Just how oil-tight are the 750 twins?

general_piffle

general_piffle

Senior member
Location
London/Hampshire
I always thought the 750 twins were largely oil-tight. Certainly in comparison to other marques of the same era.

However, my 750 GT has started dripping again after the Willersey run and back. Only a few spots, coming from the left hand engine cover now (it was the clutch side before, which is now sorted).

Don’t get me wrong, I’m not moaning as I love it regardless. But is a little bit of a leak OK, or should they be completely oil-tight? I’d like it to be the latter but with no real long term experience of these machines, I’m not sure.
 
Exmoor beast said:
Have the seals been changed or are they fifty years old? Seal behind points, gearbox output shaft will leak if the seals are pre decimal.
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Gijs van Dijk carried out the restoration of the bike in the Netherlands for the then owner. This was all carried out at the end of 2017; from his notes to me:

New big end roles
New crankshaft side bearings, including a roller bearing on the left
New point crankshaft needle bearing
All 4 gearbox bearings replaced
Replaced the prisdirekt sprocket (5th gear) with a younger one with wider cams
New clutch plates and springs
New oversized pistons with drilled and honed cylinder
Unleaded valve seats and new valves
New distribution chain and new primary chain and all tension rollers and new primary chain tensioner
Replace clutch housing rubbers
Crankcase blasted, covers polished
Electronic ignition fitted (sorry Paul)
Carburetion: new needles, needle jets, main and idle jets, choke slides renewed and all gaskets

I'd imagine the seals were done when all of this was done, as I understand Gijs has an excellent reputation.

When I bought the bike and imported it to the UK it hadn't even been run-in. I heard that after a full nut and bolt restoration the owner, for whatever reason, didn't ride it and kept it in storage for a few years (mainly throughout the pandemic) before I bought it. After all the work it had about 50 miles put on it.
 
per the Green Book, the nuts on the cam cover should be re-tightened in sequence 300 miles after an engine rebuild. I did this to mine and it,s been oil tight w a little weeping from the right side cam cover. another member mentioned minor oil weeping from the left side cam cover.
 
The modern gaskets are a far cry from the original ones from the factory, and despite Piet´s assurance that these green ones are ok, I find that the ones I have fitted in recent years are very hard by thumbnail test (laugh, maybe they have sat in storage for too long as it is not every year that I have these motors apart), and invariably require sealant where none was ever needed on original ones. My inner selector box cover needed in fact to have sealant on the gasket and left to set overnight with no oil before it was oil tight, the night before leaving for Breganze, after trying twice with sealant and filled with oil. It is still bone dry. I have found the same on other fitments.
There is also the 50+ year old aluminium parts that are no longer as perfect as they were, having been removed and replaced/tightened umpteen times most likely, as well as umpteen heat cycles.
 
Fully agree with Lee! green gaskets require a very thin coat of sealant (on both sides) and best left to set overnight before comming in contact with oil.

Gerald
 
general_piffle said:
I always thought the 750 twins were largely oil-tight.
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Hi Joel, as in previous responses, our bikes should be 100% oiltight.

As we've chatted, I have more than one 750. The only one that drips any oil is the one in the UK where it has the least opportunity for basic maintenance. None here at home drip any oil.

As you bought yours right after major intervention, after a few heat/cool cycles some minor oil drips suggest that you should retorque the outer cover bolts as the paper gaskets there may need to be squeezed tight again. Also the top of the motor nuts should be retorqued so that seepage won't appear at the head gasket.

In the case of my hard-worked, minimally-maintained, 750GTL kept in UK (it's just stored there with minimal service facilities and a true credit to the durability of the 750 bikes!!!), I believe its oil leak is from the gearbox output to sprocket seal that was cooked trying to remove the sprocket. When serious heat and a big puller wouldn't get the old sprocket off I pushed the old but not very worn sprocket back on but may have cooked the seal behind so that if filled above that level it quickly drops to that level then stabilizes, in the interim having covered my back and everything behind that point with little drops of dirty oil stuck dirt! Don't know when major renovation may be possible so, since it runs just fine, I'll accept a little oil loss and oily spots on me and the back of the bike. Otherwise, at 50 years old, many hard miles, with no major intervention in my ownership and don't believe so before, it's still completely oil tight.

Hope to see you and your GT at Baskerville - there's a little sub-group of GT and GTL owners - join us?
 
First pre-piston burn build of the SFQ, NOS gaskets from my decades old spares box, zero oil leaks from any covers. After the repair, fitting green gaskets I've had leaks from primary, gearbox selector and selector outer covers. Have just fitted gaskets to Walky's SF2 custom build (a motor that never leaked oil after I rebuilt it 30 years ago), filled with oil, never run, green gaskets, oil dripping from gearbox selector cover. It's now clear that all these gaskets will require sealant when fitting. Have never had any of my covers leak when using factory gaskets. Not particularly happy.
 
Perhaps I'm lucky to have quite a stash of old parts.
Who is the supplier of "these green ones" that perhaps we should be aware of their limitations?
 
arancia said:
Perhaps I'm lucky to have quite a stash of old parts.
Who is the supplier of "these green ones" that perhaps we should be aware of their limitations?
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Not sure - I'd just be so happy if they swapped their material to whatever the factory ones were made of. can't be that hard - they have the cutting templates already.
 
An oil leak inside the ignition housing often originates from the area where the ignition wiring passes through the upper engine case and primary cover. The holes mate up OK, but the surrounding sealing surface is a bit narrow and the gasket is a bit floppy. Inserting a rod/screwdriver into the wiring channel while mating the cover to the engine will better align the gasket, also preventing it to intrude too much into the channel to prevent the wiring to easily pass through.

I've fitted probably hundreds of the green type gaskets with the only issues stemming from warped or damaged covers. Re-torquing after a few heat cycles is a must with almost every type of gasket. Both Athena and Centauro have been supplying the green gaskets for decades.

Thrown together carelessly, Laverda engines can leak like the best of british. I've found engines with gasket remnants still present under new gaskets, old and frayed re-used gaskets, virtually litres of gasket goo of all sorts in vain attempts to stop leakages, when all it would have taken was a little care and patience. Preparation takes up the most time during an engine rebuild. Assembled with attention to detail, they can be dry as the Sahara, with only an occasional unavoidable drop from the engine breather. To avoid that, the breather hose can be routed into the air cleaner housing if present, thus re-circulating the oil fumes that are inevitably produced by the hot engine. Had the 750 enjoyed an even longer production run, this sort of mod would have become inevitable!

piet
 
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