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Home > How To > How to de-rust a block with molasses


16 Posts
Member #: 11753

Athens, GR

Hi guys,

I am currently in the engine building phase of my project car and I am getting ready to send the block to the machinist. Being in Greece for some reason I do not trust they will do a great job of cleaning it out so I wanted to do as much work as I could in advance. I read about molasses and thought I'd give it a try. I wanted to post up my process here (and the results in a couple weeks) for anyone considering the same in the future.

You will obviously need molasses (I bought 500gr for 2,3 euros at a bio food store) and not much more to do this. Whereas most people talk about dipping the block in a large enough tub to be submerged in the solution, I decided to block off the water jacket and just fill that up. The rust and crud is usually only within that space so there is no need to expose all the block to the solution.

In order to make the jacket a sealed space you should do this BEFORE removing the freeze plugs. Obviously I had already removed mine *oh well* so I made some DIY temporary freeze plugs on the lathe. I made sure they were a very tight fit and had to be hammered in.

Once this is done you need to attach the water pump and find a way to block its exit. I made a plug and wrapped it with stretch tape which did the trick.

Once the block is "sealed" you need to prep the solution. I read online various ratios, from 1:8 to 1:2 so I went somewhere between the two with a 1:5 ratio. FYI the volume in the 1275 block jacket is just about 1L so that gave me a reference of how much solution to prepare.

It is usually recommended to leave the molasses in for two weeks but I am in a hurry so I plan to check it in one week time and if I'm not happy then leaver it for another week. I also plan to tackle the cylinder head in a similar fashion tonight so I will post up my approach.

Will keep you posted.

Edited by nikollou on 29th May, 2018.

For the love of cars!!

Forum Mod

10915 Posts
Member #: 17

SouthPark, Colorado

I’m interested in the results.

But they are called “core plugs” not freeze plugs btw.

On 17th Nov, 2014 Tom Fenton said:
Sorry to say My Herpes are no better

Who remembers this first time around:

Turbo Phil

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4391 Posts
Member #: 20
My sister is so fit I won't show anyone her picture

Lake District

Thanks for sharing. I will certainly be interesting to see the outcome.




2959 Posts
Member #: 2500
Post Whore


Best and cheapest source of molasses is an agricultural feed store.

From the experience of others I believe that two weeks is the minimum soak time.

I use Bilt Hamber Deox C and this does a great job in about a week , depending on how strong a solution you use. It is a great deal less messy and smelly than molasses and can be safely poured down the drain afterwards.


449 Posts
Member #: 10396
Advanced Member Formerly mk2turbo


I just use cheap malt vinegar for rust removal

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