decking the head

[first timer]

would it be a bad idea to skim the head/jug down a little bit on my 550? It would still be running stock cams and pistons. I do realize i may need to redegree the cams but that shouldn't be to hard. I hear talk about cleaning up the squish band and that helps a lot. Basic summery of how that works?

Basically i need to reassemble the motor with or with out gaskets (with out gaskets i give myself a little buffer room) before i put the head on i put a roll of clay over the piston and valve pockets bolt it all up, hand turn the motor over let the pistons and valves squish the clay take the head off cut some cross sections of the clay where squished, measure that, take that number and subtract it from the recomended clearance you hot rod builders tell me.
then i take that number to the machine shop and have them remove just that much?

[SPARKSS]

would it be a bad idea to skim the head/jug down a little bit on my 550? It would still be running stock cams and pistons. I do realize i may need to redegree the cams but that shouldn't be to hard. I hear talk about cleaning up the squish band and that helps a lot. Basic summery of how that works?

Basically i need to reassemble the motor with or with out gaskets (with out gaskets i give myself a little buffer room) before i put the head on i put a roll of clay over the piston and valve pockets bolt it all up, hand turn the motor over let the pistons and valves squish the clay take the head off cut some cross sections of the clay where squished, measure that, take that number and subtract it from the recomended clearance you hot rod builders tell me.
then i take that number to the machine shop and have them remove just that much?

I'll bite......What's the "squish band" you're referring to? "Squish" is indeed a function of compression and how a mixture lights and burns efficiently is very much related to how this progresses - which is influenced by the shape of the combustion chamber (head) and piston top. At any rate, the only reason I'd even look at either of these operations (and not on just any motor) would be to correct a problem.....warpage usually. You're risking ruining your motor for virtually no gain (in my opinion).

Sounds like you've been reading some old time hot rod books for sure
Not sure what you're trying to accomplish but I'll assume it's more torque at normal operating range (this is what's noticed).......Increase displacement (bore would be the easiest here) - which will, with no other changes also increase compression (also good if your fuel can stand it). Cam profile and lift are the next most significant change (notice I didn't say improvement!) you can make.......be realistic about where you want the power. Any of the above, with no other changes may require rejetting to make it work (and the economy may suck as a result......ok if you're full of bucks around lots of gas stations).

Just my .02 worth

[Steve]

I'll bite......What's the "squish band" you're referring to?

I don't know if this is found much in bike engines, but in car engines, there is a narrow band around the cumbustion chamber (might be formed by a band around the chamber in the head or a ridge around the top of the piston) that significantly reduces the effective diameter of the chamber when the piston is at TDC. As the piston nears and reaches TDC, the mixture that is in this region is 'squished' into the center of the chamber, increasing turbulence and improving combustion efficiency. The following picture illustrates how Buick accomplished this back in 1965 in their 225 ci V-6.


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[trippivot]

the squish band is a area in the combustion chamber at the outer circular edge around the hemi dish and flat mating surface of the head.

if you take too much off of the flat surface of the head you will need to cut a deeper squish into the smaller comb. chamber to achieve a specific compression ratio.

stick to milling the head surface not more than 0.020" and you will have plenty of room in the engine for the overlap event not hitting the pistons with the valves. little degree change in both int and ex. very little. and be able to use pump gas (91O)

this method is easy to over power the designed strength of STOCK head bolts crank case bolts and fragile stock CAST pistons.

this is increasing the combustion efficiency of the engine and the trade off is

1) increased heat
2)increased stress on all parts of the engine (above and beyond stock pieces)
3)demands better fuel

this is 1 way to increase power
there are other ways to increase power without the severe trade offs.

[first timer]

the squish band is a area in the combustion chamber at the outer circular edge around the hemi dish and flat mating surface of the head.

if you take too much off of the flat surface of the head you will need to cut a deeper squish into the smaller comb. chamber to achieve a specific compression ratio.

stick to milling the head surface not more than 0.020" and you will have plenty of room in the engine for the overlap event not hitting the pistons with the valves. little degree change in both int and ex. very little. and be able to use pump gas (91O)

this method is easy to over power the designed strength of STOCK head bolts crank case bolts and fragile stock CAST pistons.

this is increasing the combustion efficiency of the engine and the trade off is

1) increased heat
2)increased stress on all parts of the engine (above and beyond stock pieces)
3)demands better fuel

this is 1 way to increase power
there are other ways to increase power without the severe trade offs.

So what you are saying is I can skim a little if I follow the procedures I stated above and maintain a 0.020" valve to piston clearance. Plus it would be a good idea to take more from the jug then the head so I don't effect the shape of the combustion chamber to much?

Basically to adjust the squish band either you can use different shaped pistons or skim the head/jug down to tighten up the combustion chamber area? Stock 550's run at 8.5:1 or 9:1 so bumping it up to 9.5:1 or so shouldn't be a big deal especially if I am polishing the pistons and chamber to help reduce knock and willing to run 91 or 93 octane gas. I am not to concerned about mpg since I wring this things neck all the time.

My 550 is running a oil cooler, 4 into 1 with a 1.5 competition baffle, pods, a stg 3 jet kit and accel coils, and for laughs while I have the top end apart I am polishing the pistons and combustion chamber. Plus the 550 engine is overbuilt and runs a roller bearing crank plus there are folks running 650 top ends on 550 bottoms and none of them have blown up yet.

I was just looking at simple ways to pull a little more hp from the motor by tightening up the clearances a little bit. I am not looking to go all out and since I have the head and jug off, a skim isn't that expensive and doesn't require new parts.

So the short of it is go ahead and skim some just maintain the above stated clearances? And if I do my measuring without gaskets and skim to that clearance it will actually be a little bigger when the gaskets push every thing up?

[rapidray]

You need a MINIMUM of .050 clearance on the intake valve to piston & .075 on the exhaust side. Do NOT go less that this unless you like to buy new valves. Ray.

[hampshirehog]

We used to muck about with these 550 heads all the time in search of the elusive added horses. You can easily skim the head by 20 thou but it doesn't add much, so unless it's cheap....

One of my mates had some alloy welding done to create a 'proper' squish head - his design concept believe it or not was based on a T500 head used for racing, but 'softened' (we were naive / brave / daft enough to try anything in those days). I never saw it off the bike but it really pushed up the compression. The bike ran very hot and he had plug /starting problems which proved difficult to sort. From what I remember it was a lot of hassle for not very much.

As trippivot there are loads of other ways to make your 550 perform better and in my opinion dumping weight is the easiest and cheapest way to go. (If you sold the bits you took off you could even end up in profit\\/)

As regards the bottom end strength of the 550 - from my experiences you can do whatever you like and as long as you change the oil it's indestructible deep into the tacho red area and boy did we used to go into the red zone

[Dave8338]

As long as you're not running the risk of valve / piston interference, you should be alright. 9.5:1 is going to run just fine on 91 octane. The only thing that I would want to watch closely is the pre-ignition. As head temps rise and you raise the compression, there is a greater chance that you'll encounter pre-ignition. (The oil cooler will help here) As far as blowing apart the pistons or bottom end, I find it highly unlikely...RPM's will be more likely, the evil culprit! As long as the starter spins it, you should be fine.

Typically, you don't run into problems with compression until you crest the 10.5:1 or better mark. Yes, there are exceptions and every engine is different, but that has been my experience. :?