Anyone know about checking spigot to head clearances?

[stinkwheel]

It's on my fairly highly tuned 612 enfield bullet. I'm getting oil blowpast round the head gasket which I think is either because pressure is escaping an excessively wide spigot gap OR the spigot is to high and the head gasket isn't being compressed enough.

I'm pretty sure it's the former because there is no pressurised oil passing through the head gasket, only the return from the rocker box (which has an atmospheric breather fitted) or anything which has made its way up the studs from the bottom end (which has a negative pressure breather). The high pressure oil feed to the top-end is external. In either case, there's no pressurised oil there so I can't see how it would be blowing out unless assisted by cylinder pressure.

I'm running a modern cometic composite head gasket with a fire ring.

There's very little out there on how to measure it, I'm thinking pop the head off, plastiguage the top of the spigot and re-torque. Then the question is what gap to shoot for? In an ideal world it would be zero but if it is, how do you check the head gasket is sufficiently compressed. If there is a gap, what gap is acceptable or do you skim it for zero?

The head can be skimmed to close the gap and the spigot can be skimmed to widen it.

Or am I barking up the wrong tree and there's an easier way to do this?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Robby]

What do you mean when you say spigot? I'm trying to understand the problem, but google just shows me fuel taps and the fiche doesn't show any obvious reason for head gasket to allow oil out but keep compression in.

Could it be leaking from the rocker covers and working its way down? I assume you have a higher output oil pump, but the drain holes from the rockers would be standard size.

I mention this because there still appears to be no way of persuading a Honda CB250RS rocker cover to seal, but the oil pools around the head gasket and on my left boot.

[stinkwheel]

The cylinder liner stands proud of the cylinder surface and sets into a recess in the head. In theory, the spigot seals the combustion gasses and the head gasket seals the oilways.

There's little out there on exactly what the clearance should be and how you check. In theory, the spigot should seat against the head at the exact point the gasket is fully compressed for proper function. In reality, it appears nobody actually does this or if they do, they aren't saying how. It's also probably why nearly every spigotted barrel you see is leaking oil round the head gasket.

I'm pretty sure it's leaking round the head, the sides of the rocker covers are clean and I fitted a bunded breather in one of them so there shouldn't be any positive pressure in there.

Cylinder barrel with spigot:
https://lh3.googleusercontent.com/pw/AP1GczMYsWvff3GlMST8I6urHqvFLysqp9onlqI80EoRV-ppOxY5OkFrO27E3Xcfw2iglQpqq7UIEQmpYn4q96XvCHtPOE6xTYWt2ShqBd6GhRBFsmVQVBzQF4amzVlXViI3pWZWa-dTFOJjKN457x-a1aRb=w657-h876-s-no

Cylinder head with recess for spigot:
https://lh3.googleusercontent.com/pw/AP1GczMjSSWw9Jzkpm-16LRYOkBvy9XBsk6J2AzrTxk49BPKpwaXfm6BSg6jh2RDPyVExJYy7IoO6cpEGhlZ6ST-bpwvFGh-urrzwkFCmh2K4WF1noCwMxlARlQuWpx14LfLSJ75GQ33V8GoJkFDN0M6A2Su=w1168-h876-s-no

The head gasket fits round the outside of the spigot and is sandwiched between the head and barrel in the normal way.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[virus]

I think your about right in your plan there. Id be looking at going down the same path, possibly using spacers to vary head gasket thickness to suit if its not holding back compression, on account of it being easier to throw another spacer in than do the afformentioned machining.

What I would suggest is deffo try and get the plastigauge measurement with it hot / as close to running temp as possible, test with spacers til you get the 'right' measurement then machine to suit. For something like that i'd aim for a 0-0.1mm clearance at operating temperature.


The other option ofcourse if the spigot deals with all the combustion and the head gasket is just for oilways, is to machine a small countersink into the top of the barrel and fit an O ring for the oil ways and do away with the head gasket entirely.
____________________
own: 81 xs1100g...
owned: 85 rat CG (sold), 91 GS500e (stolen), 84 gsx400f (scrapped), 81 z250 (siezed, siezed, scrapped), 83 cb250rs (sold), 84 gpz750r ratfighter (killed) 84gpz400 (sold), '80 cb650 ratfighter (wrote off) 95gsx6/12f ratfighter (killed) 91 xj900 (sold)
stinkwheel Well I just had my hands up a pigs fanny. Which makes your concerns pale into insignificance.

[stinkwheel]

[Pete.]

