Cylinder head skimming

[snoopios]

Hey guys,
This is kind of an add on to my last post but i just had a few questions about skimming. I know what skimming is but i can't seem to find a lot of info about;

1) What different methods there are
2) Possible drawbacks
3) How much should be taken off
and any other info that there might be about skimming

Thanks

[XG332]

1. A couple.

2. Higher cylinder compression, if to much is taken valves may hit pistons.

3. Enough to make it flat.

[rallycross]

method - take head off and send to head skimming place (chose a good one who know what they are doing for your make of car).

Amount to take off - depends on the car.

The more you take off the higher the compression and at some point you go too far and the head has given all it can safely be removed, if too far then possible compression is then too high and it wont run right or worse valves meet head, crunch, expensive mistake.

Whilst getting it done make sure the valve seats are all good, maybe get them lapped in.

[snoopios]

Thanks for the reply guys but i thought i should probably clear some things up. This is a project for Uni and it's all theoretical. The engine we are "working with" is a Honda GX160 - http://engines.honda.com/models/model-detail/gx160 which is a four stroke, single piston engine. What we have to achieve (theoretically that is) is 50% increase in power with 35% increase in RPM. So generally, what else can you do to a cylinder head other than skimming, porting or changing the valve angle to increase the power with RPM?
Also, any help with the original question would be much appreciated.
Thanks

[thinfourth2]

Its a fecking lawn mower engine

Don't ask us go over to the lawn mower racing forums

What i will tell you is learn how the revs are governed as it isn't by where you set the throttle setting. it has something else on the engine

I'm not taking the piss as the lawn nutters get loads power out of those

[davepoth]

Is it aircooled? excess heat will be a major concern if it is. Meth/water/nitrous injection will reduce the cylinder temperatures.

If the engine is OHC skimming the head will advance the timing too so you'll need a vernier pulley to sort that out.

-edit-

I see from the link it isn't.

-edit-

Any idea what the spring does on the camshaft?



-edit-

I know what it does. It's a centrifugal governor. I imagine locking it into the non governing position will get the power you want with no further adjustments. Once, anyway.;)

Edited by davepoth on Friday 28th January 19:00

[jsg612]

Correct that is part of the mechanism. There's also a little arm which comes out of the top of the crankcase, just under the fuel tank. Used to work on these a few years ago.

[maniac0796]

Remember if it's an OHC engine, then you may have trouble tensioning the belt or chain if you skim it too much.

However, if you're talking about a lawnmower engine, then I would guess it's sidevalve or pushrod?

[wildman0609]

i don't know what the shape of the combustion chamber looks like but could be worth adding metal (weld it) to reduce the squish area there for increasing compressing without skimming.

[B'stard Child]

Seriously - A lawnmower!!!

Pistonheads - grass cutting matters

[thinfourth2]

davepoth said:

Is it aircooled? excess heat will be a major concern if it is. Meth/water/nitrous injection will reduce the cylinder temperatures.

If the engine is OHC skimming the head will advance the timing too so you'll need a vernier pulley to sort that out.

-edit-

I see from the link it isn't.

-edit-

Any idea what the spring does on the camshaft?



-edit-

I know what it does. It's a centrifugal governor. I imagine locking it into the non governing position will get the power you want with no further adjustments. Once, anyway.;)

Edited by davepoth on Friday 28th January 19:00

I imagine that the thing labelled camshaft decompressor might be a decompressor.

It keeps the exhaust valve open at low revs allowing you to spin the engine quickly enough for it to fire easily as once it gets above 100Rpm it will close the exhaust valve and off it will trot

[jsg612]

OP, take a look at this website Chap, some useful info on there... http://www.gx160parts.co.uk/

Hope this helps and best of luck with the project.