Author Topic: Stroker build- '76 cb750k6  (Read 7848 times)

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Offline EnginebyAdam

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Re: Stroker build- '76 cb750k6
« Reply #25 on: November 28, 2013, 06:57:39 AM »
I have decided to stick with an naturally aspirated build. I'll stick to what I know for this one and see how it goes. 

I have found a set of NOS 71mm and 73mm MTC pistons for a very good price. They look a lot like the one's on satanic mechanic though I believe this picture is a 12.5:1 piston?



I have read that some here still run these pistons. One issue I have seen is finding rings. If these are worth grabbing I was told that XN style rings could be fitted. The compression ratio is supposed to be 10.5:1 based on a K model head. 10.5:1 should still give good HP with the 69mm stroke. Chances are the head will be milled so a little bump in CR would be perfect.

Any reasons to stay away from these pistons? Was told 0.002"-0.0025" clearance on the skirts, but that seemed a bit tight to me. Any thoughts?
« Last Edit: November 28, 2013, 09:19:11 AM by EnginebyAdam »
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Offline Big Jay

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Re: Stroker build- '76 cb750k6
« Reply #26 on: November 29, 2013, 11:02:45 AM »
If the pistons still have taper in them, I wouldn't go over .002. max. MTCs did not need a lot of clearance.

How much taper do they have?

Offline EnginebyAdam

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Re: Stroker build- '76 cb750k6
« Reply #27 on: November 29, 2013, 12:18:56 PM »
If the pistons still have taper in them, I wouldn't go over .002. max. MTCs did not need a lot of clearance.

How much taper do they have?

I will ask that the next time I'm talking to the seller, which would be Monday. I won't have them in my hands till after I buy them, but I do trust that any measurements would be accurate. They are brand new pistons never installed so all original design and tolerance should be present. Providing they were handled right. That being said I'll double check that they mic the pistons on their end, although I'm sure they will anyway.

I have never run a forged piston this old before, thanks for the info Big Jay. I always go with the manufactures clearance unless I know it's horribly wrong. Edit: Had some bad clearance specs on a set of pistons, 0.00425" as opposed to 0.0025" proper spec. The cylinders were bore to the 0.0025" clearance ;)

I don't own them yet but it is starting to look like I might ;D
« Last Edit: November 29, 2013, 05:50:14 PM by EnginebyAdam »
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Offline EnginebyAdam

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Re: Stroker build- '76 cb750k6
« Reply #28 on: December 02, 2013, 01:47:03 PM »
Some pictures, 10.5:1 and 12.5:1 72mm bore.




The pistons will be double checked before being sent out, Big Jay. For my purposes the 10.5:1 should do. I figure I can mill the head some and get me in the 10.75-11.25:1 range. I have to get the head I want, 77/78K, dress up the chambers and cc them.

These pistons are being quoted at 10.5:1 and 12.5:1 for 71mm, 72mm, and 73mm. I will give up HP using the 10.5:1 pistons but I feel it will be easier to tune for my first go round. I will still get 10's with this
setup so it sounds good to me.
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Offline EnginebyAdam

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Re: Stroker build- '76 cb750k6
« Reply #29 on: December 17, 2013, 05:47:40 AM »
I have been in contact with Jon over the last week, as well as HondaMan, both gave me a lot to think about.

The big factor with this build is budget. Going big, 71mm-73mm bore range, will take more money than I have to work with, as I am now aware of. The head work to support that size bore, combined with the cb900 crank, would run into the $2500-$3000 range. A used big bore block that would most likely need a bore/hone for the 71mm-73mm range most likely another $800-$1000.  No internals bought yet. I thought I had a money tree but it blew down in our last snow storm :)

One of the things that Jon mentioned was how well the F2/F3 head did on his 1080cc engine. The thought was that the stock F2/F3 head needs displacement, roughly 1000cc, to take advantage of the head. My thought is, why not build a bigger F engine? 67mm x 69mm stroke, 973cc, undersquare. This engine would be in direct ratio to the original F2, 61mm x 63mm. Pistons would be 12.5:1, so roughly 11:1-11.5:1 with the F chamber, without milling the head. The pistons are the same style of the 72mm piston pictures I posted.

