Help 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?

[cgt75b]

Is that part of the exhaust gasket face come away?

[dave411]

looks like

[Rich_T]

Hi Donnie, I sent you a PM about this when I saw your post on LCGB. Your post says the measurements are without gaskets. If you add a gasket either side of the base packer you will gain 0.8mm to the squish and head volume. This will give you 1.53mm which is bang in the money and the squish volume will be correct too.

[Donnie]

Cheers Rich, I've just checked both on lcgb and here and there's no PM im afraid

I'll try the base gaskets either side and see how it measures up though

[Rich_T]

Does the calculator you have used need to have a deck height distance?

[Donnie]

This is the one I used http://lambretta-images.com/archive/porttiming.php

[Donnie]

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?

[Rich_T]

Hi Donnie, This step between exhaust port and piston is acceptable, it is typically 1mm. The change between 58 and 60mm stroke adds 1mm to the top of the cylinder (reducing the squish by 1mm, which you get back by adding a 0.9 - 1mm head gasket) and brings the piston 1mm lower than normal.

[Donnie]

Cheers Rich