Hole in piston

[Happyrat]

Hi I have a 1968 Ace 100 and for the second time I have a tiny hole in the direct center of my piston right under the spark plug. I’m pretty sure I am jetted OK. Any idea why this is happening, is it because the bike was lean or is the plug too hot. Any suggestions would be greatly appreciated. Thanks David

[Dale]

I would suspect an air leak. Have you performed an air leakdown test? A must for any 2 stroke.

[matt glascock]

What does the plug look like?

[Happyrat]

Here are a couple photos. Two are of the current plug. Two are the piston currently installed on the bike. And two are the piston that I took out before this one. Did a compression test and it only read 45 pounds with the current pissed installed

Here are a couple photos. Two are of the current plug. One is the piston currently installed in the bike. And two are the piston that I took out before this one. Did a compression test and it only read 45 pounds with the current piston installed

[swcaudill]

Does that hole go thru the piston? It is a center drilled hole used when manufactured.

[taber hodaka]

The plug looks like a cold one, what oil are you using, show us the bottom side of the head. Does the hole go all the way through? ------------ Clarence

[givergas]

manufactured hole

[olddogs]

Lets see a picture of the pistons sides. Could be as simple as worn rings. Low compression or air leaks will cause starting and operating problems. I dont believe the hole is anything to worry about. More info on symptoms before tear down will get you help and pointed in the right direction..

[Happyrat]

Hey guys thanks for the input. You’re right the hole in the piston is supposed to be there and does not go through. The piston look really good as do the cylinder walls. With compression only 45 pounds must be losing air somewhere. Thanks for guys help

[olddogs]

This is a dykes ring Super Rat piston. The ring wears pretty quick. We used to change them after a couple of races. I dont see any signs of seizure or blowby that could have stuck the ring. That means your low compression could be a cheap and simple ring swap, or base or head gasket leak, both which you need to replace when putting it back together. I have seen mag side seal pushed out which will cause air leak and low compression. This can be changed without engine removal or tear down. Looks more investigation needed.

[olddogs]

Also check what head you have. The numbers are right on it. Rat piston needs a rat head (93)and an Ace head would start with a (92). The piston domes are different as are the squish areas. Best to make sure they match.

[MTrat]

It would run (weakly) with an Ace head over a Rat piston due to low compression.

[Happyrat]

Setting up to do the leak down test now. Question on the Dykes ring the L shape is upwards is that correct. I checked ring gap today and it’s .009 of an inch. Change the cylinder head gasket and the bottom jug gasket. When ahead and did a Second compression test today and I’m still only getting 45 pounds. Anyone know where I can get a Nother Dykes ring. What pressure do I put in for the leak down test. Don’t wanna screw anything else up. Thanks David

[Dale]

6lbs. Shouldn't lose more than one pound in a minute for a passing grade. A good tight engine will hold on 6 lbs for many minutes without losing any pressure.

[Happyrat]

Thanks buddy going to get hot on it and hopefully find the problem. Checked all the parts and they’re all 93. Trying to understand why a compression of 45 could be anything but piston, rings, cylinder, cylinder head or cylinder head gasket. I understand the bottom and will make it go lean with a leak, but it seems like to me once the piston gets above the intake and exhaust ports there should be only compression no other factors. Any Enlightenment meant be greatly appreciated.

[Bullfrog]

Are you doing the compression test with Wide Open Throttle and kicking till pressure stops rising?

[Happyrat]

I Tried that the first time and got 45 pounds. I disassembled the top end replace the gaskets check the gap on the piston ring. This time I did not hook up the exhaust or the carburetor and got 45 pounds again. I’m having a hard time understanding how compression once the piston passes the exhaust and intake ports would be affected by throttle or a leak in the bottom end. I know I’ve heard that all my life but still don’t understand it.

[Bullfrog]

Some of the possibilities here are, 1) your engine actually only has 45 psi compression, 2) your compression gauge/connecting hose and fittings reads 45psi on any engine you connect it to. Aaaaannd an important question, what problem is apparent when you run the engine? We've heard about measured low compression and the "holes" in multiple pistons - but no descriptions of engine operational problems. Are there any operational problems?

[olddogs]

When the piston goes down it pressurizes the crankcases air volume. This air is forced by the transfer ports to e space above the piston. It has no p;ace else to go.
If it is leaking out of the crankcase instead of getting forced above the piston, it is only compressing the volume of air sitting above the piston. Its all about the tranfser of pressure.

[Happyrat]

Thank you. That makes lotta sense, I’ve never really understood How the air in the crank case Works. Heading out to get hot on that leak down test. Thanks again guys

[Happyrat]

Did leak down test today and found a leak in the base gasket. Took it apart and put on an old gasket I had just to give it a try, and it leaked too. Ordered a new set of gaskets for the top end and a new Dykes piston ring. Hope to have it back together by the end of the week, do another leak down test and have a good compression. thanks for all your help

[taber hodaka]

Your old ring looked good to me. Grease the base gasket. Question though did your engine still run with the 45 pounds?? ________ Clarence