Author Topic: CB750 K3 pistons (Read 5075 times)

aejmeja1 · « **on:** January 23, 2017, 04:11:06 AM »

Hey folks,

I'm going through the engine (top end) of my CB750 K3 that I've had for about a year. I dont know what the mileage on it is, because the speedometer isn't original. The history of the machine is also a mystery.

Here's what i found when i pulled off the cylinder head and cylinder block. The pistons seem mismatched to a novice like me?

I'm assuming the 0.25 marks on the domes mean the cylinders have been bored to .25 oversize and new pistons and rings were fitted. How do they look and is it common practice to have different pistons in different cylinders?

thoughts?

Suppose the main questions is...can I run these? The bike was a runner before I started taking it apart.

Thanks!

-J

aejmeja1 · « **Reply #1 on:** January 23, 2017, 09:32:54 AM »

2 of the older looking pistons have small dents or depressions over the dome area. What could this be a sign of?

If i run these pistons will it cause damage to the engine, if the vacuums/compressions are different?

aejmeja1 · « **Reply #2 on:** January 23, 2017, 11:46:28 AM »

Here are some better pictures:

The pistons that are in better shape say "made in japan" on the inside. They are pretty darn close in size. Dont know if the other pair is heavier than the other, but I'm guessing that won't effect compression.

How easy would it be to split the cases from here to check bearings? I've gone this far...

And thanks for the replies!

aejmeja1 · « **Reply #3 on:** January 23, 2017, 11:59:52 AM »

Alrighty! Thanks again for the input, much appreciated

I'll hold off on the full rebuild.

ekpent · « **Reply #4 on:** January 23, 2017, 03:34:42 PM »

I would be interesting to see if the weight is much different in what looks like two different brand/style of pistons possibly. 4 matching cheap E-bay 836 big bore pistons might be a good idea if your doing a full rebuild.

madmtnmotors · « **Reply #5 on:** January 23, 2017, 04:40:34 PM »

Quote from: calj737 on January 23, 2017, 11:52:05 AM

since it was a runner before, why split the cases?
Unless you're undertaking a significant rebuild and/or restoration, I don't see the point.

+1

Quote from: ekpent on January 23, 2017, 03:34:42 PM

4 matching cheap E-bay 836 big bore pistons ~~might be~~ ARE a good idea ~~if your doing a full rebuild.~~
since you're already this far.

FTFY

754 · « **Reply #6 on:** January 23, 2017, 06:29:16 PM »

Is the head pockmarked too, looks like something went thru a motor to do that to a few pistons.
If the pistons measure up ok, just run them

aejmeja1 · « **Reply #7 on:** January 25, 2017, 03:28:31 AM »

Quote from: ekpent on January 23, 2017, 03:34:42 PM

I would be interesting to see if the weight is much different in what looks like two different brand/style of pistons possibly. 4 matching cheap E-bay 836 big bore pistons might be a good idea if your doing a full rebuild.

The difference was only 2-3 grams.

ekpent · « **Reply #8 on:** January 25, 2017, 05:20:05 AM »

That's quite close

madmtnmotors · « **Reply #9 on:** January 25, 2017, 05:22:27 AM »

What do the skirts look like? What is the piston to bore clearance?

aejmeja1 · « **Reply #10 on:** January 25, 2017, 10:34:48 AM »

I'll take some pictures of the piston skirts. They looked pretty clean to me tho (to my novice eyes

).

I measured the pistons under the pin with the caliper jaws on the valve depression sides...all were very close in size (three gave a measurement of 61.24 mm, one gave 61.25 mm). I supported the caliper against a flat surface to get as good of a reading as i can...at the base of the skirt.

Tomorrow hopefully I'll have time to measure the cylinder diameters and piston ring gaps.

754 · « **Reply #11 on:** January 25, 2017, 01:46:13 PM »

Just use a feeler guage..

aejmeja1 · « **Reply #12 on:** January 25, 2017, 02:53:57 PM »

So....stick the pistons into the bores without rings and use a feeler gauge?

