Measuring squish through the spark plug hole

[Guest ratchethack]

OK, some o' you Guzzi cylinder head experts. I've not had the heads off. I'd like to get an accurate read on the stock squish setting that comes from Mandello. I've got .084" solder, and can't get a "clamp" between piston crown and cylinder head through the spark plug holes at TDC. In fact, it feels quite a bit looser than this.

 

I was expecting a reading of something in the neighborhood of .060" to .080".

 

Is this possible? Am I missing the squish band entirely with about 3 cm "hook" in the solder, pointing in the downward direction from the spark plug hole?

 

If I'd ever seen the inside of the Guzzi cylinder head, I'd no doubt be able to explain this, but I'm literally flyin' blind here. Though I've been on the inside of a pile of SOHC and DOHC cylinder heads, I haven't had a hemi pushrod head off for over 30 years. The above doesn't fit my expectations.

 

Anybody got an explanation?

 

UPDATE: This gets curiouser and curiouser. I just tried .130" solder, and couldn't get a "clamp" with this either. . . . .

[big J]

I've never had mines off either,but I'd bet there's no other way.When I've measured squish on other engines,I used 4 small pieces of solder equi-spaced around the squish band area.

You may find there's a difference in the gap at different points around the head.Use two on the axis of the gudgeon pin,and the other two perpendicular to these.

When you're putting the solder in,you're not rocking the piston over are you?

[Guest ratchethack]

Big J, the only way I've measured squish previously is to do it the "proper" way with plasticene on the piston crown and head fully torqued, then taking the head off for a thorough analysis. I'm trying to "cheat" a little here without meddling with the heads, just to get an idea how it comes from Mandello. I'm not expecting to be able to do a sophisticated peripheral analysis of the entire squish band this way. I'm just after one read per cylinder.

 

I'm using a length of solder bent straight down and poked through the spark plug hole, which would make it perpendicular to the gudgeon pin. What I'm doing is slowly rotating the crank with a wrench on the crank nut and feeling with the solder as the piston approaches TDC. Ideally, I'd like to "clamp" the solder slightly as I roll it through TDC, just enough to deform the solder and get a measurement. The squish band must be just outboard of the plug hole, between it and the bore -- right where I'm holding the solder -- and yet, I get no clamping whatsoever at this location rolling through TDC with .130" solder.

 

The only conclusion I can come up with is that I must need more than ~3 cm outboard of the sparkplug hole to reach the squish band?!?!?

 

Now I gotta wait for it to cool after riding so's I can have another go at it. . . .

[Allan1956]

Okay, for the stupid

What's the "squish".

 

I thought it was just the noise that the cane toad made when the guzzi ran over it ( or sometimes ...BANG...but that depends on the size of the toad)

[mike wilson]

92mm bore? Central(ish) plug?

 

Then about 46mm from centre to periphery, of which about 35% (the only figure I can find online) is squish. You might be falling a bit short with 30mm of solder. I would drop the piston all the way down, fiddle with the solder until you find the bore wall then, using a new bit, try that length.

 

 

And hope none of it breaks off.

 

 

If it does, Blu-tack on a coat hanger works well.

 

 

Although I'm not sure you have Blu-tack over there......

 

 

 

8-))))

 

Two BTWs:

 

1. I would use a solder loop, which might reduce the chances of a piece breaking off.

 

2. Squish is the formation of an area inside the cylinder head which makes a very thin ring around the outside of the bore at TDC. This makes all the gas there try to whizz into the centre, both concentrating the chanrge and mixing it thoroughly. If it's done properly, the charge won't even explode prematurely!

[Guest ratchethack]

92mm bore? Central(ish) plug? Then about 46mm from centre to periphery, of which about 35% (the only figure I can find online) is squish.

Thanks, Mike. If there's one thing I learned well about hemi engines way back in their heyday , it's all the trade-offs and limitations of the design of the combustion chamber due to the fact that the spark plug can't be located in the center. The Guzzi head (at least from outward appearances through the spark plug hole) appears to have the spark plug considerably off center, at least halfway between center line and the bore, maybe even closer to the bore.

You might be falling a bit short with 30mm of solder. I would drop the piston all the way down, fiddle with the solder until you find the bore wall then, using a new bit, try that length.

And hope none of it breaks off.

I sort of came to the same conclusion, though I will say that the prospect of finding the bore with a bent-over piece of solder through a hole as deep as the spark plug reach, that's not so long as to become folded back against itself with a piston rising toward TDC is considerably harder than it sounds.

 

Part of my problem is that it's been so hot here that my patience wears thin on such "fishing expeditions" as this, particularly when I'm bent over, looking through a small dark hole, rotating the crank with a wrench with one hand, "fishing" with the other, and trying to imagine what I've never seen inside.

 

This will have to wait until next weekend, when hopefully it'll be cooler, and I'll give meself more time to catch a good measurement.

[mike wilson]

If the plug is to one side, try going for the squish area furthest away from it. The plug must be angled towards that.

