Leak Down Test (Don't Laugh!!)

Fast Eddie · Dec 26, 2018

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...

comnoz · Dec 26, 2018

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

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.

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 · Dec 26, 2018

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...

Just trying to get mine to go again

not bothered about speed

click · Dec 26, 2018

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]

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

Do you still use those 'fancy' base gaskets on all your rebuilds?

Fast Eddie · Dec 26, 2018

click said:
Just trying to get mine to go again not bothered about speed

Oh I think you have it running again soon enough Click.

And of course... then you’ll be thinking about speed...!

comnoz · Dec 26, 2018

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

Do you still use those 'fancy' base gaskets on all your rebuilds?

I don't have any more right now. Cometic is pretty proud of them.

worntorn · Dec 26, 2018

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...

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%

Fast Eddie · Dec 26, 2018

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%

Glen, you gotta learn to spot the

emoji matey !

worntorn · Dec 27, 2018

Saw the emoji but must defend the poor maligned MK3 first and foremost!

Glen

Fast Eddie · Dec 27, 2018

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...

Deets55 · Dec 27, 2018

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

worntorn · Dec 27, 2018

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

worntorn · Dec 27, 2018

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...

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

mdt-son · Dec 27, 2018

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%

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

worntorn · Dec 27, 2018

Not possible, I borrowed them from hobot!

The other source was the Wallace Racing CR calculator.

Glen

click · Dec 27, 2018

Ahem, meanwhile back in my workshop

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.

comnoz · Dec 27, 2018

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.

WZ507 · Dec 27, 2018

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

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.

click · Dec 28, 2018

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

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

Fast Eddie · Dec 29, 2018

Jim does make it look easy!

I don’t have his practice or experience. I still prefer to fit pistons to rods first. Then...

If the engine is out, I use finger nails and shuffling on the bench like this:

If the engine is in the bike I still prefer a stout board to support the pistons (shown below) then use piston ring clamps. I find this method utterly straightforward and fuss free...

Leak Down Test (Don't Laugh!!)

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