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MG Midget and Sprite Technical - Air filters for a Frogeye

I don't suppose many people are using H1s these days, but I am. And the standard steel wire mesh Cooper filters are still in use.
Lately I've heard that these old things fail on both fronts: don't filter much out, and restrict breathing, with the bonus of creating turbulence right in the carb opening.
What's the alternative, do you think?

I got one of the boxes down from its shelf the other day, and looked longingly at the HIF38 I salvaged some years ago. But I don't have a manifold.
Nick and Cherry Scoop

You can get K&N filers with a whole backplate that allows you to put the required holes where you want.
Alan Anstead

While you're at it, fit some stub stacks to the carb mouths. Just enough to round off the entry. I made my own from body filler, but you can buy them.
Les Rose

In combination with the K & N the sub stacks have to be low: I could not find them for the SU H1.
Flip Brühl

Nick,

Great minds - etc

Do you have a copy of Vizard's A Series bible ?

Long story short IMO - good quality changed often OEM paper filters are the way to go !

I can scan and email the relevant page

Malc
Malc Gilliver

Thanks Malc. I had forgotten about that book, ever since he described the ram pipes which came with my Frogeye as 'worthless'.

I'm going to look for it.
Nick and Cherry Scoop

Nick,

I actually found a fantastic quality second hand later edition on ebay for not a lot of cash, I have four editions of Daniels book, one bought new, the others of ebay !


The more I look at the air filter on my 65 Sprite the more I wonder about something similar to what the 1500's have - a proper cool air feed air box - then cut a hole in the nearside grill area to match the heater intake on the drivers side and get cool air to the carbs

Malc
Malc Gilliver

You have an intake hole in the grille? I've never seen that, I don't think.
Nick and Cherry Scoop

Nick,

No I don't have an intake hole, what I mean is duplicate the heater intake hole that is on the right hand side with one on the left hand side to feed cold air to a an intake box containing the filters
Malc Gilliver

So, K&N filter(s) and stub stacks seem worth investigating, thanks Alan and Les.
I'll start looking, but I expect Flip's right. Maybe I need to start practising my filler modelling skills.

Years ago I used to do cold-casting, in resin and bronze, and I've still got the gear. Les, how did you secure your stub-stacks?
Nick and Cherry Scoop

Nick,

Cold casting..... got any links ?

Thanks

Malc

Malc Gilliver

Malc, I wondered why you hadn't googled it, but when I did so, the answer was unhelpful, and didn't really cover the way I used to do it.

My brother-in-law told me roughly what I needed, and I went to Tiranti's in London and bought modelling clay, vinyl mould material and a thermometer, plus a slim leaflet on how to do it. That enabled me to make the mould.
Then you need the resin, its hardener and the powdered metal, to pour your metal substacks.

It can't be too difficult if I could do it (see the earlier evidence of my cack-handedness). I'm sure there will be Youtubes out there.
Nick and Cherry Scoop

You may not find Tiranti the same company they used to be as they were taken over a few years ago and are now Potterycrafts , they certainly don't do some of the things I used to get from them.

https://en.wikipedia.org/wiki/Tiranti
David Billington

An appropriate example of cold casting in resin/bronze.

GuyW

I glued mine to the backplate with 2K metallic resin, such as JB Weld. My logic is that the H1 is so small that anything to smooth the intake must help. I didn't go to the trouble of making a mould, I just sanded it by eye and checked with a profile gauge.
Les Rose

On my 1275 ( which still doesn't have a buyer!) I changed from twin SU s to a single HIF44. I had a K&N for the twins and they are expensive, so I made a fresh backplate to match the HIF to the long K&N and fitted a bell mouth inside that.
GuyW

Potterycrafts, formerly Alec Tiranti, seem to have started doing many of the things again such as the mould making and casting stuff and the resins and white metals. I had a look a few years ago not long after they changed and all that stuff had gone.
David Billington

Nick,

The main reason I decided against K&N with a stub stack was cost - about £400 to do the car.

The performance of a K&N is slightly better than a new OEM paper filter that costs £8, so the £120 per K&N is just not worth it IMO and figured it can't be that difficult to make sub stacks that cost £80 each.

But the cold casting - esp the high temp versions might be just what I have been looking for - thanks.

Malc
Malc Gilliver

That's a very old memory, Guy. I discovered afterwards that I was supposed to polish them with wire wool and then lacquer them. So mine is still shinier than yours.
Sad about Tiranti, David. Luckily, most of the things we need are still available.

But I was astonished by the cost of K&N filters. If I had £400 to spare, I'd get a better gearbox! All I wanted from the Sprite was the look and feel of my teenage years, I suppose. My first Frogeye (first car) was only 11 years old when I bought it. But then, with this one, I couldn't resist taking the cylinder head to Peter Burgess while the engine was in pieces, and buying a Maniflow exhaust manifold, and electronic ignition. I just wanted to give the old 948cc A-series engine its best chance. K&N could be a step too far.

Sorry to ramble.


Nick and Cherry Scoop

Nick,

Yes - had the filters been 5x the cost of OEM - ie £50 ish each then fine, but the shear greed at £120 each is just too much.

I had a shopping list - KandN's, sub stacks, bonnet and boot gas struts, electric fan, and uprated front springs and ideally a higher ratio rear axle/diff.

But then I costed it up - £400 for filters and stacks, £200 for gas struts, £200 for fan I soon decided the fan IMO is worth it for hot days in traffic - the fan will help engine and fuel evaporation, but the filters, struts have been canned and my next job is measure the free length of the original springs first !


Mal
Malc Gilliver

I have just thrown away my K&N filter as I could not get it clean, despite using the expensive cleaning materials. I have a RamAir foam one now on my single HIF44. Just needs washing under the tap with mild detergent. But that won't be needed for about 10 years at my mileage! I did think that £80 was a bit steep, although less than the K&N. That company seems to have made a fortune on the back of David Vizard's book.

I could not get gas struts to work on the bonnet. When closed they are compressed and push the bonnet forward so it won't fit. Maybe I was doing it wrong. I have just had to get fitter.
Les Rose

I remember buying a K&N for the HIF44 on my Mk2 Sprite twenty years ago, but the price couldn't have been too bad. I was poorer then, and would have noticed.

Those Coopers are looking better than I thought.
Nick and Cherry Scoop

Nick,

I have some OEM 1098 round body air filter cans, one has been cut back a bit - but they are in the loft taking up space - any good ?

M
Malc Gilliver

Malc, are they the ones that look like saucepans?
Nick and Cherry Scoop

Yes,

They are nothing to me - figure you might be able to modify them to use the better later paper filters
Malc Gilliver

Malc, that's really kind of you, but they may have to clutter your loft for a bit longer because I've had an attack of nostalgia.
This is how our Frogeye went to bed in 1982, after a major engine build by Derrington (Kingston, I think), and this is how they built the carbs. Sadly they didn't breathe again for more than twenty years, until we bought her.
I calculated that the mesh guards accounted for about 35% of the opening area, so I took them off, and as we needed filters I chucked these in a box and got the modern version of the original Coopers.

Les's post reminded me about Ram Air. I'm going to see if I can cobble a RamAir bobble onto these trumpets. Why not?


Nick and Cherry Scoop

This thread was discussed between 15/08/2026 and 19/08/2026

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