Don't have anything more accurate than a caliper. I suppose I could have them measured at a machine shop.

aejmeja1 · « **Reply #13 on:** January 27, 2017, 12:46:22 PM »

Ok.... I have a bad feeling the readings I got are not good news $:-\$

I cleaned off the pistons well and removed the piston rings. I stuck the pistons into their bores from the top...with the piston head going down into the cylinder (skirts up). I slid a feeler gauge into the cylinder before sticking the piston in. I could go as high as 0,15 until the pistons would get stuck in the bores with the gauge in there. 0.15 can't be good?

I think I had the gauge slid into the back section of the piston (facing the rear of the bike), was this the correct place to measure?

The range on the feeler gauges was 0.03-1.00 mm, so I can't be reading it wrong...15 is 0.15 mm

Here are some pictures of the chambers:

754 · « **Reply #14 on:** January 27, 2017, 02:52:02 PM »

One chamber at least had something go thru, dont know if it was a rock or small piece of metal..
If checking piston with feeler guage, cant really go from the top, as paeprt of it should have no wear. Try the cylinder upside down, slide piston in, with feeler between the skirt and sleeve..(skirts must be to front and rear of cylinder ). Try a.05... Get the skirt past the bottom , then see how feller guage feels.
.1 mm is approx 4 thou..that is too loose
.05 mm is approx 2 thou... That is near what it should be

Long feelersare the treat for this procedure..

aejmeja1 · « **Reply #15 on:** January 27, 2017, 04:25:37 PM »

Just to make sure i got this right:

I stick the pistons into the bores from the bottom of the cylinder block (pistons the normal way around: dome up & skirts down) with a feeler gauge in between...and hope for a reading of .05? Skirts facing front and rear of cylinder?

If it turns out to be too loose whats the next step? I'm short on cash right now and was hoping to not spend too much on the engine. Should i just cough up money for a big bore kit (will that require new cylinder sleeves)? Or just reassemble (since it was a runner) and save up to fix it next winter? Thoughts?

Thanks for me helping me out!

aejmeja1 · « **Reply #16 on:** January 27, 2017, 04:57:15 PM »

Are there huge quality differences? I found a Forseti kit on ebay:

http://www.ebay.com/itm/HONDA-CB750-K-SOHC-69-78-836cc-BIG-BORE-KIT-65mm-PISTON-RINGS-GASKET-/141491234430?hash=item20f188e67e:g:cpsAAOSw44BYfGmB

Much cheaper alternative from Japan:

http://www.ebay.com/itm/69-77-HONDA-CB750K-836cc-BIGBORE-PISTON-KIT-W-GASKET-10-CB750KBB-/262821780672?hash=item3d31661cc0:g:jYYAAOSwQPlV8obw&vxp=mtr

Thoughts? Or could I go from .25 to .5...Emgo any good?

http://www.ebay.com/itm/Honda-CB750-K-69-76-Piston-Kit-Rings-Wrist-Pins-50mm-Bore-18-38002WR-/151886815193?hash=item235d28dfd9:g:npgAAOSwIwhWSl~W&vxp=mtr

disco · « **Reply #17 on:** January 27, 2017, 05:02:36 PM »

Machining for an 836 kit is going to cost a lot more than the next oversize piston kit.

aejmeja1 · « **Reply #18 on:** January 27, 2017, 05:05:23 PM »

Good to know!

disco · « **Reply #19 on:** January 28, 2017, 12:00:56 AM »

Quote from: calj737 on January 27, 2017, 05:06:06 PM

Boring should be between $40-$50 per cylinder.

Except if you live in Sydney, where everything is expensive ....$200 per hole!

754 · « **Reply #20 on:** January 28, 2017, 12:39:23 PM »

Quote from: calj737 on January 28, 2017, 04:22:18 AM

Quote from: disco on January 28, 2017, 12:00:56 AM
Quote from: calj737 on January 27, 2017, 05:06:06 PM
Boring should be between $40-$50 per cylinder.
Except if you live in Sydney, where everything is expensive ....$200 per hole!
The cost of living in paradise maybe? Looks as though the OP is in Finland. Wonder what the rate is there?

If its 200 a hoke to go to 836.. Try this ..get a used 836 cylinder shipped from the US.then get the Cyclex 849 kit... Then it is a simple overbore..just like boring a stock cylinder 1 size over..

News:

Author Topic: CB750 K3 pistons (Read 5075 times)