To drill drain holes or not to drill drain holes?

[gryphon]

I have bought a subframe to fit a jaguar AJ30 V6 into my MX5. It's very sturdy and nicely made with a powder coated finish BUT it has no drain holes!

The main cross member is a closed off square section that goes across the car. It has holes with captive nuts to bolt steering rack brackets onto, for LHD and RHD cars so half of the holes are left open (although I could seal them to prevent water getting in and create a total condensation trap instead).

There are also two pipes that triangulate the subframe sitting at 30ish degrees with the top end open and bottom end plugged.

Some pictures to help the words...

I don't have any pictures of the underneath but there are no drain holes. The cross member does have two unoccupied bolt holes, but these have captive nuts on the inside so are probably closer to overflow holes than drain holes...

My options as I see them:

1. Drill holes. Two or three ~6mm holes in the cross member, then a hole each into the end of the triangulation stays. To get access to the stays I'd have to drill through the bottom of the longitudinals too, so that's 7 holes, each breaking the powder coating, which will very likely let water underneath it, start rusting, spidering and flaking off. After hole drilling I'd put some zinc primer on the fresh holes and apply liberal Bilt Hamber cavity wax inside and to the edges of the holes. My worry is I'll have just created a lot of rust on the outside trying to prevent rust on the inside...

2. No holes, just a generous helping of cavity wax. For the crossmember I would then seal the more exposed bolt holes to reduce water ingress, leaving the more sheltered holes behind the steering rack to allow it to breathe and hopefully reduce condensation.

3. Take it off again, drill holes, sand blast the powder coating off and have it finished in a good epoxy mastic. Not going to happen for time and money reasons but would be my preferred choice as I don't much like powdercoating underneath a car anyway, it never lasts long.

Am I just completely over thinking this??

TIA for any thoughts

[Waxoyling Lyrical]

I would say it all depends on how long you intend keeping the car, and what conditions you'll be using it in. The steel looks pretty thick, so unlikely to rust out quickly, even if it did get some water inside - the heat of the engine will most likely cause it to evaporate.

[duggers]

Wang it (technical term) internally with a good dose of Dynax S50 (bilt-hamber) or similar. It'll last for years.

[comfortablynumb]

This appears very similar to the crossmembers fitted to my cortina mk2 And my sons 105e
In all the years we've had the cars, which in some cases was with the car sat in long grass etc, the one piece that HASN'T rotted is the xmembers!
What gauge steel is it made from?
I don't think id be worrying overmuch. A quick blast of wwx periodically should be fine

[gryphon]

Thanks for the replies! I guess I'm over thinking it... I am hoping to keep the car for a long time, but it is solidly built. Not sure what grade the steel is.

I have a couple of cans of S50 for the rest of the car so I'll treat the subframe at the same time.

[biturbo228]

Personally I don't really see the issue with sticking a couple of small holes at low points. Given how beefy it is I doubt it'll impact any structural rigidity

[Deleted]

Option 1 on your list - every time - people forget that this is a engine bay - cavities get pretty warm due to heat transfer from the engine / exhaust etc - hence the cavity is getting hot / cool whilst ever the car is being used and largest enemy here will be condensation more than anything else - what you propose in that option is perfectly fine

[gryphon]

Sept 19, 2019 12:14:02 GMT biturbo228 said:

Personally I don't really see the issue with sticking a couple of small holes at low points. Given how beefy it is I doubt it'll impact any structural rigidity

I'm not worried about impacting on the structure - as you say it's pretty beefy. Just concerned about breaking the powder coating and it flaking off after some water gets in there.

Sept 19, 2019 12:53:53 GMT @grumpynorthener said:

Option 1 on your list - every time - people forget that this is a engine bay - cavities get pretty warm due to heat transfer from the engine / exhaust etc - hence the cavity is getting hot / cool whilst ever the car is being used and largest enemy here will be condensation more than anything else - what you propose in that option is perfectly fine

Thanks @grumpynorthener . I guess I'm drilling holes after all! I think it's what I knew I should do but didn't massively want to.

[kevins]

I'd drill the holes and treat it internally with wax.

[Splitty456]

yep as above - drill some drain holes and then cavity wax it

I'd probably also paint over the holes, don't just leave them in primer (not even zinc primer)

[gryphon]

I took a look this evening intending to break the drills out, and taking a closer look the bottom of the subframe looks double layered. There's a plate at the bottom with the square section of the cross member on top of it. I'm going to ask if this is bonded or just an air gap, I suspect the latter.

Subframe might be turning into quite the rust trap.

Agreed on painting it, might wait until I've mixed the next batch of epoxy mastic though.

[lesgrandepotato]

I wouldn’t. You’ll do more damage to the coatings than you’ll prevent by drilling it out.