Head Gasket/High Comp Piston

[morbidelli17]

Had some troubles with head gaskets this weekend; blew two of them. Sending head and cylinder out to get surfaced to make sure they are absolutely flat. But I have a question: With an 11.5:1 piston, do you still recommend a three-layer head gasket or should I go with the copper one? Thanks in advance.

At least I got a decent picture from the weekend ...

[basser23]

what was there originally? Should still work..Think Bill J will have the answer...
still diggin that fairing...looks good in b&w!

[texoot]

Maybe a copper gasket?
Nice pic!

[edfmaniac]

I'm not a betting man but I believe Bill would tell you that if you need to ask, stick with the multi-layer gaskets. Seriously though, I remember him saying that unless you were a very knowledgeable mechanic, you should stick with the multi-layer. My guess is that something is warped or you are getting some detonation with that hi-comp piston.

[morbidelli17]

No indication of detonation, I don't think; didn't hear anything, octane of fuel should have been plenty high. Mechanic didn't mention anything on the piston that looked like detonation. But I'll ask about that. Here's the report I got from the machine shop. Maybe this will tell you guys who are more machine-adept than myself something.

CMM report-1.pdf

(74.2 KiB) Downloaded 299 times

[samandkimberly]

If I read that right, you've got .004" or more warp in your head. Definitely enough to get it surfaced, but I'd also ask yourself how it got there! Did the bike ever seriously overheat, either before rebuilding or when the head gasket went out? Check for head cracks, particularly where the gasket failed. Sounds like you're using a shop with good tools, you might consider asking them to put it under a microscope and/or test the surface for cracks.

BTW- I'm my opinion copper gaskets are really only useful when you need to be pulling the head a lot and can't afford to be buying lots of head gaskets, or when you can't buy a layered head gasket to properly fit your engine. They are less tolerant of head warpage and more difficult to seal properly, requiring annealing and very careful use of the right sealers in the right situations (including none in many cases). My advice: surface the head, consider surfacing the cylinder, though that doesn't seem as bad. Assemble the head to cylinder bone dry, no sealant, and make sure the head bolts/nuts spin freely. There's debate as to whether to lubricate them; I do. Once you've torqued the heads, loosen then retorque each bolt, one at a time. Normally I don't waste my time doing all this, but if I had replaced 2 head gaskets already I'd be practicing overkill.

Sam

[morbidelli17]

If I read that right, you've got .004" or more warp in your head. Definitely enough to get it surfaced, but I'd also ask yourself how it got there! Did the bike ever seriously overheat, either before rebuilding or when the head gasket went out? Check for head cracks, particularly where the gasket failed. Sounds like you're using a shop with good tools, you might consider asking them to put it under a microscope and/or test the surface for cracks.

BTW- I'm my opinion copper gaskets are really only useful when you need to be pulling the head a lot and can't afford to be buying lots of head gaskets, or when you can't buy a layered head gasket to properly fit your engine. They are less tolerant of head warpage and more difficult to seal properly, requiring annealing and very careful use of the right sealers in the right situations (including none in many cases). My advice: surface the head, consider surfacing the cylinder, though that doesn't seem as bad. Assemble the head to cylinder bone dry, no sealant, and make sure the head bolts/nuts spin freely. There's debate as to whether to lubricate them; I do. Once you've torqued the heads, loosen then retorque each bolt, one at a time. Normally I don't waste my time doing all this, but if I had replaced 2 head gaskets already I'd be practicing overkill.

Sam

Thanks very much. The weak spot seemed to be the area between two coolant tunnels, so perhaps the thing was overheated in the past. Another theory was that with 9:1 compression (or less, if the rings were going) the gasket could deal with a warped head, but at 11.5:1, the additional compression overwhelmed the weak spot.

Will keep you appraised. Thanks for the kind words, all - M

[basser23]

Michael,
Knowing the history of this bike(followed the God Help Me thread)there probably was an overheat from previous owner...just guessing really,but it seems logical...shave the head(maybe more compression)
see what happens..

[morbidelli17]

I think you're probably right, Chip. Machine shop will be looking at that area of the cylinder/head for signs of damage.

Meanwhile, I've gotta get the front end sorted out. Still WAY too stiff for an MZ ...

Peace, yo.

[Bill Jurgenson]

I would strick with the layered gasket myself. I used Cometic gaskets in the racing engine. I also used copper successfully but I did not find any real advantage. We are not talking abe 17:1 and topfuel. What w have is only a tad better than John Deere.

My guess is that something is warped or you are getting some detonation with that hi-comp piston.

Mine, too.
What ignition are you using?

