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Head mods vs local machine shop

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i was wonderig if its worth paying to have my head sent of and done or if i could get similiar results by going to a local machine shop with a good reputation and having it shaved down. and if i get it shaved how much is a good amount for overall performance?

:lol:

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Shaving the head is a good proposal. Keeping the piston flat (no dome) is better for flame propagation. The dome is not blocking the flame front from reaching the edge of the cylinder.

There are a few important variables to consider:

Piston to valve clearance

Quench distance

dynamic compression ratio

Cylinder head shape design

Most just install a taller domed piston to raise the compression. This way any compression changes can be reverted back to stock.

To be honest I am not sure the advertised piston CR is entirely accurate. There are many factors that come into play when determining static compression ratios.

Gasket thickness, base and head. Combustion chamber cc, cc's below the top of the piston and cam selection.

Depending on the cam used the static compression ratio goes out the window. For example an engine with a cam used for top end with a 13.5:1 piston will end up at from 9.5-10:1 CR.

When trying to make more power more work has to be done to make sure proper clearances are present to prevent valve to piston contact and piston to head contact.

In order to check this the engine has to be disassembled, solder placed on the piston, re-assembled, engine rotated, disassembled and checked. Then the combustion chamber has to be cc'd, gaskets measured to find the cc and then do the math to determine the static compression. Once you have that you can enter the cam profile into a simulator and find the static compression. There are other ways to measure the above but more specialized tools are needed.

Different diameter bores, different style pistons and the condition of the engine determine the tolerances to be used. When you setup an engine "tight" you have to pay more attention to the number of runs on the engine so that components are replaced before P to V contact comes into play.

In short, if you want more compression buy a piston with a higher advertised compression ratio. It will increase torque and HP the higher you go and cause more wear on the engine.

Most anyplace can shave a head properly. Any real power increase will come from a combination of modifications. Head porting, piston CR, exhaust designed for a particular RPM range and a cam designed for the RPM range you want to run at.

One of the best areas to work on with to make anything faster is to reduce weight. It is a physics law: acceleration = force divided by mass. Unfortunately "mass" (resistance an object has to change in its velocity) is not weight alone. However, reducing weight will lower the mass if the shape of the mass remains the same.

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Pretty hard to answer, without the model of bike being stated. Unlike Rocksalt I'm going to assume that you have a 2stroke, as "shaving the head" is almost never done on 4strokes, a compression increase is generally achieved with a different piston on a 4stroke, but this would alter port timing if done on a 2-stroke.

Need to determine what the "squish" is currently at to determine a safe amount to remove. If you use a reputable raceshop they will already know what to do, for either pump gas or racegas (eric gorr, etc). If you wish to use a machine shop they will be quite cap-able of the work, but you will have to tell them how much you want removed. I can explain a suitable way to check your squish if you wish to go this route.

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bu yea i keep reading about the squish but how to measure it seems confusing to me, i cant seem to grasp the concept

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, as "shaving the head" is almost never done on 4strokes, Not true a compression increase is generally achieved with a different piston on a 4stroke, True but this would alter port timing if done on a 2-stroke. How? Isn't he talking about a cylinder head?

Need to determine what the "squish" is currently at to determine a safe amount to remove. If you use a reputable raceshop they will already know what to do, for either pump gas or racegas (eric gorr, etc). If you wish to use a machine shop they will be quite cap-able of the work, but you will have to tell them how much you want removed. I can explain a suitable way to check your squish if you wish to go this route.

Sounds like good advice to me. I second just sending it to a proven race shop. They have already put the time in and learned from their mistakes and you will benefit from it.

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Using the solder is acceptable, but it's more accurate to measure the deck height at TDC, then add or subtract any inset or relief in the head, and gasket.

anything you squish has a certain amount of spring back, the more its squished the more error, so try to select solder that is close to what your measuring.

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on your bike,you dont want to shave the head.you will need to re-machine the combustion chamber.and there are several options availible for the 250 yammie.the most important thing is to get it done by a known good engine buider.and be sure to tell them what you are trying to acheive.just removing material from the sealing surface will only result in a motor that will want to detonate and burn up your piston,and damage the head.

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I would suggest talking to Ron at RB Designs. He could tell you if a head mod will get you what you are looking for.

Do a search in the Yamaha 2-stroke section, he has done tons of YZ heads for people on here.

He's done a couple Honda heads for me, and he does excellent work.

http://www.rb-designs.com/

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06 yz250 my bad

2 strokes respond much better to polishing up the ports, slightly higher compression and oxygenated race fuel.

More power comes at a cost and that cost is higher service intervals. So yes, you can get more power out of that motor if you set the squish to 1.5mm and run oxygenated race fuel. But the motor will last a few hours before you damage the bearings. So that means, rod, bearing and piston replacement every few hours, not a pretty sight.... but if you're building a race bike, thats the price ya gotta pay sometimes.

So my suggestion is to just sent the cylinder out, get it ported and set the squish a tiny bit tighter then previously set. You want at least 2mm of squish for the motor to run reliably.

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You want at least 2mm of squish for the motor to run reliably.

That's more than stock.

.040"/1mm squish and the bowl re-cut to maintain stock compression would be a normal mod if you want to run pump gas.

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That's more than stock.

.040"/1mm squish and the bowl re-cut to maintain stock compression would be a normal mod if you want to run pump gas.

Yea, I tend to run them a bit conservative, but then again, I keep the head stock. All in all, 1mm of piston to head clearance might sound like enough, but as the bearings wear and the rod stretches, you will get piston slap. Stock setting at 1mm sounds like they are willing to risk longevity for power.

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There is no rule on what squish should be!! There is a lower limit, but there are so many factors involved that it is misleading to just say "set the squish at .040" (which is very near the lower limit of safe squish). That's why an experienced engine-builder's input is needed. I could be wrong but I've used .035 as the lower limit on 125's, more on bigger bores.

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There is no rule on what squish should be!! There is a lower limit, but there are so many factors involved that it is misleading to just say "set the squish at .040" (which is very near the lower limit of safe squish). That's why an experienced engine-builder's input is needed. I could be wrong but I've used .035 as the lower limit on 125's, more on bigger bores.

The OP is specifically asking about a 2006 YZ250.

20 pages of late model yz250 squish discussion: http://www.thumpertalk.com/forum/showthread.php?t=573021

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oems will often leave the squish clearance wide for reasons of production tolerance and assembly. In other words no matter how the finish machining is done on the case, cyl and head surfaces, all the parts go together "safe". As an "adjustment" they will make the squish band width wider than they would if they had set the clearance tighter.

More compression is not always better. more complete combustion is what makes more power. Compression is only part of that equation along with a whole bunch of other factors.

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