F4/F4i:

It seems Honda makes 2 different thicknesses of head gasket, a thick and a thin. The problem is I can't seem to find the thicknesses anywhere. What are they?

Using the thin head gasket, what is the max mill on the head before PtoV clearance becomes an issue?

Chris

 

Well right after posting I thought to check cometic, they make the following gaskets for F4/F4i:

Kit Type Stock Code OEM Bore Size Material Displacement Price
Head Gasket C8636 12251-MBW-013 68.0 mm MLS/C.O.T. .030" 617cc $84.36
Head Gasket C8671 12251-MBW-013 69.0 mm MLS/C.O.T. .030" 635cc $84.36
Head Gasket C8572 12251-MBW-013 67.0 mm MLS/C.O.T. .030" 599cc $84.36
Head Gasket C8572-018 12251-MBW-013 67.0 mm MLS/C.O.T. .018" 599cc $96.37


I would guess that makes the stock gaskets 0.030" and 0.018" as well? 0.018" seems really tight unless the pistons are not reaching the top of the bore (I don't know if they do or not as my motor was apart when I got it).

 

u can check the current v to pist clearance like this.just bolt the head back on whth the old gasket. install cams, chain and tensioner. then put a short length of solder through the spark plug hole and place it under the valve. then turn the motor over untill it squishes the solder. then u measure the new thickness of the solder. i like to start with a thin peace of solder and work my way up, rathere than starting with a thick piece.
u can also measure the piston to head clearance this way. but i use 2 pieces of solder at once. its a bit tricky. i put 1 over each wrist pin , at the cylinders edge. this is so u dont get an inacurate measurement due to piston rock.

with the head off, u can install the pistons and bring 1 up to tdc, then measure how far down it is in the bore with a dial indicator or vernier caliper.
if the stock gasket was .030" the .018 would give u more compesion , providing u have enough clearance. i wouldnt go less than .035" piston to head clearance unless u got a good number from a trusted source. i have run .035 on a big my cc 2 valve motor. not sure what u need for v to piston. i run .080"exht, .06" intake, on my 2 valve.

 

Thanks for the info/help. My motor is all apart right now so I was hoping someone had already done milling and could tell me from experience, otherwise I'll be measuring it myself. I use the same process you mentioned, though I prefer to use clay. With clay as I can cover a large area of the piston crown and then cut the clay with a razor blade to actually measure thicknesses.

I dug the old head gasket out of the box today (previous owner already had the motor 50% apart) and it's the 0.030" version. Looking at the carbon in the cylinders it looks like the piston stops below the deck so I could see where the 0.018" would still work. I'll need to measure piston/deck height once I get the bottom end back together (before I order gaskets).

0.035" is the standard that I have always used on my motors, though was going to ask before I built this motor (I have never built an engine for more than 8K). Likewise I always used .100" exhaust/0.080" intake, though motorcycle valves are quite a bit smaller than car valves so there should be less chance of float. What size valves are on your 2 valve motor (trying to do a comparison of valve size/weight)?

Would you happen to know if the F4i valve springs will drop into the F4 head? I like the idea of having a bit more spring pressure to prevent float (call me cautious.... bent valves due to float before).

Thanks again,
Chris

 

I use play-doe. lol
i use 37.5mm intakes in my KZ1200cc motor.
i dont have any experience inside a modern motor. my f4i trackbike is running too good to take apart yet. i have been upgrading the suspension instead.
check out APE and see if they show valve springs for f4's . maybe they have the same part # for F4i's ?
maybe a rev limiter would be a better option, stronger springs reduce power.

 

I have seen play-doe used at engine building shops. It's cheap and gets the job done; that's what counts. Any form of clay that won't harden quickly (and doesn't need water to keep it soft) will work.

I know the F4 and F4i springs won't be the same part# because the F4 is a single spring vs the F4i is a dual (inner and outer). A rev limiter won't do any good as far as preventing valve float unless it is set quite a bit lower (which would limit RPM range and thus performance).

