can you explain how they can leak but not show it at the seal area, do they leak somewhere hidden? I know that shock oil does not evaporate with no trace like petrol (gas) does, it leaves an oily residue and runs all over the place.
If they DID leak, and it was visible, the British MOT tester would fail it, or at least issue an advisory.
I have never seen a visible leak, and have never had to top up shocks, in many years and many 000's of miles.
David Smith

David, according to Peter the shock is designed to keep the shaft to housing contact area damp with oil and since the "seal" is just a jute type packing it will absorb some oil and could leak some slowly over time. As you say, it's usually not a visible amount and since the shock is filled with more than enough oil to function some loss won't affect the operation so most should last for many years without need for topping off. The problem with the jute seal is because it absorbs oil it also attracts dirt which then can cause wear on the shaft in the seal area and cause more leakage. Peter replaces the jute seal with a lip type seal that won't leak and attract dirt.
If the area around the seal is damp or has a lot of road grime adhereing to it then the seal is leaking too much. The front shock on my car in the photo shows the accumulation of residue and dust from a leaking seal as well as some oil I spilled in that area, one reason I bought rebuilt units from Peter.

B Young

This thread was discussed between 01/08/2011 and 02/08/2011

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