Topic: Big Bore pros and cons

[thelowmax]

Hi, Anybody with experience want to weigh in on the pros and cons of Big Bore systems? What size upgrades are recommended for what situation/ Is power the only benefit? Are major jetting and exhaust adjustments necessary? Other than the bore itself, is this something that a novice builder like myself could tackle? Any recommendations on quality/price/manufacturers? Anything will help. Thanks.

[iron_worker]

I haven't done a big bore but I can answer a few of your questions ... first of all you should tell us what bike we're talking about. lol

1) Are major jetting and exhaust adjustments necessary - Unless you're going huge and lots of head work, changing exhaust, intake, etc then probably not much change required.

2) Is there power benefit only? Depending on what you buy you may use pistons that are stronger (forged perhaps?), lighter? etc

3) If you're rebuilding the engine anyway then going to a big bore isn't really any different ... unless you start going really big bore.

Others will chime in on this I'm sure.

IW

[thelowmax]

I'm working on a cb750k7.

[bjbuchanan]

Big bore will, with nothing else changed, give i think something like 3~ more hp, but that really depends on the kit.


The con is expense and downtime. The old downside was that you couldn't clean it up, like a small step up if it grenaded, you had to get another set of cylinders or go real big. Some older ones consumed more oil but nowadays they don't. 836 won't have heating problems either, you haven't gone big enough yet to bring that deal on.

You likely won't have to change jetting because the engine breathes a little more efficient and sucks more fuel thru those jets. Think the reverse of the pod effect, sort of. Some guys have dropped to 38 pilots, but all are a little different.

If you want to go balls out then send the head to Mrieck. That will be a different build tho, that starts to get more serious tho he does offer reconditioning, you don't have to go crazy. Is tempting tho. If you want to go low budget with a healthy engine then lap the valves in and slap it together.

I am one of the ebay 836 guys so I have first hand experience with some of the changes. Others can chime in too. I rebuilt my 836 outside (no garage/shed/anything convenient lol), more or less basic hand tools and I took it down to the point of splitting the cases. If you aren't ham fisted and can follow pictures and directions you are fine. It seems like a joke but really, the text is important not just the pretty pictures. I wish PO's would read more a lot of the time.

[Retro Rocket]

Big bore will, with nothing else changed, give i think something like 3~ more hp, but that really depends on the kit. Since yours is the later engine you, to the best of my knowledge, will have to get a more expensive kit not the cheapie ebay. Ebay 836 domes will be too small, you will have terribly low compression.

Loss of compression, on an engine that isn't exactly pushing the envelope will make it run miserably. Maybe Wiseco ($$, 500~) will have a kit, they will have better figures, something like 10.5:1. That is if they make it for the more domed later engines, there 'older' kits with early heads spec around 10.5:1.

The con is expense and downtime. The old downside was that you couldn't clean it up, like a small step up if it grenaded, you had to get another set of cylinders. Some older ones consumed more oil but nowadays they don't. 836 won't have heating problems either, you haven't gone big enough yet to bring that deal on.

You likely won't have to change jetting because the engine breathes a little more efficient and sucks more fuel thru those jets. Think the reverse of the pod effect, sort of. Some guys have dropped to 38 pilots, but all are a little different.

If you want to go balls out then send the head to Mrieck. That will be a different build tho, that starts to get more serious tho he does offer reconditioning, you don't have to go crazy. Is tempting tho. If you want to go low budget with a healthy engine then lap the valves in and slap it together.

I am one of the ebay 836 guys so I have first hand experience with some of the changes. Others can chime in too. I rebuilt my 836 outside (no garage/shed/anything convenient lol), more or less basic hand tools and I took it down to the point of splitting the cases. If you aren't ham fisted and can follow pictures and directions you are fine. It seems like a joke but really, the text is important not just the pretty pictures. I wish PO's would read more a lot of the time.

Ok i'll clear up a few things here, the K7 has the same head as the F1 {392}, so all K model piston kits will fit with none of the problems associated with the F2-F3 engines with their different heads. Most big bore kits {street} are around 10-10.5 to 1 compression so they will be fine. The 836 ebay kit , with its lower compression may give a 3hp boost but the other kits, Weisco, cycleX  and APE's kits will give better figures than that {depending on size}, no one would bother doing 5-700 dollars worth of work for 3hp.  Off the top of my head , aftermarket kits available are 836, 850, 970, 1000cc and the cheaper ebay 836 kit with its closer to stock compression  .With any displacement change, to get the extra power on offer you should  rejet, especially if you go with an aftermarket performance pipe. Other things to consider are the top end components, better quality springs and retainers would be advisable, valve guides {and valves for better quality} , seals and a better camshaft, the cam will also add to the HP increase if mated to the engine correctly, some mild port work will give a bigger increase in HP due to increased flow and would be money well spent, you don't have to go wild as the ports are quite rough standard and cleaning them up will make a noticeable difference, carbs mods would be paramount to this all working as a well put together package to maximize the parts being used...  Posting this question in the High Performance section would probably help you with advice....

