I've bought a new bottom bracket for my late 2000's indoor trainer bike but think I might have messed up

On the one that's fitted all I can see is the following markings ..

FSA
Mega Exo
BC1.37x24T

After a bit of Googling I ended up ordering a Shimano SM-BBR60, it turned up the other day so I set about installing it. Old BB came out fine, gave everything a good clean and installed the new one, first thing I notice is the external diameter of the new BB is smaller than the original, is that a concern? Should the BB match the frame shell diameter?

Next problem was I went to slide in the spindle and it doesn't seem to want to go in, I'm afraid to whack it incase it gets stuck, so next question, are BB internal diameters the same? If not how do I know what I need to buy?

Any recommendations for a replacement if this Shimano is no good?

EDIT:
Done a bit more reading and now I'm thinking the problem is compatibility between the FSA crank and the Shimano BB

The external diameter of the shell doesn't really matter other than you needing a separate tool for each size - Shimano have three different diameters for example.

Spindle diameter - Shimano are usually 24mm, can't remember if they do a 30mm yet but that should be obvious. SRAM GXP has a tapered spindle (WTF??) which is 24mm on the drive side and 22mm on the non-drive side. SRAM DUB is 28.99mm (see a pattern here?) FSA Megaexo is 24mm.

SM-BBR60 is Hollowtech II so takes a 24mm spindle.

So your new BB and your crank's spindle are both 24mm and should work together but...

There are two variants of Megaexo, one works with Shimano BBs, the other doesn't A bit of searching gives:

The ones that will work are the lower range Gossamer and Omega cranksets that use the same pinch clamp design as Shimano.

The higher range K-force and SL-K designs use a tension screw to lock onto the spline on the spindle. This requires a metal sleeve that comes with the MegaExo BB's to support this 'compression' inwards behind the bearings. Else you won't get your lock screw tight enough or, you will get it so tight, the BB bearings will seize.

All a bit of a minefield.

Having worked in many cycle shops over the years and having carefully measured axles,widths etc.
I recommend always fitting the exact same make and model as removed bottom bracket bearings if possible.
It saves time ,money and frustration!
The same ,almost ,applies to steering bearings!

Time for another noob question, sorry folks, this time on headsets

I've noticed mine is starting to feel a bit grainy, if I don't ride the bike for a while it'll almost lock up too, I'm guessing the bearings are on the way out. I'll ride it for a bit longer though but I just wanted to get a steer (sorry about that) on what I should be looking for in a replacement.

Current one is this .. https://www.planetx.co.uk/i/q/HSSSELC18 ... SSELC1815D

What I can surmise from that page is, I need ..

Integrated
1.5" bottom - 52mm ID
1 1/8th top - 42mm ID

Probably a bit to Gucci for me but am I on the right lines with a Chris King Dropset 5 ...
https://www.sigmasports.com/item/Chris- ... adset/TWXG

And the cheaper Dropset 2 ..
https://www.sigmasports.com/item/Chris- ... adset/NYIO

The only difference I can see between the two is the 5 has an included baseplate


Or do folk just buy replacement bearings instead?

Your cranks - they almost certainly fit. Lightly grease and tap with a rubber mallet. It will be fairly obvious if they are wrong (i.e. they're clearly too 'thin' or obviously won't go in).
Headsets - You can replace bearings, but take them to a bike shop - angular contact bearings can be tricky to order correctly. Chris King are fantastic headsets, but you can get something 80% as good for 40% of that cost (I just made those figures up, so don't quote me). Hope are great, or I've found Cane Creek excellent, with easily replaceable bearings.

This one should fit Rich. Won't really go wrong with it and much better than the Selcof imo. https://www.spacycles.co.uk/m16b0s115p3 ... -42-ACB%29

NB. It's the quality of them cartridge bearings that made them feel norchy and locking up so quickly. If you're careful, you can also service the cartridge bearings yourself (but I'd not on PX bearings and now they're notchy its too late anyway). Itsgenerally all drop in stuff here so shouldn't be a problem and won't need special tools.

You may have also overloaded (preload) your bearings at some point by cynching the top cap a bit much (though if your stem was already tightened onto the steerer this wouldn't have done much but might explain your plug pulling itself out.

Bike maintenance - best getting used to doing it yourself especially when you're regularly going out very far... Measure twice, cut once (ie. Get The Big Blue Book).

I have had Hope ,Chris King, FSA, Cane Creek and unbranded ones - all of them just worked irrespective of price and I am not inclined to pay the premium for hope or King personally.

I just replace headset bearings if its required - cannot recall the last time i had to do headset ones - dont get anything like the abuse of frame, wheels or bottom brackets

A good bearing shop should be able to get you what you need [ almost always cheaper than a bike shop] though not certain with these as they are angular bearings

Ditto what lazarus said, and the price of the selcof one seems high for what they are. I've had £12 brand X ones last ages and still going strong for me. Never had issue with cheaper drop in headsets, and you can also get spare 'hi-fill' bearings new on ebay which i've done in the past but can't say I noticed any difference. I think if you keep an eye on preload then no need to fork out much.

I stripped down my headset this morning and found the grindy culprit to be the lower bearing, it was a rusty coloured mess down there.
I've cleaned everything up barring popping the seals and regressing, and instead ordered a set of replacement bearings from a company just down the road.

I'm up in Aberfoyle to bash out some gravel.miles in a couple of weeks so should be good to go by then.

Thanks folks, saved me a wedge of cash again (if I've ordered the right ones)

If they aren't right (angular bearings can be tricky), you could just go for the lower half of a Hope mix n match. The sealing is better.

Add a neoprene sleeve around the bottom bearing cup to keep out muck &emoisture.
Add a bung in the bottom of the crown also helps, for the same.
Just my two penn'th ,
.