Setting Squish on an off centred plug hole head

[savage sausage]

I have finally rebuilt my engine and assembled everything I used all 3 base gaskets so the piston crown is flush with the exhaust port @ BDC.

My bike is a counterbalanced engined and the plug hole is off centred, after some discussions with ron black he advised me he cannot perform the head mod as he used the plug to attach to the lathe.

@ TDC there is quite a bit of clearance from the piston to the top of the cylinder, I tried the solder but maybe it was too thin ill buy some thicker stuff to see if that works.

my question is what can i do to try and correct the squish, remove some material from the head face ?? will that work or am i risking the piston hitting the head.
has anyone ever cut the mating face of the head, if so what sort of figures am i looking at.

Im on the final furlong now after what seems light years rebuilding my engine so any advice will be greatly appreciated so i can go ride my fresh rebuilt motor

[motopsycho87]

You need to find out your current gap first. Easiest way is with a vernier, take the head off, rest the base of the vernier on the edge of the cylinder with the depth gauge poking into the bore, crank the engine slowly so it pushes the depth mic up. This is measurement a. Next measure the depth of the recess of your squish to the mating face of the head, this is measurement b.

A - B = Squish

If it's between 1.2 and 1.6mm, leave it. If not, you need to space out with gaskets if it's too small (keep an eye on port timing), or machine the head if it's too large (avoid machining the cylinder, the nikasil will chip).

If the head is off center your options are to either mill on a rotary table, or hold very gently in a 4 jaw chuck on a lathe.

[Zman]

I see several options.

1. Cut the mating surface of the head (you will need to find a machinist that will mount the head via another means other than the plug hole.

2. Cut the mating portion of the cylinder but that could be more of a challenge as you will also need to cut the O-ring groove.

3. Take out the extra gaskets from the base. You will move the rpm range lower and will have more bottom end torque at the expense of a little over rev.

4. If removing the gaskets in item 3 does not do get your squish corrected you can also cut the cylinder base to get you were you need to be.

It is not critical to have the piston at the bottom of the exhaust port because the piston spends so little time there and because the majority of the exhaust flow is already gone. Ideally you want to set the port timing for your riding style and desired rpm / power deliver range. If you want more bottom end and mid range drop the cylinder down. If you want more top end, raise the cylinder up.

You can do some web searches for port timing to find ideal degree measurements. I was told that the GG cylinders are close copies to early 1990s CR250.

[(F5)]

I agree. Flush bottom of ex port is not a consideration. The only factor is where that leaves your port timings (given that you are not changing the ports themselves) suit the power you want. Rule of thumbs can be dependent on the size and relationship to the other ports and pipe.

Either way you can ruin a good setup with piss poor squish or the wrong compression.

As above you can skim head in a 4jaw but sod of a thing to grab. A normal head you could mill, but due to the step you'dneed to cad model it and cnc.

I've recently done a offset head but no step so milled. Then I was lucky and the chamber was concentric to the plug hole so i could open the chamber to get the right compression using sparkplug mandrel.

I'dgive up and put back to std. / or see if a latter head fits. Squish is best set to 1mm and head machined quite a bit to drop ccompression back to only slightly higher than std, assuming good petrol.

[savage sausage]

thanks for the reply guys i appreciate it, as much as i would like to cut the head i decided to reduce the base gaskets, as where i live is very difficult to get anything done without a major headache with language barriers.

a 4 hour drive in total just to get the head cut and even then they will probably not listen to what i am trying to say.

so for now i'll leave it and just ride it, maybe in the future look into it

[(F5)]

Ahh. Yorkshire.

[savage sausage]

Originally Posted by (F5)

Ahh. Yorkshire.

A cud talk to thee in't yorksher accent but dunt know if thal understand.

So tell me what do you know about yorkshire??

I'd love to have a rip on that cheetah engined RZ /RGV especially in the yorkshire dales on a epic road along the buttertubs pass, just gotta watch for sheep, the little wooly fuckers pop out of no where.


where i live in lithuania everything is a headache due to the language, i know what i want but try to translate to them in my very limited lithuanian sucks, so i just used the 0.5mm base gasket instead as i thought in reality how often will it be balls out.
Apparently the housing for the water/coolant is different on the newer cylinder heads, plus the coil is in an awkward place.

[motopsycho87]

Have you got a picture of the head in question? If it's got the same stub for the engine mount as mine, a L shaped bracket could be milled that could also be held in a 4 jaw. Rigidity isn't much of a concern as ally is soft as...

I did manage to hold my KDX200 head in a 4 jaw, just need to be very gentle on a bigger lather.

[(F5)]

Haha, sorry total stab in the dark , whats the chances? we've had a few at work and an ex gf.

Never been that far up, only London area and at some pubs English was the minority of conversations i heard. Kinda cool but odd for us sheltered remote islanders.