diff lash, long read, sorry!

[dylan0451]

bought someones track car mx5 last year. i've wanted to turbo it for a while, but predictably, not had the money so decided to do it in stages.

since the flywheel was knackered, i had fitted a new one that was about 60% of the weight of the original, and a beefy clutch to go with it. the flywheel dia. has gone up, as they're 1.8 items on a 1.6 car, so in all honesty, it hasn't made a huge difference to the character of the car

however

it has brought a really, really horrible rattling noise on overrun out - around the 3k rpm mark


i firstly assumed diff lash - since this was a track toy afterall - went to mazda techs to verify and they said it was fine, the diffs don't wear out (?) got the feeling they weren't interested

since then, i've replaced the gearbox for a low mileage later one - noise remains the same

replaced halfshaft cv's with any movement and regreased, replaced the uj's on the prop - noise remains the same

replaced the diff bushes with poly ones - much taughter, crisper feel to the driveline, brought out more vibration, but the rattling, clattering remains the same

had the entire exhaust off and apart - could find nothing loose, no rattling baffles etc. and it doesn't contact anywhere

heatshields now off/checked for no movement/reverberation

so,

now i'm back to the diff - it's the only part of the driveline i haven't changed. i know it's probably original, and since the cars import in 06 it's been used exclusively as a track toy. doing a hamfisted down change coming off the clutch quick does result in a lot of spongy back and fourth movement

i realise due to ring gear/pinion design there will always be the feel of some apparent lash, but how much is considered lash, or a lot of? a few mm free play in the prop?

thanks if anyones read this far, i know its a bit of an epic essay but i'm getting to my wits end. with the amount of other work i've done to the car it's got to be worth £3k but with this rattle, well, as a buyer i'd assume the diff or gearbox was about to go bang, so offer a stupidly low price or walk away

any help/hints would be really appreciated before i consider pulling rear subframe, roll bar, fuel tank etc. etc.

[Pumaracing]

Are you saying the noise was never there until you replaced the flywheel? If so then one must presume the two events are connected. Could the flywheel ring gear be fouling the starter motor gear or something else inside the bell housing as things move around on the overrun?

[dylan0451]

Pumaracing said:

Are you saying the noise was never there until you replaced the flywheel? If so then one must presume the two events are connected. Could the flywheel ring gear be fouling the starter motor gear or something else inside the bell housing as things move around on the overrun?

well, i guess it would be naive not to suspect the replacement flywheel/clutch, it's just that i 'assumed' i was moving a resonance frequency of something worn into the useable rev range which presumably mazda worked out of the car by using a heavier wheel

i would've thought (i don't know) any connection of flywheel to starter would be pretty catastrophic at speed? and to sound perhaps a little dense, would there be any movement within the bell housing of any parts?

did think about the thrust bearing rattling, and clutch diaphragm 'buttons' but the noise remains with gentle to moderate clutch pressure.

forgot to say, on downshift letting the clutch out fast, when the driveline appears to jerk back and fourth for a while, the rattle comes and goes with each jerk - only becoming audible when pulling, or slowing the car

[Pumaracing]

The crankshaft has a small amount of endfloat which may become greater with wear. The flywheel can move a little respective to other things because of this. I suggest you see if the flywheel is very close to anything else which you may have missed on the installation.

[dylan0451]

ahh, i see what you mean now, just seems odd it remains silent on overrun from high rpm, goes through 3k like a grenade's just gone off in the gearbox/diff then goes completely silent again. i guess removal and inspection is the only course here

managed to replicate the same pitch rattling sound, albeit at a much slower repeat by letting the clutch out repeatedly about half way in quick successions while rolling back in reverse gear

does that sound as though i have huge float from previous track abuse, and am essentially hamering the crankshaft into the engine?! :-s i would imagine the noise would not be the crankshaft/fly themselves or i'd have mashed the bottom end by now!

[Simon says]

Knackered/compressed or damaged cush drive springs in the clutch driven plate possibly? this problem can give severe back lash & chattering in the trans was the bigger clutch assy s/hand new or what?

Edited by Simon says on Saturday 10th July 11:29

[dylan0451]

was a consideration i have/had. the clutch was new and bought with the flywheel:

http://www.flyinmiata.com/index.php?deptid=4531&am...

looks standard in design, just uprated to hold 318 lb/ft - considering it's got little more than 100lb/ft going through it at the mo, i would've thought it should be ok

was wondering if the springs themselves were rattling, but they're still under load on overrun, and as i said above, still does it with light pressure on the clutch, so it seems to be ruled out unless i'm overlooking anything?