[bjbuchanan]

Revised the post retro, had the heads/years mixed. Don't want the wrong stuff out there

[Retro Rocket]

Revised the post retro, had the heads/years mixed. Don't want the wrong stuff out there

Thats ok, there's plenty of guys here that look at the year, like 77 for arguments sake, and think all of the late model Honda's had the F2-F3 heads, the first model F bikes had the first of the 392 stamped heads, those same heads were used on the K7and K8, they are good heads to modify, I have one sitting at MRieck's atm....

[750cafe]

From several examples of personal experience, keep the compression ratio at 10.5:1 as the maximum for an air-cooled engine.
I have a few that are 10.75:1 and that is pushing it a bit too far for a street bike on 91 octane pump gas.

Eric

[Don R]

Prepare to learn about ring gapping, valve/piston clearance checking, finding a reputable machinist to bore to a precise spec, determining what those specs are, A few guys dove into it without checking clearances and had their engines sieze when hot.

 I'd consider buying a second engine to rebuild so you can keep riding in the down time. Or, maybe you're real sharp and can bang it out no trouble, that's for you to figure out.
 I built mine tight for compression, it ran awesome but I had too much fun, missed a gear and a piston tagged a valve. So what to do? Get some clearance, a new valve, springs and retainers, gaskets, a big old cam and make it faster!

[thelowmax]

 I'd consider buying a second engine to rebuild so you can keep riding in the down time. Or, maybe you're real sharp and can bang it out no trouble, that's for you to figure out.
 I built mine tight for compression, it ran awesome but I had too much fun, missed a gear and a piston tagged a valve. So what to do? Get some clearance, a new valve, springs and retainers, gaskets, a big old cam and make it faster!

I actually have, in addition to the engine I'm tearing down, another complete extra engine and a third that is mostly there in parts. I would n't even be thinking about it if I didn't. Thanks.

[Jerry Rxman Griffin aka MuthaF'er]

Pro - more power
Con - may have to play with jetting

[PeWe]

"Big Bore" the natural step when a cylinder is damaged and need to be rebored. If the pistons are very fresh, replace the damaged sleeve only.
Higher compression will give a wilder engine.

I understand why the very 836 cheap pistons set from Thailand is very popular. I should use that if my bike was in OEM look with 4-4, not much done with the head and mild cam.

[thelowmax]

So the ebay 836 is a decent product? I was wary.

[PeWe]

There is a long thread about them from cruzinimage
http://forums.sohc4.net/index.php?topic=89779.0;nowap

The 836 gasket 65mm is very cheap, shipping too.

[bjbuchanan]

The ebay 836 is good for what it is. I'm running it with no issues along with a bunch of other guys. Towards the end of the thread that PeWe linked is more testimonials

If I was dropping the money on a performance buildup like a worked over head, mild cam swap...blah... I would go wiseco, or something like that.  You want an economical oversize kit that is reliable and will have you down the road with some extra power easily then ebay 836 it is. You can make a strong argument for everything under the sun tho

[thelowmax]

I'm actually doing my rebuild as more of an educational project more than anything else. These cb's are the only mechanical thing I've ever really worked on and each time I do repairs I like to go a little deeper and more advanced. Since I did my first complete strip down and have the engine out, I figured I would see how far I could take it. I think an extra 3 hp will be worth the learning experience and at the ebay 836 price, I feel it's worth the effort and money. Realistically, If I can just get it running again, I'll be happy.

I think I could do it with just a manual but I'm so glad that you guys are here for support. Thanks everybody.

[iron_worker]

If your bores were old and out of round (as many of them are) then just having a fresh round bore with good rings will gain you a bit more power too I would think.

IW

[thelowmax]

Does more power translate into significantly more speed or just more torque?

[KJ790]

From what I have seen, the ebay 836 kit will end up giving you about 8-9 more hp on a healthy K motor without changing anything else other than getting the jetting right. If your motor is tired and down on power then you will see a bigger power boost. With a few other changes you can get more power out of these easily. Just remember that while the parts themselves may be cheap, you will have to pay for the machining to complete the task.

[bjbuchanan]

Many boring services out there, I got mine done thru ebay for 150$ before shipping costs. Just over a third of the price the shops want locally

If you don't go wild you can really hold prices down. It is the slippery slope of why don't I do this, oh and this is old too.. that runs the price up

[750cafe]

I go to a Boat and Marine machine shop in the winter months and they love to have the business in their off season
and give you better prices right about now besides much faster service. 

Eric