My plan to build a new 97mm flat-top motor was in the toilet.

I needed to clean up the cylinder head to eliminate the crevice caused by the detonation.  If I didn’t get rid of that crevice, I was just settin myself up for more trouble.  Bad enough that there’s already an inherent crevice between the piston and cylinder wall.

I took a .010” cut on the head surface.  That didn’t completely eliminate the crevice, but it came close.  Unfortunately, it also reduced the combustion chamber volume by 1cc.  Now it was 54cc.

The only other option was to do a proper trench cut on the piston to increase the volume of the dish.  I needed to get that dish to about 10cc to achieve a reasonable CR.  Lucky I had the old 97mm flat-top to practice on.

It pumped 210 psi.  That’s more reasonable, but still pretty high for pump gas.  Best I can do over here is 92 octane.  As you can see, I got a new gage.  One of these days I will see if I can salvage the old one.  Have to find some stout hose.

I had mentioned this detonation issue in another post regarding oil system mods.  Michael Moore and Zevenenergie had pertinent comments.  To keep things a bit more cogent I am pasting those comments here along with my response.

On 7/9/24 Michael Moore Responded:

Mike, I think the erosion on the piston and the slight groove on the head have the same cause since they are both in the same area.

I'd expect detonation to not be localized like that, but instead taking place all around the edge of the chamber.  But there might be some very slight differences in the piston/chamber that aren't readily visible that caused the detonation to be limited to those areas.  Maybe detonation starts there but is extinguished before it travels to the rest of the bore?

On 7/9/24 Zevenenergie Responded:

I'm no expert, but when droplets of gasoline are deposited on the cylinder wall, they are collected in the space above the upper piston ring.
This can cause corrosion above the piston ring and in the squis area of the head, just like you show.
A squis that is too small increases this effect because the gasoline evaporates more poorly during compression.

It doesn't have much to do with the compression ratio.

This effect can be caused by the mist from the carburetor being incorrect.
You may have an optimal mixture adjustment, but the mist may have too large droplets.

Also the valve seats of the inlet valve can be too fluid in shape, which prevents further atomization. They must always have sharp corners for both a 3 corner and a 5 corner valve job.

The exhaust valve seats can, be rounded.

My response to those comments:

Michael Moore, thanks for your comment.  I also was surprised that the damage is confined to the intake side.  I assumed that would be the cooler area and less prone to detonation.  But I guess detonation must occur where the fuel is, and fuel certainly will be on the intake side.  The ignition source will be the rapid rise in pressure, and that pressure will be uniform throughout the combustion chamber.  I was able to find some YouTube vids of a Ducati that had almost exactly the same condition as mine.  I have no idea if the fellow that made the vid is an authority, but what he said made sense.

Zevengerie, I am confident that this is detonation rather than corrosion.  The ring land is deformed across the back side of the piston.  You make a good point about fuel droplets and fuel hiding out in the crevice above the top ring.  Smokey Yunik recommended reducing that crevice as much as possible without bringing the ring too close to the top of the piston.  He felt that the fuel trapped in the crevice could be good for something on the order of 5% more power.  Otherwise, it's simply wasted.  I may be aggravating the condition with my port configuration.  I increased the short side radius to improve flow on the bottom of the port.  That in turn results in more charge being directed at the area in question.  But I don't want to give up on the improved flow, so I'm gonna rely on the tried and proven recommendation to keep the quench as tight as possible, and reduce CR to a more appropriate level (as close to 10.5 as reasonably achievable).

I don't take any extra effort to blend in my seat angles.  I cut them and leave the corners as they are cut.  I do use four angles (75, 60, 46, & 30).  Thankyou for your comments.  They are very helpful and give me more to ponder.  

See my most recent post on this detonation issue.

On 7/12/24 Zevenenergie responded:

Tanks Mike, I feel compelled to share a little more.
(Life is all about sharing)
I don't want to sound too decisive, like this is how it should be, but you can also think about this:

There is also some power to be gained if you round off the squis edge, so that the flam front can enter the squis and crevice more easily when the piston descends.

