Topic: Big bore kits, how big? Streetability?

[Dunk]

I need to pull the engine from my K1 to reseal it. Pucks and a weeping base gasket, countershaft seal leaking, and I should probably do something about the trans popping out of 3rd with increasing regularity. Last time I was in this engine for a top end rebuild, porting, and valve job back in about 2013 the taper on the bores was so small I just did a hone and new rings. I wanted to do an 836 but couldn't justify it with the overall restoration cost and the bores being nearly pristine. It now has another 10k miles and I have another 750 to ride every day, so I'm thinking this is a good time for more displacement.

I see various 836 options from anywhere from under $100 ebay kits to more reputable and costly options. I also see up to 1026cc such as Dynoman's offerings. I'm tempted to go all out for 1026. It does open the rabbit hole, which I'm not opposed to but question if it's worthwhile. I also wonder if going to such a large sleeve will cause streetability issues with cooling or anything else. I'd like to keep this bike looking fairly stock with good manners but as much power as reasonably possible while maintaining good manners. Thoughts?

[craz1]

The 836 kit has been a popular one for quite some time. My view on big bore kits is why. It opens a whole can of worms with drivability. You would need to take care of that transmission for sure. 1036 kit would require work with the carbs.  I would stick with just an 1mm overbore and possibly a street cam or at the max the 836 kit.

[Don R]

 A small cleanup bore helps round out the cylinders. Anything over 836 requires new sleeves and cylinder block boring which can cause the fin gaps to be exposed, then the inside of the sleeves get bored. CycleX has some good info on big sleeves and the machine work required.
 

[Jerry Rxman Griffin aka MuthaF'er]

Anything above 836/850 will not only open up the rabbit hole but also your bank account. Rods and case boring minimum. Then you will want to do head work, crank work, carbs, etc. Not necessary but.....  My BIG buck 915 with goodies is very streetable (so far) and mild mannered but comes on very hard. One thing I would suggest is to not go over a 10.25/10.5:1 compression for streetability. Save the 12.5:1 for the strip. 836 requires cylinder boring only. Put in a cam while you're at it. Very inexpensive compared to going bigger. Next above that simple suggestion would be to do head work also. That will double your investment.

[Stev-o]

My K4 has an 836 kit and an aftermarket KH cam, very streetable.

[Dunk]

Jerry, about 10k miles ago during restoration I did some port work along with new guides and three angle valve job. Not expecting it to need any head work after that but I can do the needful if it shows wear. I regretted not going with a larger cam afterward, as it felt like it would keep pulling if not for stock cam and springs.

After looking some more I'm thinking 998cc, as it gives room in the sleeves for an overbore. I was looking at the Dynoman 998/71mm kit with 10.25:1 pistons. I'm curious what cam would be best for this displacement to maintain a stock-ish relatively smooth idle. I had a CB750 that ran rough below 2500 RPM, came on around 5000 RPM, and revved past the end of the tach... while it was a hoot, a bit more mild mannered would suit me better these days. What octane does your 915 require? Compression ratio? Cam?

[grcamna2]

subscribed

[MRieck]

I ran a 915cc kit with Megacycle 125-70 for 25 years.....it was very dependable. I'd highly recommend replacing the primary chains and rods with heavy duty pieces if you go over 836cc. Even at 836cc I'd recommend replacing the primary chains and rod bolts with late F2 bolts.

[Jerry Rxman Griffin aka MuthaF'er]

One of my bikes is set up like Mike's. Plus it has his stage 4 head in it. 125/75 which is the same grind as his 125/70. 10.25/10.5:1. I have not ridden it much except during testing and down to the dyno and back. Seat time is coming when I return for the summer. Mild mannered UNTIL you light it up. Hang on!

My other bike is still in shakedown stages. It's solid. I gotta sort out the RS34 carbs at elevation still so I really can not comment on the streetability yet but from what I see it is also mild mannered. It has a 1000/71mm kit from APE. We had Big Jay source out 8 kits since none others were available then. It is 10.25/10.5:1. We then had him source out 8 Crower rod kits due to the displacement. I am trying the Dynoman RC315 cam in it.

I fill both bikes with premium gas even though they are not really high compression. At elevation it is usually 2 points less. Neither have a rough idle.     

[ekpent]

 A nice set of Wiseco 836 with 10.5 comp can give a nice improvement in performance even with a stock cam if you want to keep the increased lower end power/torque. A buddy had an 811 Wiseco when they made those and that bike ran very well. Just depends on how much you want to spend.