Leak Down Test (Don't Laugh!!)

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Fast Eddie said:
Be sure about what Click...? The 850 DID NOT HAVE a gasket from the factory. So, unless your pushrods have been shortened to accommodate the thickness of a gasket by the previous builder (rather unlikely I’d suggest) then by your own argument, you’re better off without one...!

Jus’ sayin’...
Click to expand...

Thanks for the reply, what I've found is that the 850's up to the MK3 did not use a base gasket, but they reinstated the use of a base gasket on the 850 MK3 (which is what I have)
The main reason they reinstated the use of the base gasket, owners were using too much 'gunk' & plugging up the oil return hole!

I get your point so no need to shout :rolleyes:;)

Some guys have rebuilt without a gasket & have had no issues but some have had niggling leaks, which when rebuilt with a gasket have gone away.
 
Fairy Nuff... I didn’t know that about mk111s. Good to set the record straight.

The loss of CR probably explains why they were so slow...;)
 
click said:
Thanks for the reply, what I've found is that the 850's up to the MK3 did not use a base gasket, but they reinstated the use of a base gasket on the 850 MK3 (which is what I have)
The main reason they reinstated the use of the base gasket, owners were using too much 'gunk' & plugging up the oil return hole!

I get your point so no need to shout :rolleyes:;)

Some guys have rebuilt without a gasket & have had no issues but some have had niggling leaks, which when rebuilt with a gasket have gone away.
Click to expand...

I would use a gasket.
I would put a thin coating of anaerobic sealant on both sides to keep it from slipping out of place in the future. [locktite 515 or equivalent]
 
Fast Eddie said:
Fairy Nuff... I didn’t know that about mk111s. Good to set the record straight.

The loss of CR probably explains why they were so slow...;)
Click to expand...

Just trying to get mine to go again:( not bothered about speed;)
 
comnoz said:
I would use a gasket.
I would put a thin coating of anaerobic sealant on both sides to keep it from slipping out of place in the future. [locktite 515 or equivalent]
Click to expand...

Thanks Jim, I've ordered some gaskets, just have to wait:rolleyes:

Do you still use those 'fancy' base gaskets on all your rebuilds?
 
Fast Eddie said:
Fairy Nuff... I didn’t know that about mk111s. Good to set the record straight.

The loss of CR probably explains why they were so slow...;)
Click to expand...

Not slow once the breathing restriction is removed.

The effect of Cr alone isn't much


Increasing from 5 to 6 to 1 gives about 3.5% power increase.
>From 6-7 = 2.5%
>From 7-8 gives 1.4%
>From 8-9 gives 0.7 %
>From 9-10 gives 0.6%
>From 10-11 gives 0.5%
 
worntorn said:
Not slow once the breathing restriction is removed.

The effect of Cr alone isn't much


Increasing from 5 to 6 to 1 gives about 3.5% power increase.
>From 6-7 = 2.5%
>From 7-8 gives 1.4%
>From 8-9 gives 0.7 %
>From 9-10 gives 0.6%
>From 10-11 gives 0.5%
Click to expand...

Glen, you gotta learn to spot the ;) emoji matey !
 
Fairy Nuff Glen.

But I therefore feel I gotta defend poor maligned compression ratio...

I’ve seen those kinda tables before, but its never sat right with me.

When did you last see a race winning engine with 7:1 CR?

If there are so little gains, why is just about every single modern petrol engine up at CR levels that our Cdo’s would need methonol for?

Even your ‘grey porridge’ 4 cyclinder family hatchback is well over 10:1 these days, even though they’re turbo charged. The bike world is far more serious, your 955i = 12:1, R1 = 13:1, Norton V4 = 13.6:1, Ducati Panigale = 14:1, etc. And these are all box stock production engines, on pump gas, achieving modern emission ratings...
 
Do you still use those 'fancy' base gaskets on all your rebuilds?[/QUOTE]


Click,
When you contact Cometic ask them about the outer primary gasket. I have one and they work pretty good. So far I have had mine off twice and it is still in good shape and leak proof. I forgot to see if they made the valve cover gaskets. That might be something to ask them too.
Pete
 
Cometic made a complete engine trans set (minus head gasket) for my BSA A10.
I sent them a new paper set and they copied it.
Price was quite reasonable, I think around $200 or so.
They are happy to do this as they now have those shapes on file and can repeat.
They also have most Vincent gaskets on file.
I wonder if they have all of the Commando gaskets on file?
Their gaskets are excellent.

