[JamesPal]

Ronnie, looking good. Any thoughts on the piston selection? Will you use a CB500 crank?

 

[JamesPal]

I haven't verified this yet, so it only a theory... Honda moved the wrist pin hole higher on the 500T pistons from the 450 DOHC pistons. The distance would be 1/2 the difference in the stroke. Cb450 stroke = 57.8; CB500 stroke = 64.8. 1/2 the difference is 3.5mm. This is assuming the same deck height. This way they could use the same rods. Back in the day, General Motors would do this. I know Pontiac V8 engines from 326 to 455 ci all had interchangeable rods. The bore and stroke varied. The pistons would not interchange even with the same bore.

You got me going on this. There might be a big engine in my future. Have you though about cooling? There were some big Triumph Twins back in the day. I have heard they would run hot.

 

[JamesPal]

How would wrist pin position affect the displacement? The stroke's determined by the offset between the center of the crank and the crank pin. wrist pin position on the pistons and con rod length will affect power due to leverage but if you're after more displacement the con rod length or wrist pin position won't affect this. It will affect the compression ratio and leverage of the piston on the crank

This is about the deck height, not the displacement. Deck height from the top of the piston (in the case of a CB450 the top of the squish band) and the top of the cylinder block. I've never measured it, but on the 450 the piston looks about 1-1.5 mm lower than the top of the cylinder block.

Starting from the center line of the of the crank shaft (at the main bearings) 1/2 the stroke plus the length of the rods plus the distance between the center line of the wrist pin to the top of the piston gets you to how high the cylinder block needs to be. Of course this is a mute point since Ronny is using a CB500T engine. But it is something to consider if he would want to use a 450 head and pistons

 

[JamesPal]

On a 500T piston the wrist pin or Gudgeon pin as we know it is 17mm OD the height to the deck from the top of the pin is 18mm so the centre of the pin to the deck is 26.5mm. I hope that this is helpful information.

Nige, what is he dimension on a CB500T? BTW- thanks for the translation to the Q's E!

 

[JamesPal]

Ronny, Please clean it up, this is about motor cycles!

 

[JamesPal]

Nice to see you have a 500 engine. What are your ideas for pistons?

 

[JamesPal]

Wiseco has a Z1100 1.5mm forged piston. The critical item will be the location of the wrist pin bore in relation to the piston crown. I think it takes it out to a 74mm bore. I spoke to them when I was putting my 450 together. My rod small end was over tolerance for the diameter. They had a machine shop that could grind a 18mm wrist pin to any size I wanted. I would think they could put the wrist pin hole any where you want it.

http://www.wiseco.com/ProductDetail.aspx?ItemID=4256M07400&ModelID=175&ModelYear=1981&AppID=7901

 

[JamesPal]

Ronny, I have been thinking a lot about doing what you are starting. But I need to finish my 450 K5 and I have a '81 GL1100 Standard that needs to be done first. I had a conversation with JayinNY on a similar proposal last November. My comments are applicable here. The difference is you will have a larger displacement engine. That will shift the calculation slightly to a higher compression engine. I apologize if you have already read this:

The dimension from the wrist pin bore ont the 500 engine to the top of the piston is shorter than on a 450. it is exactly 1/2 the difference between the 450 and 500 stroke. The CB450 stroke is 64.8 and the 500 is 67.8. the difference is 3mm so the wrist pin is 1.5mm (~.060) closer to the crown. This would affect the squish band and compression ratio.

The new compression ratio could be calculated by using the formula for a volume of a cylinder; Volume Calculator = 5775 Cubic MM or 5.77 CC. since a stock 450 is 444 CC, each cylinder is 222 CC, at a 9 to 1 compression ratio, the stock 450 DOHC combustion chamber volume is about 25 CC. Adding another 5.8 cc would make the compression ratio about 7.3 to 1.

To deal with using 450 pistons in a 500 the height of the cylinder block could be shaved by 1.5 mm. this would allow the stock compression ratio to be maintained, the piston crown would need to be shaped to match the 450 head. Another compromise would be the cam timing. Since the cam centerlines are 1.5 mm closer to the crank centerlines, the tensioner will need to take up the difference. I'm sure it could, but it would push the whole chain length difference to the back (or slack side) or the rotation. This would advance the cam timing off by 1.5 mm rotation of the (calculate: cosine of 1.5mm at the radius of the cam) cam chain sprocket. I've had enough math for one posting but it would be around 3-4 degrees.

Of course your situation is a little different. A 7.5 to 1 compression ratio would be easy to live with, but I think it would feel like a tired engine that is down on compression. I'm watching with very high interest. JimP

 

[JamesPal]

What bore size will you wind up with? I will calculate the compression ratio.

 

[JamesPal]

I discovered a couple of errors in my thinking. I hate it when that happens, especially in an international public forum.

I used the incorrect stroke for the 450 in the example above. Also, I am postulating the is wrist pin hole in the CB500T is closer to the crown of the piston than in the CB450 and the length of the connecting rod is the same between the two engines. My error was thinking the 450 pistons would have a lower deck height in the 500 than the standard pistons. Based on my assumptions, the opposite is true.

The difference in the stroke is 7mm. If Honda made up for the difference by making the piston shorter, in the 450 piston, the wrist pin bore would be 3.5mm closer to the crown than in the 500. I have a set of old 450 piston out of a K6 motor. Using a caliper I measure the center of the pin bore to the top edge of the squish band area at 25mm. The squish are of the crown slopes about 1mm to the domed area. I'm very curious to know what this measurement is in the 500. Every comparison picture I have seen showing the 450 and 500 piston show the 500 piston significantly shorter.

 

[JamesPal]

Hi Ronny, good to hear from you. I don't see the pictures either. Keep us posted.

 

[JamesPal]

Ronny, the nearest I can measure my K5 is 7.10 Inches or about 180 mm. It is difficult to measure without disassembling the bike. I measured across the top of the bridge, the tangent points of the cap nuts. I believe all of the disk brake models have the same fork bridges.