I know that oversquare engines are typical for drag racing, I've built a few, but my theory is the natural midrange of the engine coupled with the right cam, would allow me to access the top end as well. HondaMan said this combo should make for a better launcher than a oversuare engine, I agree. What do ya'll think?



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Offline joe p

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Re: Stroker build- '76 cb750k6
« Reply #30 on: December 17, 2013, 07:02:33 AM »
I check in to this to and no way I've got that much money. If I build a motor it will be a 970 or 1000 with a F head, super rod and thats about $3,000 and thats me doing eveything except the bored job. I had a motor like this in a street bike years ago and It would pull the front wheel off the ground with no problem ;).
« Last Edit: December 17, 2013, 08:25:12 AM by joe p »

Offline EnginebyAdam

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Re: Stroker build- '76 cb750k6
« Reply #31 on: December 17, 2013, 08:40:58 AM »
I check in to this to and no way I've got that much money. If I build a motor it will be a 970 or 1000 with a F head, super rod and thats about $3,000 and thats me doing eveything except the bored job. I had a motor like this in a street bike years ago and It would pull the front wheel off the ground with no problem ;).

If I went with 71mm-73mm pistons plus the cb900 crank, the issue is getting the engine enough air. The way I want to go now I should be at $3500 for the engine.
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Offline joe p

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Re: Stroker build- '76 cb750k6
« Reply #32 on: December 17, 2013, 09:54:18 AM »
I had to go back and read all the post, I some time do not read thing right. It would cost me a lot of money to come up with the parts you have. and I'm not trying to go that fast. I sold my S/C 1428 KZ drag bike back in Oct. I have my old Honda drag bike that I raced years ago. I'm just looking at building a 11sec Pro ET bike. I agree with Dragracer go with the guys that know what there doing with this build Jon Weeks, Bear, Benton.
« Last Edit: December 17, 2013, 11:02:02 AM by joe p »

Offline 754

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Re: Stroker build- '76 cb750k6
« Reply #33 on: December 17, 2013, 11:12:29 AM »
The pistons above, are not for stroker...correct?
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Offline EnginebyAdam

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Re: Stroker build- '76 cb750k6
« Reply #34 on: December 17, 2013, 12:34:17 PM »
The pistons above, are not for stroker...correct?

No they are not stroker pistons, will be using a spacer/copper gasket under the cylinders.
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Offline EnginebyAdam

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Re: Stroker build- '76 cb750k6
« Reply #35 on: December 17, 2013, 12:39:19 PM »
I had to go back and read all the post, I some time do not read thing right. It would cost me a lot of money to come up with the parts you have. and I'm not trying to go that fast. I sold my S/C 1428 KZ drag bike back in Oct. I have my old Honda drag bike that I raced years ago. I'm just looking at building a 11sec Pro ET bike. I agree with Dragracer go with the guys that know what there doing with this build Jon Weeks, Bear, Benton.

From speaking to Jon Weeks, the 973cc setup should yield a low 11's bike that can look at the 10's.
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Offline TurboD

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Re: Stroker build- '76 cb750k6
« Reply #36 on: December 17, 2013, 12:47:00 PM »
Adam I have read through the posts and there is a lot of good info here. Now going back to your original post, what exactly are your goals (to fit a certain class, ET, MPH ,HP) etc?

Myself also new to SOHC Honda performace engines, I had posted up a thread asking about what could be expected in terms of both horsepower and cylinder head numbers flow wise. The cylinder head flow numbers will directly give an idea as to how much horsepower a given engine will produce. When it comes to the IC engine one simply cannot "cheat" the engine into making more power than the head will provide. Barring nitrous use or turbo.

A few years back I built a Kawasaki KZ 1428 with a completely stock head (head flowed about 70 cfm @10). It ran great with killer monster torque, but at the end of the day it simply was not very fast. That head was going to produce roughly 120 crank HP reguardless of how many CC's I put under it.
« Last Edit: December 17, 2013, 12:49:09 PM by TurboD »

Offline jweeks

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Re: Stroker build- '76 cb750k6
« Reply #37 on: December 17, 2013, 01:44:59 PM »
Any pin height piston can be used with a more than stock stroke. It's a question of how you get the right deck height assuming that the combination makes sense to build.

