102 cc question

[fishinworm]

hi I have a yamato 102 and need to know how to cc this engine and what it must be to race also how much can the head be decked???

[Raymond]

Assuming you are asking this question relative to the APBA Stock Category, this is what I know.

Go to the APBA website and click on the ‘Resources’ tab. From the ‘Resources Type’ drop down menu select ‘Technical Info’ and click the ‘Apply’ button. Now you can scroll down to the ‘Stock Outboard Technical Manual’ where you can find the information you are looking for on page 15. You would then take the information gained from doing this procedure and look at the spec for your motor on page 71. I pasted a pic of it below; I will explain why later.

This resource will describe the official recommended method of measure but it won’t tell you how much material can be removed: that would be a function of the current state of your equipment. For more general information on the measuring process, search terms like CC Combustion Chambers or Measuring Combustion Chambers at the University of You Tube.

I pasted the pic to help set up a question I would like to ask. When you look at the spec you see two numbers 14mm 25.5 and 18mm 26, is the mm reference to the spark plug and the following number the minimum CC volume for that sized head? I believe this to be true but would love to hear the response from a certified inspector.

A second question I have (which might be a different way of asking the part of the original question I could not answer is) what is the correlation of material removed to reduction in chamber volume? I understand that due to the shape of the chamber the mathematical slope of the line is not straight all the way down to the extreme top of the chamber but… given the starting point is the as manufactured volume, how much material would one remove to reduce the volume by 1cc?

[Raymond]

Sorry I forgot the pic in the last thread

[ZUL8TR]

14mm refers to the 14 mm spark plug head and 18mm refers to the 18mm spark plug head that has a bigger plug hole..

[fishinworm]

Ahh yes the answer I was looking for 25.5 now I can go to work making this motor have more POWER..!!!!! thank you very much...

[Jack Stotts]

The squish ring must be retained.

[BabcockBros38A]

fishinworm, First off I would CC your engine with the uncut head on it. You can compare volume of each cylinder to use when you set up the head to be cut. To give you a rough example of volume of the cut, I use the ''H'' dimension in Inspection manual and a .010 cut will remove between .868 to .888 cc. And as Jack said, the squish ring has to still be evident. An Inspector can use a scribe, or edge of a pocket machinist scale, or even a finger nail to check for the band edge. Like I said check the cc's before you cut the head.
Good luck
Pete
Mod Inspector

[fishinworm]

so if I put the engine at tdc number one cyl and then tilt the engine so the spark plug hole is level towards the sky then measure the ccs of oil I inject?

[BabcockBros38A]

fishinworm Yes your headed in the right direction.
Page 15 of the S.O Inspection manual has the complete procedure. You will need a 50cc burette for measuring the volume. you also will notice that a 50/50 of mineral spirits and TCW 2 or TCW 3 oil, is used as the fluid. this mix is less likely to trap any air in the chamber.

Pete

[fishinworm]

pete thank you... would a 100 cc syringe work for the measurement?

[BabcockBros38A]

It would be close. the burette is gradutated to .1cc giving more accurate reading if your syringe has the smaller graduations it will give you what you need.

Pete

[fishinworm]

pete where do i get a burette?

[BabcockBros38A]

Look on amazon.com or go to diagger.com they are a lab equipment supplier. There are inexpensive ones as well as more pricey ones

[Hydro doc]

Don't expect much improvement from changing the head. How the pistons fit is most important. Our pure stock motor is the fastest.

[ZUL8TR]

Don't expect much improvement from changing the head. How the pistons fit is most important. Our pure stock motor is the fastest.

What have you done to maintain pure stock on that engine?

[fishinworm]

ok thanks everyone for the help. What else can I do to this stock 102 to make it rev higher??