900GTS big end life

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kerrin
Mariana
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900GTS big end life

Post by kerrin » Sun Jan 10, 2010 9:29 am

I`m just starting to feel my way into rebuilding a cosmetically trashed 900gts & have talked to a couple of UK suppliers about parts availability & ballpark figures for costs. What suprised me was to be told to expect a big end life of between 10,000 & 20,000 miles, is this a reasonable figure? The upper figure is with regular maintenance & a redline at 6000rpm. I wasn`t expecting modern levels of longevity but am having problems getting my head around such a low figure.
KERRIN

wdietz186
Cagiva Alazzura
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Post by wdietz186 » Sun Jan 10, 2010 2:21 pm

They are entirely wrong! I have a 75 750GT with approx. 50k on it [speedos aren't quite as long lived] with absolutely no problems from the bottom end. It has had the oil changed at approx. 1.5k intervals and hasn't been babied by any means.

BEVEL DAVE
Diana
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Post by BEVEL DAVE » Mon Jan 11, 2010 3:51 am

I know many with over 100,000km on big end assy's.
One guy had about 230,000km on an S2 900 when it was pulled apart & it was still fine, he never revved above 6000rpm & always had fresh 50w oil in it.

Dave.

kerrin
Mariana
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Joined: Sat Oct 17, 2009 11:47 am

Post by kerrin » Mon Jan 11, 2010 10:53 am

Thanks guys, I felt like I had dropped into a parallel universe where I was going to spend more on bearings than petrol.
KERRIN

mgh1955
Mariana
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Post by mgh1955 » Mon Jan 11, 2010 12:43 pm

Kerrin,

main thing (and hopefully previous owner/s adherred to this) is oil changes every 1,500k, change filter every 3,000k and keep carbs balanced so that load is balanced across both rods. Also worth investing in a dipstick that has a magnet attached to end to catch any stray metal that might pass near it. A bit random but it helps. Steve sells them in his shop.

Mick

kerrin
Mariana
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Post by kerrin » Wed Jan 13, 2010 12:15 am

The previous owner had the engine rebuilt about 1500 miles before bending a valve & putting it into the back of the shed for 25 years. If the motor hasn`t suffered from internal corrosion I should have a good starting point. The magnetic dipstick sounds like a sensible idea, I`ve used a magnetic drain plug in the oil tank of my Trident for a few years & it always picks up a coating of really fine debris, much better than having it going round the motor. I should also say that it`s probably a `75 860GTS rather than the `79 900GTS I originally thought {thanks Craig}.
KERRIN

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Craig in France
Cagiva Gran Canyon
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Re: 900GTS big end life

Post by Craig in France » Wed Jan 13, 2010 1:14 am

kerrin wrote: ... & have talked to a couple of UK suppliers about parts availability & ballpark figures for costs. What suprised me was to be told to expect a big end life of between 10,000 & 20,000 miles, is this a reasonable figure?
That wouldn't be a certain company in Dudley, by any chance?

Ciao

Craig

BEVEL DAVE
Diana
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Post by BEVEL DAVE » Wed Jan 13, 2010 1:48 am

If its been parked 25yrs just bight the bullet & pull it apart now.

kerrin
Mariana
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Post by kerrin » Wed Jan 13, 2010 3:52 pm

I know the motor has got to be stripped, just keeping my expectations low & hoping there are decent cams, pistons,bores & gearbox. Anything else will be a bonus but the weather has got to improve before I make a move into the garage & start pulling it apart! The 10/20000 mile suggestion didn`t come from Dudley but as it`s possible I missunderstood a phone conversation I won`t name the other party. It was about 5 minutes after I put the phone down that I realised it meant the engine could need rebuilding after a couple of good summers, & thats something I would find hard to justify financially. Thank for your unbiased advice & the benefit of your collective experience
KERRIN

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Craig in France
Cagiva Gran Canyon
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Post by Craig in France » Thu Jan 14, 2010 11:30 am

kerrin wrote:The 10/20000 mile suggestion didn`t come from Dudley ...
Ta - good to know :-D .

Btw, big-end failure in the big twins seems to be ultimately related to wear in the thrust washers. (Note that I say seems 'cos the BevelHead jury is not unanimous on this ... :) ).

This wear allows the rods to tip slightly, which allows the outside edges of bearing cages to catch on the rod eye. Eventaully, this causes the cages to break, scattering the pins out of the bearing.

Causes of thrust washer wear are:
1. Lubrication break-down.

2. Poor original assembly.

3. "Everything put together sooner or later falls apart".

Also:

1. Imminent big-end failure cannot be ascertained by pulling the rod up and down; however it can be predicted by measuring side play in the rod.

2. If your big-ends fail, you should also replace the oil pump 'cos, like as not, it too will be knackered.

Ciao

Craig

kerrin
Mariana
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Post by kerrin » Sat Jan 16, 2010 3:29 pm

Cheers for the cheerfull scenarios.What has got me thinking that the big ends will need replacing now [rather than very rapidly in the future!] is the tendency of ball & roller bearings to mark their tracks when they are left sitting in one position for a long time [25 years]. The minute indentations cause the bearing to fail soon after the engine is restarted, or maybe I`m being paranoid. Anyway, the weather`s getting warmer & tomorrow I am going to the garage to investigate.
KERRIN

BEVEL DAVE
Diana
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Post by BEVEL DAVE » Sun Jan 17, 2010 12:19 am

No you are correct about failing shortly after but its usually corrosion that starts from sitting once the oil drains & dries up.

kerrin
Mariana
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Joined: Sat Oct 17, 2009 11:47 am

Post by kerrin » Sun Jan 17, 2010 4:01 am

Dave, I don`t know where I picked up the information about old bearings, or if it is anymore than an old wives tale, but unless every part is checked & OK I will not feel happy. 25 years of storage in uncertain conditions has to have caused some internal damage as well as perishing every oil seal. I will let you all know what I find & then ask for advice about what to do next.
KERRIN

kerrin
Mariana
Posts: 33
Joined: Sat Oct 17, 2009 11:47 am

Post by kerrin » Sun Jan 31, 2010 1:48 pm

Finally got to the garage & started dismantling; first good news was loads of decent oil & no water. The clutch looks brand new with no wear on the basket, unmarked bores at 86.6mm & good pistons. Both heads have what looks to be a welded repair to the exhaust thread, also welded up are the inlet manifold mounting stud holes. These have been relocated to suit 40mm carbs. The rear head to liner mating face is quite badly marked, how do you damage that? More to the point, how do you repair it?
KERRIN

in-two
Diana
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Post by in-two » Fri Feb 19, 2010 6:02 pm

just joined the forum, I had an 860 GTS bought new in 75 or thereabouts, and did do a big end in about 20k miles, this seemed due to the conrod thrust washers wearing as previously mentioned because of plonking it around at low revs commuting to work in central London. I had a special set of thrust washers made to the correct dimensions of proper shim steel (sorry can't remember the spec.) and put a tooth or two on the rear sprocket, no further probs until I sold it at 60 odd k.
I subsequently bought a 1975 900ss, this has quite deep concentric, original machining marks on the horizontal head/cylinder joint but I think the main gas seal is the spigot joint between the top of the liner and the vertical face of the head joint, so they don't really matter. If it does leak I think your only option is machining or building up with weld and then machining.

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