I have a 1987 Trek 400 with a steel fork. I just installed a set of 700x32 tires, they are undersized and measure about 28mm wide.

The tires clear the chainstays, seatstays and both brakes with ease, I can fit a 5mm allen wrench at these points.

The front tire spins, but has almost zero clearance under the fork crown. I can't fit a 1mm allen wrench at this point.

Can I do anything to the fork increase the tire clearance at this point?

There nothing you can do about it , maybe try a different size tire .

I had the same problem when I set up a track bike for street use about 45 years ago. My solution was to fill the top of the fork slot with 2-3mm of auto body filler, with a short piece cut from a spoke trapped at top dead center both as a gauge and as reinforcement.

This safely pushed the wheel down, and it's been that way ever since. You can't get much more than that because you have to keep the axle and nut (or QR) faces well supported in the flat part of the dropout.

But if 2-3mm is all you need, this is simple enough and works very well.

On some bikes you might also be able to file some away at the front of the crown. Because the front wheel is forward of the crown it comes higher at the front than rear. Filing the crown to follow the rising contour of the wheel can buy you another few millimeters. This will often not work with a caliper brake, since the brake projects forward where the tire is yet higher, and often the tire will touch on the brake while clearing the crown.

Lastly, consider that there's no rule saying you need to use the same tires front and back. You rarely need more section in front (for road) so 28mm should be fine. That leaves you free to use a larger tire in back where the axle loads are higher, and you want the extra width.

Use of different tires front and back has been SOP in high performance and heavy duty auto and truck applications for decades, and serves well. There's no reason for cyclists to lock themselves out of that option.

you could also try to mount those tires with a wider rim
this might bring the outer diameter down a bit
or it might not
just a thought

The difference will be miniscule, and replacing rims isn't a cheap option.

In any case, widening the rim will increase the overall circumference of the cross section which would increase not decrease the diameter. A simpler option is to carefully shop tires with the biggest actual size that clears, regardless of nominal size. But that seems to be where the OP is already.

I would add the spacer as per FB and/or file the bottom of the crown to gain some clearance if its an alloy or steel fork.

grind away metal from the under side of the fork crown? replace the fork tips ?

Obviously New fork with longer blades ... Might be a Buy a whole new real 'Fatties fit fine' bike frame ..

with that tight a clearance a stone sticking to the tire on your Gravel Grinders will jam in there tight..

anything else is wishful thinking .. spend , it boosts the economy .

if replacing the fork
it makes much more personal economic sense
if not macro economic
to get an old ten speed fork designed around 27 inch wheels
which will have ample clearance for 700 x 32 c tires
rather than send a hundred bucks
to be split between a chinese factory and their american distributor

Whatever you might do the front at the dropouts, you would have to also do to the rear dropouts to keep from changing the head tube angle. Also you would have to watch out for the brake calipers being too short on the new setup. As for taking metal off the underside of the fork crown, the place where all the twisting stresses are concentrated, well, best I can say is I do not think that would be a good idea. This is so dangerous, it is absolutely not funny. Oh hell, yes, yes it is funny.

If you do that, you will change the head tube angle and the handling of the bike. You have to fix the rear as well.

well
you dont have to

if the change in fork length is fairly small
then the change in handling will also be small

also
there is no reason to assume that a small change in geometry and handling
will necessarily be negative

but what you said before about brake reach
is right on the money
no matter what means the op is using to push his wheel further from the crown

Thanks'

Strangely enough the front brake has plenty of clearance. I'm going to hand file the crown just to remove any flash or unintended material. I'll consider the smaller tire options if that doesn't do it.

Well, if the fork has curved blades, you could straighten them a bit and then bend them further up to restore the rake...

Although that's a pretty tongue-in-cheek suggestion; probably doable but not easy.

No, you don't have to. But why potentially screw up the handling of a bike just for this totally unnecessary modification. Only my opinion, of course. And a strong one, also of course. I just think this is all half baked.

all good advice.

but as i read your post, the front wheel spins freely and the tire does not touch the fork crown. if so, i don't see any problem. as the tire wears there will be more clearance not less.

in the future, i may look at using a tire with a slightly lower profile, or, as has most likely already been mentioned (but i didn't take the time to scour all responses), not mounting the wheel fully into the fork. it can create several millimeters of clearance.

just don't forget, like i did, that you did it and then wonder, like i did, why, after fixing a flat and reinstalling the front wheel fully into the fork, the clearance that i previously enjoyed was now gone.

many track bikes have surprisingly little clearance between fork crown and tire. and, IIRC, dedicated tourists used to buy special contraptions that mounted on the brake bridge to brush off road debris to reduce punctures. or at least, i think that was the reason. you've got a simpler, cleaner, and more efficient one for free.

strongly held
but not that strong

it would take a pretty major change
to even come close
to screwing up the handling

fbs idea of using body filler and a peice of spoke
for instance
would bring the front end up 2mm
which could be less than the difference between a new tire and a worn one

Okay, but why not do the rear too? Same method.

I hope your tongue is firmly in your cheek, and that you're careful not to bite it.

2mm change in front height vs 1,000mm wheelbase, so we're looking for a change in angle = to one with a sine of .002. This is roughly 1/10 degree, hardly worth thinking about. (I couldn't find a sine table for angles less than 1°)

Anyway, if the OP takes my advice and uses a larger tire in the rear than front, this would change the head angle more. (in the opposite direction). Maybe if he combines both the bike will retain the original geometry.

A really wide rim and the tire may not be as tall..

I stand corrected. I was thinking of the change from 27" to 700c I made on a frame I was also having cut for S&S couplers. The frame builder insisted on building up the crown race seat of the new fork to keep the top tube level. Talk about ugly! That was more of a change, but still probably no biggie. My bad. Thanks for the gentle let down.

Right there. It should only be the leading edge that needs a little grinding. I've done it many, many times.

Choose 1:

I agree. It looks like the steering tube extends below the lugged crown by 2-3mm. filing or grinding the steerer won't weaken anything.

Thanks'

Filing the underside of the crown is common practice with track forks, where tight clearances are considered de rigueur. Just realize that the back of the crown needs less relief than the front, and take care not to compromise the brake mounting hole.

Tighten all the spokes 5 turns.................