If you are using the stock DENSO box, you probably have detonation with that high compression. (Been there done it) Especially if have one that is doctored to a higher limiter since that process also increases the spark advance. And if you have a TDM box, it is bound to blow up. Both the doctored box and the TDM box are only just barely usable with a stock engine. The spark advance is far too extreme for anything more than say 9:1 - if that. Go back a few years and read what Wonky had to say about this.
You will need to get either one of those rare and expensive OVER ignitions used (they have a number of preset curves to choose from) or something programmable. I use and recommend the Ignitech Sparker TCIP4 but there are others out there as well. You need one that works with a battery ignition if you intend to keep the board electric basically standard with generator, battery, and starter. THE XTZ/XT660R/SZR engine does not have a CDI ignition.
If you are serious about the track, then you should ditch all that and run a constant loss battery ignition. Then there are several to choose from. Or you can go for a magneto and ditch the battery, too. I ran a PVL magneto for all but the last season

here with Barker's milled side plate on my engine; no flywheel at all.
Barker's championship winning SZR (back a bit!) ran with a PVL magneto, a downdraft head and ram air airbox.

then I changed to a SilentHektik battery ignition

the same ignition mounted on the 1200cc 3 cylinder Laverda I race sometimes

You see the small induction wheel and the pickup mounted on an adjustable ring to set static advance.
The magneto has the advantage of being selfcontained and not needing a battery but it is hard to start since the spark is very weak at starting RPM. Without a starter machine you have no chance. The battery ignition is much easier to start, the main reason I changed but you have the added weight of the battery; with a 4,5AH LiFePo that is only about 300 gr. The PVL electronics are very delicate as well and turning the engine over without a sparkplug plugged in is enuf to blow it.

Just to set a frame of reference: the PVL has a curve like this:

This is the box I used; it is for GM grass and dirt track engines, alcohol burning 500cc singles. It has a max spark advance of 22º to which one adds the static advance, i.e. the position on the crank. We set it at 5º BTDC static, giving a max advance of 27º which remains pretty constant to the usable red line of 9500 (no limiter) which was usable on our engine.
Compare the DENSO box:

37º!!! which advance to about 40º if the box has been doctored to raise the limiter from the stock 7200 to 8400. It can only be doctored in whole steps of 6 so 7800 is also possible. The TDM box goes to 40º in stock condition. BTW, the YZ450 advances to all of 24º.
I don't have an advance curve for SilentHektik at hand. The PowerBlock is not programmable but it has 16 different curves to choose from and I used one going to max of 32º.
The bike ran best (when and if we got it started) with the magneto and we were getting nearly 80hp at the rear wheel.


This is the curve I use with the Ignitech Sparker.


A bit of a compromise that approaches the magneto while still taking the auto deco on the cam into account.
A serious racing engine should not have this at all so it doesn't need to be accounted for.

[morbidelli17]

Thank you, Bill! So the TDM box I have isn't suitable for the 11.5:1 piston? Is the stock box useable until we can get into total-loss land?

[Bill Jurgenson]

you can use the original ignition is stock condition without much danger. Of course the 7200 limiter pretty much rules out any really quick riding on the track, especially with the original cluster with its wide spread.
that said, Here in Germany, really good riders (such as pors riding as guests outside of the points) run lap times with Cup Skorpions (on short circuits) dangerously close to real motorcycles.
they have to use the OEM box and tuning is not allowed. The engines are rated on the dyno and them sealed. More than 52hp is not allowed. Neither are other forks and struts, better wheels or other tire dimensions, in fact they all ride the same tire. It is a Cup class ans these are based on over the counter bikes.
I never road in that class; I never ever owned an eligible Skorpion. I owned up to thre at one time but they were all tuned and otherwise modified at a time when the Cup class did not yet exist.
Mostly, I rode in the "Sound of Thunder" class since the SOS class has only very recently been revived in Germany; too late for me due to my age. The SOT is a sort of follow up of the BOT class, open only to 2-cylinder bikes from 1986 on and originally divided in two divisions: a 2 and 3 valve for older Ducatis and Buells and some specials and a 4 valve division for newer Ducatis, Aprilias and SP2s. they let me ride in the lower class because I had fewer valves (5) than the max allowed (6). The really old Ducs (bevels,and Pantas, etc) run in an 18" (or larger) class along with the Laverda I ride. A well set up Skorpion SOS racer is good match for those 2 and 3 valve Ducs and even for 916s and 996s depending on the rider and track of course.
It's like Chris Hunsicker once elatedly wrote me, ' today I was able to pass the SVs on the straight for the first time. Till now that was only possible in the corners."
This was after he had finally fitted the much lighter and wider TZ250RR wheels, 3,5" with 120/70 and 5" with 160/50 SuperCorsas.

his last season - he has moved on to other interests - saw GalenMiller engine mods and a Barker 2-2 exhaust.

[morbidelli17]

Thanks, Bill. If the switch from a 7,200 rpm stock box to an 8,000 rpm TDM box really does make a difference with this piston, I will switch boxes back from the TDM one to the stock one.

I am a gentleman racer, and as such plan to stick with stock ignition, starter, and flywheel. But if I can find a box that will allow me to get a proper spark advance curve without getting into total loss, that's my plan. I am a bit of a geezer, and quit racing the SV650 classes because they are far too competitive for me - I have no desire to put $5,000 worth of titanium valves and porting into each SV head!

Take care, all. Will keep you appraised - M

[samandkimberly]

The Sparker Bill talks about is exactly that-a fully programmable ignition that it very close to a drop in for the stocker. I'm still mulling over buying one; maybe we should consider setting up some sort of group buy?

Sam

[morbidelli17]

I'm listening. When you say "close to a drop-in," what do you mean?