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MG Midget and Sprite Technical - Sills-Foam Injection, good or bad idea?

Hi All

Its a Monday, time for silly ideas so bear with me!

Early Mini's had terrible problems with water ingress, it was caused by a seam being lapped the wrong way, but before that was discovered various things were tried including injecting polyurethane foam into the sills.

It didnt stop the leaks but it did significantly increase the torsional strength of the shell and when some cars were inspected years later the sills were rust free.

My car has nice new sills, all hollow and tinny sounding!

Has anyone ever tried foam injection into sills?

Any thoughts?

Mark
M Davis

Well you have educated me because I have never heard that before. The story of the sills being constructed incorrectly and injecting foam acting as a cure for rot?

IMO this would be possibly the very worst thing that could be done, effectively it would prevent any natural drain and also hold moisture against metal, both less than ideal.

On another front welding up the sills on any damage or rot would be catastrophic?
Bob Turbo Midget England

Expanding (dry) foam can be used to increase strength on hollow sections (used on racing cars, for example), but agree with Bob re potential for rust.

Also ... not so good if you want to weld in vicinity at some point in future.

Waxoly would make the sill sound less empty, and proof against rust...

A
Anthony Cutler

BAD IDEA!!!! Don't do it as some water will inevitably enter the sills and be trapped between foam and metal with obvious and rusty results. Moreover, if any of the surface of the foam is damaged or perforated then the foam will act as a sponge and retain water with even worse results. The best treatment of sills (if sound to start with) is a good quality cavity wax (there are lots of different makes - dinitrol, Waxoyle, Bilt-Hamber etc - try Frost for a range of products)applied every year or two (but making sure any drain holes are clear so any water entering the sill can drain out straight away.

Chris
Chris H (1970 Midget 1275)

Hi Thanks for the feedback!

Ref to the foam was in the book Mini by Rob Golding, I'll dig it out tonight to refresh my memory, the non rusting was an unexpected side effect!

Yes agree well vented/draining sills with Waxoly def the best way to go!

Curious to know if any racers out there have found it improves handling!

Mark
M Davis

Years ago I had one of these minis that had foam filled sills, and they were pretty sound as I recall. It was a dark green Traveller - proper one with wood battens like the Moggie.

I must admit, I have occasionally wondered if it would be a good idea or not. To be effective one would need to waxoyl or similar first although I guess the foam wouldn't then bond to the metal and would be less effective at increasing rigidity. It would also be good if one could inject oil into the foam so that it was dispersed through the foam cell structure. Without one or other of those "additions" I think it is probably not such a good idea.
Guy

I did it on the Blue car when we first did the body work. It's away being done again as the rear wings and A posts were looking a bit worse for wear. The sills on the other hand looked perfectly fine....

Well on the outside they were, they'd rotted from the inside out and were extremely thin due to the moisture retention caused by the expanding foam.

Andrew
Andrew McGee

I once did some work on a Mk1 Granada Coupe and the lower rear quarter panels were foam filled, presumably to stop the long panel from drumming with it being a two door, and they were absolutely rotten.

Bernie.
b higginson

Well, expanding foam works to fit double glazing frames in place, instead of using that tiresome method of bolting the frames to the brickwork,-- so it must be ok on a Spridget too :P
Lawrence Slater

I read the same story as Mark and the foam was used to prevent water ingress so presumably was water resistant. Scrapped Minis were later inspected and found to be rot free in the sill areas (but not in rear subframes...) The reason for stopping using the foam was because it was toxic and causing health problems for the workers applying it. I think the story was recalled by one of the production engineers.
I remember my Dad's 1600E having foam strips in the rear wheel arches between inner and outer wings which absorbed water prodigiously with the obvious results. However I think it depends on the type of foam you use - the expanding hard foam mentioned earlier doesn't absorb water and combined with a good anti rust product, should both stop the rot and stop the tinny sounds. It's your choice.
Jeremy
Jeremy 3

Hi Jeremy

Yes thats pretty much the story I read!

It shouldnt absorb water as we use it for sealing windows, gaps etc in our old wooden house (yes I know purists hate it!)

Anyway interesting application, I'll probably not do it as a buyer in the future may assume the sills are made of filler!

Mark
M Davis

This thread was discussed between 21/05/2012 and 23/05/2012

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