Detonation in the squis cleft is a bit strange. If the squis is not larger than 1mm, the piston and head cool the mixture sufficiently so that it does not self-ignite.
When you increase compression you have to look at the squish space and optimize it to prevent detonation.

You cannot make the gap too small because the connecting rod stretches.
And you also have to pay attention to the speed of the gases that are squished because they can blow the plasma away from the spark. Sometimes a tapered squis slit is therefore better.

I have another question Mike, I see on your pistons those parts that have no carbon residue (on the intake side).



My question is what does the piston look like on the inside?

Is the piston blacker there as you would expect ?

Because detonation makes the piston hotter and causes burn marks on the inside?

Zevenenergie: "My question is what does the piston look like on the inside?"

The underside of the piston is totally sweet.  Absolutely zero coking.  That is one of the parameters I always inspect.  I want to know how hot things are getting.  I view this sandblasting as a relatively minor failure.  It's not catastrophic.  I think it indicates that I have been running the thing right on the edge; that fine line between OK and total destruction.  I believe the reduction in CR was the right thing to do.

Dave, regarding the dual plugs.  That setup on the DR is strange; both plugs on the same side of the combustion chamber.  That engine only has 9.5:1 CR.  I guess it shows the lengths the OEM has to go to in order to deliver a reliable motorcycle.  I see from the spec sheet that it had a CDI ignition.  That probably means the primary windings on the coil will have significantly lower resistance. The kiss of death for TCI.  But I bet I could find a Dual-Fire Dyna coil with compatible primary resistance.  I think I could probably pull off the second spark plug in the location shown.  The increased compression should in itself require a little less spark lead.  I might eventually pull two degrees out.

Thanks for the pics of the DR head.  Do you have that in your possession?

DragBikeMike wrote on 07/13/24 at 22:54:29:


I'm new to 4 tack tunning but with two strokes there is no doubt about it. NO
The piston and the head have a cooling effect on the mixture and this prevents detonation where it first occurs.

The entire combustion chamber form is based on this principle in a two-stroke engine.

This is what comes to mind:

Since your piston shows no heating at all on the inside, I don't think there was any detonation going on.

The light spots are caused by a blast of evaporating fuel. Is my conclusion.
The cylinder head gets the full blast because the blast media comes from the crevis.

A smaller squis gap counteracts this effect because the pressure is higher in a smaller squishy gap.

but also cooler because it is cooled by the piston and cylinder head.
I know counter is intuitive, but
I would increase the compression ratio rather than decrease it in this situation.

Anyway I would opt for as small a squis slit as possible.
But in my opinion the real cause lies with the carburetor.

But the ignition time could also be looked at.
In a two-stroke, an ignition with a variable ignition time has a lot of influence on the power band. It gives more pulling power over the entire speed range instead of just peak power at top.

And optimal ignition timing at all speeds, contributes to optimal combustion and prevents complicated situations associated with the complicated pattern of evaporating fuel in the cylinder, combustion chamber and crevis.

In my opinion, the crevis above the upper piston ring should be kept as small as possible.
This also shows that a bumper due to wear of the cylinder should never be removed.

My knowledge comes from boosting small two-stroke engines. a compression ratio of 1:14.5 with petrol station is possible. (effective compression ratio 1: 8.4)

But at the same time, a 2-stroke has pressure filling due to the resonance in the exhaust. So the pressure in the combustion chamber is higher as the engine enters its powerband.

So that makes the situation about the same as in a 4 stroke.

But because the fress mixture in a four-stroke comes into direct contact with the cylinder wall, the problem of condensation is much greater and this is clearly visible on the piston image on the intake side.

So the size of the droplet in a four-stroke is of decisive importance for the power developed. (lets say more than whit a 2 stroke)
Experimenting with different carburetors and their setup is worth it.

You can transfer the $64.000 to my bank account that I mentioned in the PM.