Bill B. ran a 10.5 to 1 compression 836cc stock wheelbase Honda at the Manufacturer's Cup with 12.4 dial-ins. With 12:1 compression and a lockup clutch, he would be knocking on 11's door. 915cc?  11 second time slips.  A longer swingarm would help the launches. Yes, the head airflow limits horsepower, but the torque curve also has a big effect on ET.

There's very few surprises in building what has been done over and over for the last 30 years. Improved heads from better valves, better porting,  and more $$ spent welding up better ports to flow more air are most of the changes. The internet has made sharing build results far easier. There's a lot of answers on this forum. It should help your confidence when you start your next build. Remember, we want pictures! ;)

                                                                            Jon W.
« Last Edit: December 17, 2013, 05:48:43 PM by jweeks »

Offline EnginebyAdam

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Re: Stroker build- '76 cb750k6
« Reply #38 on: December 17, 2013, 04:50:59 PM »
Adam I have read through the posts and there is a lot of good info here. Now going back to your original post, what exactly are your goals (to fit a certain class, ET, MPH ,HP) etc?

Myself also new to SOHC Honda performace engines, I had posted up a thread asking about what could be expected in terms of both horsepower and cylinder head numbers flow wise. The cylinder head flow numbers will directly give an idea as to how much horsepower a given engine will produce. When it comes to the IC engine one simply cannot "cheat" the engine into making more power than the head will provide. Barring nitrous use or turbo.

A few years back I built a Kawasaki KZ 1428 with a completely stock head (head flowed about 70 cfm @10). It ran great with killer monster torque, but at the end of the day it simply was not very fast. That head was going to produce roughly 120 crank HP reguardless of how many CC's I put under it.

My goal has always been a 10 sec sohc Honda drag bike. I wanted something that would make the crotch rocket crowd at the drag strip give a second look to a 30 + year old bike that lays down similar numbers to their factory bikes. As I dug into the build I realized we don't have the cash to build this engine big enough to run 10's consistently. So racing heads up is out of the question.

We don't have a motorcycle drag community around here, it is coming and this build will be a big part of that. My partner and I ( I handle the engine design, he handles the FI and chassis) are racing our selves for the most part. One of the regular drag racers runs a extended swing arm, air shift, 1400 ninja. I know him fairly well, the bike has no engine work. 9.42 @ 146MPH. There are a lot of "drop in" in guys running factory bikes on average mid 10's due to lack of skill, not engine. Our goal is to see what we got in the 11's. This is all about pushing our selves to see what we can come up with design wise.

A 1996 GSXR fell in our laps this mourning, I just finished striping it down, looks like we got a swing arm, front end, wheels and brakes to work with. Have to dig into it more tomorrow to see if we can use it all. The swing arm can defiantly work and that rear wheel gives us a 180 size tire to start with ;D

On the point of the head flow, this F head can support a 1080cc engine flow wise. So flow wise it should support the 973cc. The 67mm bore should work nicely to unshroud the large valves in the F head. Large valves and in the case of the F head, nice intake ports, are not typical of a undersquare engine. The natural characteristics of the undersquare engine combined with a head that flows on a undersquare engine, in my mind, should produce an engine capable of 100HP and low 11 sec passes on a good chassis.

As Jon said, the torque curve should be the ticket here. HondaMan also mentioned that with a undersquare engine, as these were stock, a turbo charging effect comes into play in the stock engines around 6500 RPM with the 28mm carbs, and about 7700 RPM with the 31mm. He also mentioned that as the intake valve gets bigger so can the carbs. So with the stock valve size of the F head being 34mm this should move this effect even further up the rpm range. We will be running gsxr throttle bodies so will have to see how this relates.

Cheers,
Adam
« Last Edit: December 17, 2013, 05:08:23 PM by EnginebyAdam »
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