loose crankset - carerra kraken

[westview]

hello folks
looking for some help please.
i recently bought a second hand carerra for my lad. the bike is 3-4 years old in reasonable condition and seemed ok when bought. Not been ridden much recently due to weather but a few days ago my son reported a loose crankset. i took the bike for a spin and quickly noticed the 7mm crank bolt was loose on chainring side. i took the whole crankset off, had a wipe over and tapped both sides back on then tightened 'securely'. ten miles later loose crankset again! looking closely i suspect previous repairs/adjustments may have been attempted and poss rendered the set useless. Bottom bracket seems ok - no play, axle looks square etc.
How can i diagnose the exact issue? Should i swap out the front crankset (its a 3 ring, welded, one piece budget shimano -so new arms would be needed too). i have got a similar truvative set on an old carerra frame in my shed.

[landsurfer]

Apply more torque .... your "tight" is possibly not my "Tight".
Mmm .... "tapped" ... not sure what that means ....

[mercalia]

and use some loctite that acts like a glue to stop bolts and nuts coming loose?

if none of the above works then think about replacements?

[cycleruk]

Before re-fitting the crank heat it in some boiling water or equivalent. Makes it expand and allows the crank to slide further on a bit. This was the only way I could get a crank to fit tightly on a Dawes I had.

[willcee]

If in previous ownership or indeed while it came loose during your young chaps cycling the damage to the alloy taper sizing will have been done, and I suspect that's why it was sold..for peace of mind and safety get a cheap set new and fit it, I know what a loose crank can do, it caused me a severe smack with the ground in front of traffic one Christmas while crossing a busy junction..it had come loose during the ride, i hadn't the 8mm to tighten it with me, I stood up to put more speed on, the crank parted company with the bracket when it was at 12oclock and I fell in a heap under the bike.. took a couple of weeks to feel right again although i wasn't actually bleeding, just bruised..never mess with safety..will

[Brucey]

if the bike has been ridden with the RH crank loose the socket in the RH crank has probably been damaged. Compare it with the socket in the other crank and see if it looks (or feels when the spindle is inserted) different or not.

Sometimes you can recover the situation (the crank may be reshaped by repeated tightening so that it is snug again) but more often a new crankset is the way ahead. Since different cranksets often require different BB spindle lengths, expect to perhaps have to buy a new BB too.

cheers

[westview]

Many thanks all
i have tried tightening up again after tapping home with a rubber mallet, but this seemed to make the crankset reluctant to turn which i suspect may be due to it touching the frame? winding back quarter/half a turn on the 7mm bolt feels like its going to be too loose.... maybe crankset has been previously over tightened and/or is not the orig part.
I wish i wasnt so tight, id just take it to LBS to sort out!

[westview]

quick update with thanks to you all
following advice from Brucie, Willsee and Landsurfer i took a good look at the inside of the crankset and decided it was probably past it.
i had and equivalent truvative isoflow crankset on an old frame in the shed so i moved that to the Kraken it went on nicely and is better quality too! i removed the granny cog first and converted/modded the Kraken to an 18 speed rather than 27. next task is to swap the pedals over and yes, the ones on the 'new' crankset have bloodied my knuckles and turned the air blue already.....
Best of all though - repair done for free! 80)