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There's a number of options if you change over to a disc brake rear axle. You would want to install a rear brake force regulator if you did that. Ratios 3.75 and 4.08 ( drum brake axles ) or disc brake axles 4.08 3.92, 3.69, 3.46 and 3.27 come to mind. I wouldn't go too high in gearing and stick to 3.92 and lower gear ratios for a heavy sedan with a low HP engine. I think the 230S came with 4.10 ratio or maybe 4.08 as standard equipment.
 

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Discussion Starter #3
Thanks for your reply ,can I put a 280se diff in my 230s and what year 280 se would fit, I am aware the ratio would help with the high reving and give me rear discs with a new brake regulator . It is possible to buy a 280 se car for less than a new ring and pinion for my 230s. Sorry, the whole reason I ask these question is because my diff is making a whiring noise
 

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Any year of 108 would work. Stay away from V-8 cars because the gearing would be too high for your application. A ring and pinion change isn't a good idea because you will need to take the diff apart and then you will need to be able to set the back lash and without the proper tools and know how it won't happen for you - this NOT an easy DIY job. So yes, you should be able to find a rusty car for 500 or less which is less than half of the price on a gear set.
 

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Discussion Starter #5
Thankyou doc ,you certainly know your stuff.As you know I am looking for a good second hand diff to put in my 230s but still not sure what year models would fit .You say any model w108 but are there any other models that would do the job.Can you give me a list of w and year models that would do the job .Many thanks for your insight and experience .
 

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All 250S and SE sedan or coupes, all 280SE sedans except 3.5 and 4.5. from 1967 until 1970 should work. I do axles in my shop. Ideally, a 3.92 would work well for you.
 

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Discussion Starter #7
Thankyou for the info ,just have to find a car with a good diff now ,is it possible to use a keyhole camera to look in a diff through one of the drain plugs to see if the crown has any rust spots without pulling it apart. Most of these cars have been sitting around and seem to absorb condensation and rust
 

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Rust inside old diffs never seem to be a problem. The outside case will rust but I've yet to see any real amount inside. There are a number of tests we do before I even take one apart or even consider one for transplant. You would want a healthy person before accepting an organ donor, yes?

It's very possible to install a used one that's worse than the one you have right now.
 

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Discussion Starter #9
Have taken some photos of my ring gear and it shows rust pitting on about a third of the teeth and we think this causing the whiting sound. The pinion has no rust pitting.can a chemical reaction occur inside old diffs that have been sitting around for years
 

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That ring gear is damaged for sure. Looks like rust did that but it's not on all surfaces so I'm not going to say that's the only reason.
 

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My Ponton converted form Diesel to petrol has a diff ratio of 3.7:1 and pulls very well top. It may be different story with full load. I think it's around 20 MPH per 1000 RPM in 4th gear
 

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That sounds about right. Diesels usually have a bit higher gearing because they have more torque at the same RPM as a gas engine car.
 

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Discussion Starter #13
Still working on the diff and have found a replacement,will I notice much difference between a 3.92 and a 4.08 diff as far as high revving goes.Are there any tests I can do on a second hand diff before purchase or is it pot luck
 

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There are a number of tests and things you can check before installing an axle. Hold each axle shaft from turning and then move the pinion flange back and forth only as much as you can feel any play. If the movement is small, say less than 3 mm as measured on the outside edge of the flange you should be OK for back lash on the crown and pinion.
Now hold the pinion flange and the left axle shaft steady. If you have any excess movement on the right axle it could be something broken inside the diff. Save yourself a whole bunch of grief and move on if you can't identify the problem.

A change from 4:08 to 3:92 is 160 less RPM at the same road speed. It's not a lot but it is noticeable.
 

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Discussion Starter #16
Something I foregot to tell you guys is my merc is an auto.With the 4:08 diff that is standard 1st gear doesn't seem to get used and when on the highway it revs high in top gear and feels like it needs another gear.Sorry if I'm backtracking but i want to get this right!
 

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I had my first Ponton in 1961, it did not feel under geared then as all cars were like that. Some UK cars had an overdrive as an option.
 

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Discussion Starter #18
got onto a 250se diff 1968 ,does this have a 3.92 ratio and I'm confirming this will fit my 230s 1966 but I will need to change the brake master cylinder.
 

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Diff ratio is stamped on the bottom side of the main central casting. Looking from the back of the axle, it's on the bottom left side near the front on a flat surface. You might have to scrape the area clean to see the stamped in numbers. It should read 3.92 and then any numbers after that are serial.
You may not need to change the master. I didn't in my 230SL but I did add a brake force regulator to prevent the disc brake axle I used from locking up during heavy braking. The back of the car will become light enough that the rear wheels will quit turning. Good way to end up in the pumpkin patch, upside down, wondering what the hell just happened.

Later 108 cars didn't use brake force regulators and had a different MC. Maybe someone knows a bit more about this.
 
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