 

 

 

 

I think.

 

You could do it at night as you are already working in the dark. 8-)))

 

Sorry, couldn't help myself.

[Guest ratchethack]

If the plug is to one side, try going for the squish area furthest away from it. The plug must be angled towards that.

This makes perfect sense, Mike.

 

It would seem to give a near-optimal angle of approach, without having to bend the solder into messy contortions.

 

Many thanks!

 

Will advise.

[mike wilson]

This makes perfect sense, Mike.

 

It would seem to give a near-optimal angle of approach, without having to bend the solder into messy contortions.

 

Many thanks!

 

Will advise.

 

I'd still use a loop so that, if it breaks, it should be attached at one end.

[Guest brucev11]

OK, some o' you Guzzi cylinder head experts. I've not had the heads off. I'd like to get an accurate read on the stock squish setting that comes from Mandello. I've got .084" solder, and can't get a "clamp" between piston crown and cylinder head through the spark plug holes at TDC. In fact, it feels quite a bit looser than this.

 

I was expecting a reading of something in the neighborhood of .060" to .080".

 

Is this possible? Am I missing the squish band entirely with about 3 cm "hook" in the solder, pointing in the downward direction from the spark plug hole?

 

If I'd ever seen the inside of the Guzzi cylinder head, I'd no doubt be able to explain this, but I'm literally flyin' blind here. Though I've been on the inside of a pile of SOHC and DOHC cylinder heads, I haven't had a hemi pushrod head off for over 30 years. The above doesn't fit my expectations.

 

Anybody got an explanation?

 

UPDATE: This gets curiouser and curiouser. I just tried .130" solder, and couldn't get a "clamp" with this either. . . . .

 

Re the squish band , attached is a picture of the cylinder head 'cut away' . This may help in the 'fishing' with the solder.

[Guest ratchethack]

Re the squish band , attached is a picture of the cylinder head 'cut away' . This may help in the 'fishing' with the solder.

Most helpful, indeedy, Bruce. This is one case where the photo is worth a thousand words. Many thanks!

 

I assume the dual plug head dimensions are about the same as the single plug heads. I note that the outboard spark plug is exactly where I expected it to be relative to the combusion chamber, but no wonder I couldn't reach the squish band by going straight downward -- not only is it more of a stretch than I had imagined, it's also more of an acute angle to reach it than I'd expected. As Mike suggested, it's nearly a straight shot across to the other side. This is what I'll shoot for next.

 

Strange that the inboard plug hole locates the electrodes closer to the centerline of the bore than the outboard one, and that it's also at a steeper angle relative to the bore. Different diameter plug -- no doubt different heat range also.

[dlaing]

 

Re the squish band , attached is a picture of the cylinder head 'cut away' . This may help in the 'fishing' with the solder.

hmmmm...I can just imagine the Mike Rich pistons snugging up for nice tight squish right by the squish band.

[luhbo]

I was always asking me why all the US Guzzis seem to be so prone to pinging. Now, what if they haven't sold you any squish area at all?

 

Hubert

[Guest ratchethack]

I was always asking me why all the US Guzzis seem to be so prone to pinging. Now, what if they haven't sold you any squish area at all?

 

Hubert

Why, in this case, Hubert, I'd say us Yanks might've been sold a bag o' goods. . . . . After all, we paid for a squish band -- we darned well better have one! Now if we didn't get one at least in the neighborhood of .060" to .080" (1,5 to 2 mm), I'd expect even worse pinging than we tend to get with 91 RON pump gas.

 

After all -- in the case of such a development, much like the .6 kg/mm Marz fork springs we got that were rated for riders of 135 lbs (61 kg) it's easy enough to make right what the factory got wrong. Now depending on what we've been given to start with, by getting it set to .040" (1 mm) -- I would expect such a change could decrease pinging considerably -- and possibly yield a bump of some kind in mileage, even power output. . . . .

[motoguzznix]

 

 

Ratchet

 

The shape of the V11 cylinder head is different to that on the picture sent by brucev11. I tried to draw that into the picture above. As you can see, the breva has the classic hemi with squish again.

The V11 heads have a conical shape in the area from the cylinder bore to the hemispherical chamber. So there is only a small area of approx. 1 - 2 mm squish remaining on the ouside of the combustion chamber.

Squish on my V11 was at 1,7 mm.

You have to mill approx. 1mm off the heads to make the conical area disappear. Then the hemi area blends into the then larger squish area, which at my opinion makes a better combustion chamber design.

When you further machine approx. 0,5 mm from the cylinders to bring the pistons to the top, you are 1,5 mm shorter with head and cylinders, so the pushrods should be shortened by the same amount, to prevent the valve train geometry getting asymmetric.

Furthermore, the compression ratio raises and the valves come nearer to the pistons. Valve clearence has to be taken into account and the pockets in the pistons might have to be deeper. I would look for a CR not too much above 10:1 without twin spark.

Unfortunately I am only half that way with my V11, but when I pull off my heads next time, all that will be done.