Is this a viton coated MLs gasket or a more traditional type? The MLS gaskets won't have any discernable amount of compression IMO. If truly the spigot seals on the head with no gasket then IHow else ar you going say it will have to do that at the exact same time that the gasket is clamped up solid. This is the only way that you can be sure that the top face of the spigot is fully seated with the recess face in the head. With the four bolts outside the sealing ring it would be all too easy to pull it down cocked otherwise.

I think I would build it up dry with no gasket and clay/plastigauge the gap under the head where the gasket would be. If the clay mic's out the same thickness as the gasket then you wil be pretty sure. If the clay is thicker then it's riding on the spigot. If the clay is thinner then it's riding on the gasket.

Another method is if you could find an old very large micrometer you could turn a plug for the bottom of the cylinder and mic the distance to the spark plug land with the gasket fitted and not fitted. if the distance is appreciably less with the gasket fitted then it's not sealing on the spigot.

If you were nearby to me I would fit them up in my lathe and measure the distances with the DRO and a dial gauge.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[virus]

[stinkwheel]

[sickpup]

The traditional thing to use would be solder, a line laid around the head to be compressed by the liner but anything with a low surface tension so low springyness or low memory could be used.

[jeffyjeff]

You can get a preliminary idea of what you are dealing with by measuring the depth of the counterbore on the head with a depth micrometer, and subtract the protrusion of the cylinder "spigot" to find the cold clearance. Measure the thickness of the "fire ring" on your head gasket, and you would expect it to be close to cold clearance dimension. Might be that your counterbore clearance is insufficient. Thermal expansion at operating temperature might actually be lifting the head and relieving the crush on the head gasket. Could that actually be happening with six head bolts clamping the head and cylinder down to the case? I don't know; might depend on the coefficient of expansion of the materials comprising the cylinder and head bolts. But that is something to consider.
____________________
History shows again and again how nature points out the folly of men - BOC

[virus]

the steel of the bolts and the cast iron of the liner have a simmilar CTE of around 1-1.3 microns/mm over 100C change. The alloy of the head and barrel is closer to the 2um/mm range. so if the liner was 100mm long and got to 200c it would grow to 100.2mm and the barrel with an assumed length and temp the same would be around 100.4mm.

obviously this is heavily rounding on the maths and even then Im probably still wrong in my maths because ive not had to do CTE calcs in years, but it suggests that the alloy barrel and head will expand more than the liner so close the gap because the steel head studs will be holding preventing the alloy expansion the other way.
____________________
own: 81 xs1100g...
owned: 85 rat CG (sold), 91 GS500e (stolen), 84 gsx400f (scrapped), 81 z250 (siezed, siezed, scrapped), 83 cb250rs (sold), 84 gpz750r ratfighter (killed) 84gpz400 (sold), '80 cb650 ratfighter (wrote off) 95gsx6/12f ratfighter (killed) 91 xj900 (sold)
stinkwheel Well I just had my hands up a pigs fanny. Which makes your concerns pale into insignificance.

[Robby]

Many assumptions I have.

I assume this is a very old fashioned design to compensate for head gaskets being a bit shit when the engine was designed, back before my dad was born. The spigot and machined cutout on the head do the majority of the sealing for combustion gases. The head gasket should only need to deal with whatever gets past, and sealing the oilways.

I also assume that a bit of oil leakage was no big deal back then, because everything leaked oil.

But you're using a modern gasket with a fire ring. That should do the job the spigot is doing.

If I was in your shoes I'd certainly try to measure it all accurately, but realistically I would want to machine down the spigot as an experiment. If I took too much off, no big deal, the gasket has a fire ring.

[stinkwheel]

The amount of oil is what I'd call excessive, it's landing up on the front of the rear mudguard. I though it might have been pressurising the rocker area and forcing oil out past the gasket that way (there isn't much of a gap as you can see on the head gasket)but it now has a breather there so it can't be pressurised.

I do know if the spigot clearance is excessive, it can lead to errosion of the cylinder head over time.

I'm going to measure up this weekend and I'm just off out to go see an old school mechanic i knowa dn ask him about it. He'd be doing any machining/skimming if it's anything beyond what can be achieved with glass and emery paper/grinding paste.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Robby]

Is the amount of oil excessive as measured by the dipstick, or as measured by the mess it makes of the engine and your boots?

I ask because the CB250RS makes a mess, it looks like a lot of oil. Turns out that a small amount of oil makes a mess. It also doesn't have top end pressure problems, oil is just really good at finding very small gaps and leaving through them.

What doesn't help in my situation is that the rocker cover contains the upper half of the camshaft bearings, so there is always something exerting force on it. I assume there's a teeny tiny amount of flex happening every time the camshaft spins.

