MadaboutKitcars

I made a leap of faith and ordered the Dual 7" servo from Ducksville yesterday, and it arrived at 9am this morning.
The above shows that the trial fit is very good news. The studs attaching to the pedal box are on 85mm centres compared to the Metro ones at 90mm, so only minor hole fettling is required to make them fit.
The maximum diameter around the seam is identical to the Metro servo, so it fits the space available between engine and side panel.
The length front to back is 2" longer than the Metro, but the secondary chamber is 1" smaller in dia., so remains inside the bonnet line.
I am reasonably confident that after I have made a new adaptor plate and extension pushrod between servo and master cylinder it will still fit under the height of the bonnet.
And if I can get away without new brake pipes it will all have been so easy its untrue.
I am well chuffed - Thanks Jason for a very good spot!

Well - now we know it fits. All we need now is an on the road assessment of performance!

I may have to get one myself. I might have to splash a little extra for a chrome one though, to bling up the engine bay.

Hey Mike that is good news for all of us with doubts about brake performance. What about the connection between the Servo and pedal - does that look OK as well?

Cheers

Robin

Jason/Robin

You are welcome to borrow mine if you would like to "try before you buy". There will be a bit of work as it will need a new adaptor plate, and push rod conversion, and probably an extension piece between servo and cylinder?
I will make permanent alterations to mine as I do not like Marlin's double pivoted extender, but I can do all that later if you are up for giving it a test?

I shall not have my car on the road for a few months yet, so I'd be interested to see what benefit you guys who have your cars on the road feel it gives. Let me know.....................


I thought about Chrome but ................American chrome? They do a polished Stainless one though.
Mike

Robin

I will post some photos of the new Dual 7", but it will require some adaption to make it fit the pedal. I plan to weld an extension to the push rod as I did with my Metro one, so there are no potential pivot points, and I will now have a back up servo if this one ever goes wrong.

My Metro servo adapted




I would say the Dual 7" servo seems a lot more sturdy than this Metro - typical American engineering - big and heavy!
Mike

Bah! Duksville are out of stock now.

Thanks for the offer of a try before you buy - but as you're going to weld / modify the pushrod - and no two sportsters are the same... I'll get one of my own once they're back in stock and modify it so it fits Vikki.

I suspect the new servo will need a longer extension between servo and cylinder than the Metro, as the output push-rod is inset deeper within the servo. The adaptor plate will be bigger too as the fixing hole centres are wider. The push rod is threaded and more substantial, so it will be easier to make this rigid, yet adjustable, by making a threaded extension with a fixed yoke. This will be better for adjusting/setting the final position of the brake pedal after driving it for a while.

Wow - seeing the metro and the dual side by side really reinforces how much more assistance you'll get.

My calculations, based on the surface area of the diaphrams, assuming there is one at say 6", and one at 5", suggest around 55% more boost.
However, I read in one of the adverts the gain is only 25% more?

I'll eagerly await your proper road test!
Mike

Jason

Just a thought..... could the Sportster take an " 8" " booster on the basis the 7" is exactly the same maximum diameter as the Metro?
It would be very tight in the Cabrio, but might have been possible?

You are welcome to try mine before I alter it, as I'll be several months before I am ready to put it in for the dreaded test.

Yes, but 25% more compared to what? I'd assume that's against a stock yank wagon booster - which when you look at their engine bays (Not small), and the size of the cars (enormous), it wouldn't be a tiny metro-ish servo.

I've just found this whilst googling:

How much pressure should I be getting to the wheels with a power booster?
Typically you should expect about 1000 psi. to the wheels for a disc brake system. A disc brake system requires this amount of pressure so be careful when using a smaller 7" booster that puts out only 900 psi pressure. Use our AC2004K Pressure Tester Kit to get a pressure reading at any point in the braking system.

What size booster do I need?
For lighter weight vehicles like street rods a 7" booster will work fine for disc/drum set ups. For a 4 wheel disc system a 7" booster will not give enough assist. An 8" dual diaphragm booster is the ticket for 4 wheel disc.

Hmmm - proof is in the pudding, but I'd put our cars in the lightweight 'street rod' catagory. And does the 7" they talk about there have dual or single diaphragms?

This is interesting - it is obviously a true "dual". Even though the 7" is not a 7" diaphragm.

Therefore, as the diaphram is the same as the original Metro it ought to offer double the boost?

You didn't have to get the angle grinder out Mike...

Just rang Duksville. No more Duals until after Christmas, as if he orders them he has to get 50 at a time. Rats.

I knew I should have kept this to myself...

I am totally bemused by this now.
I found an American site showing a 7" servo as 7" across the diaphragm chamber - which is what you would expect.
Yet the Dual 7" that I have measures only 6" across the diaphragm chamber.

So what have I received?

Maybe the 8" is actually the 7", and people are taking the external diameter and selling them as such, rather than the more accurate internal diameter.

I spoke to Paul at Ducksville and he says the 7" is a 6" diaphragm. He reckons these Dual ones are used on big American Chevys etc., so it should stop a little old Marlin.

Interesting stuff, this is what happens when I don't look at the forum for a few days!!!

I have begun the process of making adaptors to allow me to fit the Dual 7" servo:



The steel backplate is made from 6mm mild steel, and turned out on a very old lathe.
The Metro push-rod, seal and nipple are from a standard servo. I plan to use the yoke to attach the servo to the pedal.

Useful tip: there are plenty of Metros in scrapyards, but removing a servo is incredibly difficult. Buy one cheap from ebay, or.......
Go to scap-yard - find metro - remove clevis pin to pedal: remove master cylinder ( 2 x 13mm easy access nuts): then use brute force and intelligence to drive a sharp chisel into the servo and smash the plunger out the bottom. 15 minutes start to finish - trying to remove the servo could take hours - if you don't give up in total frustration!..... (Like me!) Total cost of parts - £2

Had a similar experience trying to remove a Metro server in a scrapyard - the scrappie became increasingly colourful with his language
Still, we got it out in the end...

Mike. I suspect that there may be an avalanche of Marlineers interested in this upgrade. I certainly am for my disc/ disc Hunter. Keep the info coming please. You could even manufacture and sell an adaptor kit of parts for us?

I assume this is the adaptor plate between the servo and the master cylinder? Looks as if you've cut a rebate for the sealing o-ring into it...

Rally Design supply a threaded yoke - 3/8" UNF: £8 delivered
http://www.rallydesign.co.uk/product_info.php?products_id=14609