head squish

[Norrie Bodge]

lads
im on fitting my Reed-valve Mick Abbey tuned iron 200 barrel
ive fitted my Cylinder head with the thinnest base gasket AF-rayspeed had 0.1mm
& have no head gasket fitted & im getting 1.9mm squish clearance
this is a head i used on my previous setup.... Ralf Saxelby stage 4 tune

i have an untouched std head
which dealer best to use who would turn me a head down to achieve 1mm - 1.5mm

cheers

[fairspares]

Depending on were you live most of the countrys scooters shops will be able to help, but its not just a matter of machining the head, you will be best taking the whole engine so they can check your timings and were best to machine and which gaskets to use , if you have gone to the trouble of having it ported and fitted a reed valve get it set up correctly . which reed block are you using and has the piston been cut to suit the reed? By the way did you give Mick your casings to check the timings , ideally he should of had the head also.

[Norrie Bodge]

I just sent him the barrel & piston to tune
He has cut the boost port in barrel & holes in piston to suit

You are right I would be best taking the whole engine to him m8

[Adam_Winstone]

If the squish band has the correct dome profile then recutting the head will only increase compression, not change squish.

Cutting the head will only reduce squish by making the radius smaller, removing the bore + 1mm diameter that most heads are cut to so that there is sufficient float so that it doesn't overlap the bore and allow the piston to his it. This might help if your profile has been cut to greater than +1mm (e.g. if you're running a 69mm diameter squish profile on a 66mm bore, where cutting into the head to take the diameter down to 67mm could help).

The above is providing the head has the correct profile and does not have a straight section at the edge of squish profile, which is sometimes used on long stroke motors or to get rid of gaskets.

If your head is the right diameter and profile, squish is determined by casing mouth length, barrel length and gaskets/packers (head and or base).

I hope the above makes some sense... I'm tired!

Adam

[Norrie Bodge]

thanks on the reply Adam
i was thinking could i send my standard head to a dealers ?
& they can turn the head down to suit what i need??

[Adam_Winstone]

If the head you currently have is correctly machined for diameter and squish (regardless of compression) then work to the head may make no difference to the squish. If the casing and barrel length are causing the squish issue then no amount of machining the head will resolve it.

To better explain what I mean, look at how far the piston crown comes up to top of barrel when assembled with the head off. If the crown is too low in the barrel then work to the head is irrelevant.

Adam

[Norrie Bodge]

Right i get what you mean Adam ..
its using a thin ringed Asso thats about 2mm ?? remaining at TDC

do i have any other options?

[rosscla]

If you have 2mm of barrel above the piston edge at TDC then as Adam has said nothing you do to the head will fix that. The only way to do anything that will. It alter your port timings would be to take a bit off the top of the barrel.

You could always get a 60mm crank or one with a 110 rod which might improve your chances.

[Norrie Bodge]

thanks on reply m8
ive measured the barrel & its 105mm from base gasket to the top... are some barrels longer at all?
as the Saxelby tuned one is the same length 105mm

& a 60mm crank it may av2 be then?

at its current squish at 1.9mm will i be ok or not?

[Solerunner1]

If you have 2mm clearance at the top of the barrel with the piston at TDC, then you'll have to machine the barrel if you want to close the squish clearance up. You can plug your port heights into a calculator to quickly find out where you're at.

If the timings sit good where you're at, then machine off of the top of the barrel. If you measure a squish clearance of 1.9mm, then have a machinist take 0.4mm off the top and you'll have 1.5mm squish clearance. Or 0.5mm and you'll have 1.4mm squish clearance. If you wanna go back up to standard gasket thicknesses, then just adjust the math accordingly.

There's a number of reasons why the barrel is sitting too high, but the bottom line is that if it's too high you've gotta bring it back down within range if you want the squish within the standard 1-1.5mm range.