Glen
 
Last edited:
Fast Eddie said:
Fairy Nuff Glen.

But I therefore feel I gotta defend poor maligned compression ratio...

I’ve seen those kinda tables before, but its never sat right with me.


When did you last see a race winning engine with 7:1 CR?

If there are so little gains, why is just about every single modern petrol engine up at CR levels that our Cdo’s would need methonol for?

Even your ‘grey porridge’ 4 cyclinder family hatchback is well over 10:1 these days, even though they’re turbo charged. The bike world is far more serious, your 955i = 12:1, R1 = 13:1, Norton V4 = 13.6:1, Ducati Panigale = 14:1, etc. And these are all box stock production engines, on pump gas, achieving modern emission ratings...
Click to expand...

I would have guessed the gains would be greater but perhaps not. The Wallace Racing CR change calculator gives similar values.
Those modern bikes with their super high CR also rev to the moon- that's mainly where all the extra hp comes from.
The Daytona below 5000 rpm is no more powerful than the Vincent. Below 3500 the 8 to 1 Vincent is stronger than the 12 to 1 Daytona.
And I also have a 9 to 1 Vincent but the 8 to 1 bike is a bit stronger...

Glen
 
worntorn said:
The effect of Cr alone isn't much

Increasing from 5 to 6 to 1 gives about 3.5% power increase.
>From 6-7 = 2.5%
>From 7-8 gives 1.4%
>From 8-9 gives 0.7 %
>From 9-10 gives 0.6%
>From 10-11 gives 0.5%
Click to expand...

Hi Glen,

I think your figures are wrong. Using the “ideal” Otto-cycle model calculation and a polytropic ratio of variable heats for real gas (1.25) as the exponent (power), we derive at figures in the order of 3.8% per point increase in CR (+2.5% for high CR's), which is in agreement with experimental values. Actually the mathematical model estimates a bit on the low side.

http://victorylibrary.com/brit/compression-c.htm

-Knut
 
Not possible, I borrowed them from hobot!

:)

The other source was the Wallace Racing CR calculator.

Glen
 
Ahem, meanwhile back in my workshop:rolleyes:

I couldn't be further away from concerns over combustion ratio's;) but interesting discussion all the same:)

Good news, RGM sent my gaskets today, might have them on Monday if I'm lucky or Weds/Thurs if postal service works as usual!!

I'm going to fit the pistons/rings into the barrels over the weekend & try out the method Jim uses in the link I posted. I won't be fitting the gudgeon pins, just want to see how it all balances out & how easy it is to get to the rods etc.
 
Back when I was racing a well developed 750 Norton and running VP high octane race fuel I experimented with compression on my dyno.

I found that I did not see any more peak power gains once I was over 10.5 to 1.

Over 11-1 I actually lost a little peak power but gained a little in the midrange, and high cylinder head temps became a problem.
 
mdt-son said:
Hi Glen,

I think your figures are wrong. Using the “ideal” Otto-cycle model calculation and a polytropic ratio of variable heats for real gas (1.25) as the exponent (power), we derive at figures in the order of 3.8% per point increase in CR (+2.5% for high CR's), which is in agreement with experimental values. Actually the mathematical model estimates a bit on the low side.

http://victorylibrary.com/brit/compression-c.htm

-Knut
Click to expand...

The table below provides Otto cycle efficiency comparisons over a slightly broader range of compression ratio than shown in the above link at Victory Library. The calculation used here to generate the data table is identical to that described at Victory Library where exponent K = 1.25 was employed. Thought it might be of interest. Hope it is large enough to be legible.

Leak Down Test (Don't Laugh!!)
 
Did a test fit of a piston & rings. Realised I've no nails so had to improvise!!

Had some computer/mobile phone tools that were perfect for easing the rings into the bore.

Jim makes it look so easy in the video, he's obviously had more practice:rolleyes:

and of course a photo of a piston & rings fitted in the bore;)

Leak Down Test (Don't Laugh!!)
 
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