The difference is that I don't care enough to go down the route of plastiguage and machining. I would rather leak a bit of oil than seize a cam.

[stinkwheel]

It's a pretty small amount given how much it breathes and burns through the unsealed valve guides so oil consumption is what I'd call acceptable. It is however making a real mess of the bike which then gets all over both me and my clothing and luggage.

Happily it's an overhead valve engine, the cams are splash lubricated in the bottom end and the valve rockers are seperate units which bolt to the head and are directly lubricated via an external oilway. As such, any head work I do will only affect compression and valve to piston clearance. The former can be bealt with by setting deck height and the latter you cold drive a bus through (but will be checked anyway).

I have another bullet which does weep a little oil round the edge of the head gasket but it's nothing to write home about. Certainly not enough to get on the back mudguard and luggage which this one does after a few hundred miles. Admittedly the other one is running at much lower compression but is also used for trials so it gets really hot. Both are using the same head gasket. It might be the spigot clearance is bang-on but I do want to check it.

I'd post a photo of just how much oil is flying about but I pressure washed it yesterday in anticipation of todays tear-down..
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[virus]

if the problem is more to do with the oil issue than any running issue id be sorely tempted to counterbore and seal the oil ways with a squish o ring rather than mess with anything compression related if its got no issues tbh.
____________________
own: 81 xs1100g...
owned: 85 rat CG (sold), 91 GS500e (stolen), 84 gsx400f (scrapped), 81 z250 (siezed, siezed, scrapped), 83 cb250rs (sold), 84 gpz750r ratfighter (killed) 84gpz400 (sold), '80 cb650 ratfighter (wrote off) 95gsx6/12f ratfighter (killed) 91 xj900 (sold)
stinkwheel Well I just had my hands up a pigs fanny. Which makes your concerns pale into insignificance.

[stinkwheel]

Well, initial teardown is informative.

Looks like the fire ring is pretty much holding. Also clear that oil can passively get up/down the studs into the vicinity of the head gasket but isn't blowing through in a massive way. Piston colour looks good, maybe slightly the lean side of perfect but no hotspots and no evidence of burning significant amounts of oil.

There is quite a bit of carbon on the face of the spigot suggesting clearance may indeed be too large. At the 2 o clock position on the spigot on the picture below (front left of the engine), there's an area of more completely burned carbon which might suggest an area where hot exhaust gas blowthrough is breaching the spigot. This, combined with oil going down the studs might explain oil blowing out of the left side of the head gasket.

https://lh3.googleusercontent.com/pw/AP1GczN1Rx1774Usz8-6o57fbcKncDojnVQxX6g02t4iLDdeenJgxcTpidYcIFKEZgWY53FfpncBB1UjjVW3XeswwzEKG3Tp7PpHH81J2nGMwSZseXx6Wl_IWZ_An72_USZh4PvgEttUSpazzDjD4rCZTkAP=w1557-h876-s-no

You can see the corresponding area on the head (again, about 2 o clock on the picture below) and a fair bit of carbon on the spigot area. You can also see more brown swirl marks where the hot combustion gasses have been running and burning off the carbon on the head. To me this indicates my plan to double plug this head is probably a good idea. maybe get a more even burn.

https://lh3.googleusercontent.com/pw/AP1GczM7b6K59CSElFnknUV22uD_fUmAfu2uZCualLhJa2sW1_vIIddABAyG-jXot7ibxqZUnLOU7DEGk53exyKg12P9Nk26RgR1m93CQVd6UbuM8CMzpVRAl0XsGpPwMAFGZ8JJyPJh1IzhoVQTJtccpjBB=w1557-h876-s-no

Swirl marks highlighted:
https://lh3.googleusercontent.com/pw/AP1GczN0skocxAr3iOh8mEmxC6bXR-i2t2VtZ_M3KIvwqYcjj2RUbtoo69HLq9Re-C4YtmHnMDspWcNoTEM76mbp-WlzxBdDd0MZnWliAJ10ShUdPOjUecHTTEygIxHmv_RedNAmvxxFXSVPkDf93lt67aEm=w1557-h876-s-no

Other things of note are that my breather system seems to be working well, there's distinct resistance to turning the piston up and you can hear air being slowly sucked in through the breather hose/valve as it approaches TDC, then a nice blowoff of air as the piston goes down. It's to the point it's noticeably sucking the piston back down as you turn the engine up towards TDC.

I'm going to pop out now to see if I can speak to an old school mechanic I know about what to do next. Then my afternoon will contain a lot of carbon scraping in preparation for measuring that spigot clearance. All looking like it'll need a little bit off the head gasket face, the question is how much.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stinkwheel]

Well, spigot gap is relatively huge. By bolting down onto the old gasket, it didn't register on the plastigauge. I got a really good measurement with lead solder though.

