Getting the GT186 compression right after last years issue!

[Donnie]

So last year this happened to my GT186 kit

Untitled by Donnie Canning, on Flickr

Untitled by Donnie Canning, on Flickr

After some thought about what to do and chatting to Rich Taylor I managed to get a local mate that does great work to sort it, yes, the exhaust ports rewelded and I got a new piston / rings at one oversize (64.5mm) and he rebored it for me too.

I mentioned to him that when Id built it up last year Id ended up using a 2mm head gasket to get a squish of 1.1mm (from memory) which he thought was odd, so on further measuring and looking at everything it turns out that what I thought was a 58 stroke crank is in fact a 60mm! Bonus!

Of course now, I am trying to get it all rebuilt so I can get run in over the summer!

(all measurements are without any sealant or paper gaskets)

So, in that quest (and at his suggestion) I've got a 1mm base packer and a 1mm head gasket as when we looked at it all together this left the exhaust port / transfers at a point where I should get improved performance without it being too peaky and then double check what squish that gives me.

However, I've just built it up and the squish comes out at .73mm which is too small and this is with the 1mm head gasket.
Also I have checked the combustion chamber volume with it built up, TDC and filled to the bottom of the plug hole thread gives me 22.2cc and all that inputted is

Screen Shot 2017-05-13 at 12.38.15 by Donnie Canning, on Flickr

This will be with a 30mm PHBH, MBGM V3 Clubman, static timing at 17 degrees BTDC and mainly for touring use but wanting some good fun speeds too. Im unlikely to go back to an expansion to be honest though if I every do will likely be a franspeed tourer.
Fuel wise I run 3% fully synthetic and standard unleaded.

So, Im thinking that my best option will be to go for a 1.5mm head gasket rather than add .5 to the base? Increasing the head gasket by .5 should give me a squish about 1.2/1.3mm.

Thoughts?

[Donnie]

no one? Looks like Im going to have to go over to scooterotica at this rate.

[jonw]

no one? Looks like Im going to have to go over to scooterotica at this rate.

Rich T has posted on Rotica before stating packing and timings using various stroke/rod cranks for both 186 and 200 kits.
I'll see if I can find it.

I run a GT186 using 60/110 with 2.5 packer plus base gaskets and 0.5 copper head gasket as recommended by Rich. Squish is 1.1mm.

[burnside]

My GT186 with 60/110 crank was built with a 2.5mm packer and 1mm head gasket giving a 1.5mm squish as advised by Richard. This was the email I received from him at the time

You don’t need to alter the port timings with a 60 stroke crank. You only need to build with the packing plate and use a 1mm head gasket. You should target a 1.5mm squish.

[Donnie]

cheers, he replied over on scooterotica about 2 base gaskets either side of the packer and that that should sort a proper squish so Ill try that first and see how I go from there.
My worry about using too much on the base is that it might produce too much of a lip at the exhaust port at bottom dead centre and i dont want to create a hotspot there?

[jonw]

I used two base gaskets either side of the packer and have had no issues so far.
Here is the info I referred to.

[Donnie]

thanks for that jonw, all good information.

right, with a base gasket either side of the packer, Im getting a squish of 1.38mm, but here's the exhaust port with piston at bottom dead centre where its now got a "lip" of 2 to 2.5mm whereas before it was only just measurable?

IMG_0822 by Donnie Canning, on Flickr

Im usually paranoid about this sort of thing as it could cause a hot spot and lead to problems such as a seizure? Or would this be acceptable?

This is why initially I was thinking of increasing the headgasket to a 1.5 from a one?

[Warkton Tornado No.1]

I used two base gaskets either side of the packer and have had no issues so far.
Here is the info I referred to.

One of us has got it wrong

The majority of two stroke top ends have the piston level with the bottom of the transfer & exhaust ports. Lambretta kits in particular tend to do so. I only state that for completeness.

Regardless of that, in the scenario of the piston undershooting the transfer & exhaust ports by 1.0 mm - as in the table above, - there remains 1.0 mm additional piston movement toward the head.

The consequences are for a 60 mm stroke in a cylinder to lower the transfer & exhaust durations.

[jonw]

This info was straight from the horses mouth...

https://www.scooterotica.org/forum/view ... 86#p243575

[Warkton Tornado No.1]

This info was straight from the horses mouth...

https://www.scooterotica.org/forum/view ... 86#p243575

I realise that However, I am trying to help here with an issue....

There are many examples of the maths involved online, but one of the 'easiest' calculators to check port durations can be found here:

http://www.csgnetwork.com/directporttimingcalc.html

To convince yourself, just input some figures:

The variations of Engine Stroke are 58 & 60.

The Rod Length is 110 in both instances.

The Piston Height above BDC is a guestimate but needs to be the same in both instances. I used 14 but this is purely to prove the effect of varying the stroke on port durations.

You should concur with me that port durations will decrease with the increase in stroke. However, by allowing the piston to undershoot the bottom of the transfer & exhaust ports it appears that the duration will be increased with the longer stroke. That will be proven by inputting 15 for the Piston Height above BDC with the Engine Stroke of 60.

Although the increase in total port duration is greater, I am surprised that the piston is allowed to dwell for 1.0 mm below the bottom of the port, but each to their own.

So, I now feel that my initial assertion of the variation in stroke has any effect upon the problems with pulling the FDR. My apologies

Nonetheless, as an aside, I would say that an engine is likely to gain from having the bottom of the ports aligned with the piston @ BDC.

[jonw]

"Nonetheless, as an aside, I would say that an engine is likely to gain from having the bottom of the ports aligned with the piston @ BDC"

Agreed.