I have discussed with with some prostock builders over the years and the consensus is that that springs actually DO NOT reduce power because of conservation of energy. Here's why:
- Even though it takes more energy to push the stiffer valve spring open (lets say 200lbs of force needed), on the closing side of the lobe, that energy (200lbs of force) is pushed back into the system by the spring attempting to return to the closed valve state. So in the end it's really negligible.

 

i'm not sure that i agree with your theory. as the spring helps to push the cam lobe down, it would translate that energy through the cam chain and oppose the piston trying to come up. i always thought that it would be negligible.


i just read that the newer 05? rr's have a lighter valve that allowed lighter springs. this allowed an increase in rpms from 14200 to 15000 over the f4. this allows for more over rev room.
i hope i got that right, i am going from memory. i read that a long time ago, i think it was yesterday lol.

That is fairly significant.

in the end , if a stronger spring allows u to over rev, and u go faster because of it, then it doesnt really matter if u lose a bit of power does it.

 

Nope, it would help the piston travel up the bore (you are forgetting that at any point in the rotation, if one piston is traveling up the bore, another is moving down the bore.... but different subject).

The cam lobe is getting smaller (as the cam advanced, the valve is allowed to close). The natural tenancy of the valve/valve spring is to reach the closed position. In order for the valve/valve spring to reach this closed position, the cam must continue to rotate in the "forward" direction to allow the lobe to advance. This "forward" direction of camshaft rotation is the "forward" direction that it would be pulled by the cam chain, crank, and ultimately the "piston." So, as I argued before, the valve springs are actually putting their energy/force back into the system. Conservation of energy. Now while one valve is closing, at least anther is opening, so they tend to cancel each other out. And yes there is frictional losses, inertial losses, etc. BUT, the point still stands, valve spring stiffness DOES NOT really matter in regards to "loosing power."

However, stiffer springs do cause problems elsewhere, namely cam lob wear and cam bearing wear. Because the camshaft lobe and bearing must resist the physical pressure of the spring, high pressures cause more wear on these parts (also going to stretch chains a bit, add to noise, etc...). Reason being is the valve spring does not want to compress. When the cam lobe comes around and tries to compress the spring, the spring tries to:

a- push the cam away and the bearings have to resist the load
or
b- wipe the lobes off the cam (make the cam a smooth round bar with no lobes) and therefore no be compressed

Also there will be increased force on the valve, valve stem, and valve seat and face.

So there are certainly reasons to not use excessive valve spring pressure, however it is a matter of longevity rather than power. Don't know if you have ever seen one, but this is why GM went to a beehive shaped valve spring in the LS1s. The beehive shape caused a non-linear natural frequency (don't ask me to explain this one, I dropped out of mechanical eng before I got to studying natural frequencies) which reduced tendency to bounce/deflect and therefore reduced valve float. Prior to the beehive shape, GM was using large, higher pressure, sometimes dual springs. With the lower pressure beehive, more aggressive cam profiles could be used while extending gaining increased longevity.

Make since?

Chris

 

Well that is a pretty good explanation.
Thanks for taking the time.

i cant say i have ever thought about it in such detail.

i just recall reading article's over the years, and it seems common to hear that spring pressure robs power. it just seems logical to me that anything applying preasure against the grain, has to be a negative .
not something i have ever tried to overcome, just accepted.

 

Yeah, things that resist rotation of the motor do rob power, though springs (to an extent) give the "energy" back to the system when they "push" the system on the closing ramp (it's not 100% but nothing ever is). Piston rings, piston skirt clearance, bearing clearance all rob HP. Heck even oil on the bearings takes HP which is why lighter weight oil shows more ponies.

The article is right in saying don't use any more pressure than you need. Heavy springs cost more to buy, they often require modification to the head to fit, they hurt longevity as they stress parts, they hold a higher tendency of cracking, etc... a whole host of issues.