The gap is 1.457mm measured over 6 points with a maximum deviation of 0.032mm. (with a micrometer). At least it's relatively level and concentric.

My engineer friend couldn''t come up with a definative measurement to aim for either but we both agree this is far too big. "on the plastiguage" is going to be a working minimum but shooting for 0.1 - 0.2mm range is going to leave enough slop for the head gasket to fully compress.

Options are a thinner gasket or skim some off the head. Off to measure the gasket thickness and gasket surface clearance with the head bolted to the spigot. Perhaps aim for zero then lap the head onto the spigot a bit to produce a tiny but consistant gap.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Pete.]

Easier to turn the top of the barrel casting than skim the underside of the head IMO, and you won't rik thinning the bottom fins. I'd definielty lap that head flat though.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[virus]

Yeah I'd agree with Pete there, Id personally be turning 1mm off the barrel face then seeing what difference that made tbh. Worst case scenario is its not as much of an improvement as you like so its cost you a days tinkering and a gasket to swap.
____________________
own: 81 xs1100g...
owned: 85 rat CG (sold), 91 GS500e (stolen), 84 gsx400f (scrapped), 81 z250 (siezed, siezed, scrapped), 83 cb250rs (sold), 84 gpz750r ratfighter (killed) 84gpz400 (sold), '80 cb650 ratfighter (wrote off) 95gsx6/12f ratfighter (killed) 91 xj900 (sold)
stinkwheel Well I just had my hands up a pigs fanny. Which makes your concerns pale into insignificance.

[stinkwheel]

Done more measuring. The head to barrel clearance with the spigot fully seated is 0.307mm mean over 6 points with a maximum 0.375mm and minimum 0.265mm.

Once again, the lead solder did a very neat job for measuring this.

The current composite gasket is 1.1mm compressed.

If i fit an 0.5mm copper gasket (which are available), this will take the spigot clearance to 0.25mm.

Using my calculations from when I built the engine, that would give a compression ratio of nearly bang-on 9:1 which I think is the working maximum for these engines on pump fuel. Twin plugging it will probably help a cleaner burn (and replacing the decompressor valve with a plug will marginally drop the cylinder head volume).

I could also lap the spigot true (although it seems pretty damned close from the previous measurements, wont need to take much off if at all, I'l sharpie it up tomorrow and see how it's looking contact-wise). Then take 0.1mm or a touch more off the head here using the old fashioned sheet of glass method to take the spigot closer and throw an extra base gasket in to keep the CR sane. Should be golden.

Then re-assemble and take it apart a couple more times to check everything is how I expect it to be.

Seems to have been a lot of faff, buit that spigot clearance was way too big. On cleaning up the head, I can see nibbling round the edge of the head on the edge of the spigot recess. This is probably where gas has been coming round the spigot and hitting the fire-ring.
https://lh3.googleusercontent.com/pw/AP1GczNu_heMiQe76GCbGK3kJTuS6gMBMKNjZPxxtZPGkTWaRyAmCch6unI8Zoc-cN8rvj0CjSIhekUOWKs55aFvFbjPV9_C9vCR74gDr6EL6Jzp6qEbbOmdp9oMeSBgRV8xmCd4r-J6ztILNZDIWpjv582K=w1557-h876-s-no
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[jeffyjeff]

[Pete.]

[stinkwheel]

[stinkwheel]

Well, that escalated quickly. Having skimmed a bit off the head, I took the barrel off to adjust the deck height.

Umm, yeah, I'm pretty sure that's not supposed to be like that:
https://lh3.googleusercontent.com/pw/AP1GczOT-JG_jEA8xNF7YykiTrPg83uSl7WN55uinWu4alyFFSFp71rPUlBWYKfmphgyhi3MEluz6h9z29NUaM6y-lD2z6dlqi2qXpUxz5XepqWeaXm0yx3riHpbvOt3ExiCKpMswXh-Eys-gMgK62yw5Tq-=w1557-h876-s-no

I hate doing jigsaws but needs must...

I'm pretty sure this happened quite some time ago. I had an incident where the engine got absolutely smoking hot and it turned out the decompressor cable was trapped under the tank making it run lean as hell.

That's the top ring. Amazingly, it seems the second ring was holding compression, absolutely no blowpast below it. Piston is in good order after a clean (thankfully, they are stupidly expensive forged omega pistons). Even a set of rings for it was £47.

Bike was running fine before disassembly.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.