Topic: CI pistons not central in bore

[disco]

I’m also gonna get the oil drain galleries in the head & barrel ‘upgraded’. I swapped out the Honda cylinder studs for APE heavy duty studs. After that, I noticed that the amount of oil that was in the head when running was a lot! A real lot! Basically my valve guides are submerged in oil!

The APE cylinder studs are not ‘waisted’ like the Honda ones. So effectively the cross sectional area for oil drainage back to sump, has been reduced. The APE studs measure about 7.9mm. The drain holes in the barrel are about 10.8mm. That leaves about 1.45mm around each stud for oil to drain. I’m gonna bore the holes out to 12mm. Then I’ve got 2mm clear all around the stud, which must be getting close to how Honda originally designed it.

[754]

If the scuff pattern is slanted, its a bent rod, not sure it ran long enough to show that.
 Stock rods should bend easily for straightening, its easy to do. I have done it on other engines.
 Are these rods from a motor you ran or  did the top holes get a bushing ?
 If they came from a motor you ran, you will see it on a piston.
If they bushed the tops, they should be checked after.

[Terry in Australia]

No worries Disco, all the best with the rod straightening. I had NOS RC H/D studs in my hotrod K1, the RC rods were 8mm like the Ape studs, but had special "waisted" studs to allow for oil to run back down.

Having said that, I don't know why yours wasn't allowing oil to empty normally, because I've done a couple of other bikes with APE studs with no issues, and no mods. I reckon Ape has sold thousands of sets of their studs over the years with no issues either, but your mod sounds interesting, oil pooling over your exhaust valve guides would certainly account for oil burning, I'd reckon.

Have you checked your oil tank for a blocked vent? Cheers, Terry.

[disco]

Hi Thanks Frank & Terry!

Frank this motor ran for maybe 100 miles. There is no scuffing on the bore, but pistons show wear on top LH side and bottom RH side. No bushing. Machinist said he would fit an arbour to little end and check straightness? A bit above my pay scale, I’ll leave it to the expert!

This has certainly been a learning experience for me. I just figure while I’m in there, I might as well upgrade the oil drainage back to Honda specs while I’m at it. It might not be an issue, but I might as well take care of it.

I’ll let you know in a couple weeks the outcome.

Terry, that’s a good point. I know the hose to tank is good. It’s brand new. I get what you’re saying though. Crankcase Pressure needs to be able to vent. I’ll do check the oil tank itself tomorrow.

[enwri]

When I pulled my 750 apart I noticed uneven pattern of wear on the bottom alternator side and top points side of no. 1 piston.
I imagine it got tweaked when someone undid the alternator bolt.
Just happened to have a long perfectly sized stainless rod, when put through the no.1 little end, it was about 3mm low when trying to fit it through the no.4 little end.
Wound crank around to BDC and chocked the side of the rod against the crank web, gently pulled the bar up until I could thread it through No 4. as well. Was surprised just how soft and easily it was able to be straightened. was expecting it to be stronger, maybe brittle, but was more like thick hard leather. Same pistons with new rings went back in. not enough tilt to wear the piston or bore, just had uneven carbon deposits.
That was over 60000 klm and many many redlines ago.

[754]

The procedure is easy for rods in engine.
 Make a checking pin, that just slides in small end, long enough to cover the top of cases each side.
 Then you lower the rod and check pin with paper strips or feel led guages. You make a tool that fits the rod snugly below  small end...tool is on a long arm. Then you pry on the  arm and straighten the rod.
 Stock rods should bend easily.

[disco]

Hi enwri, what you described with the stainless bar is almost exactly the fix my machinist described to me. Thanks!

Thanks Frank too! Good to know this stuff.

Makes me wonder how many CB's are running around with rods that aren't perfectly straight?

[disco]

Terry, you may be on something....

I should mention that I'm running a K6 oil tank on this bike. It doesn't have that circular device on back of it, that earlier bikes did.

Anyhow, I removed the hose of back of gearbox & tried to blow thru it. It offered some resistance. I then removed the cap/dipstick & easy to blow thru. Put cap back on & blew a lot harder. I could now hear some gurgling & air coming out inside the engine. I'm guessing that was coming out the scavenge side of the oil pump?

So is the cap/dipstick thingy meant to vent to atmosphere. Is that what the spring business & tabs under the cap are for? 'Cause mine sure ain't venting too good. I can't see how the later K6 oil tank can vent any other way but thru the cap. There's only 3 connections I think & 2 of those have oil running permanently thru them!

[PeWe]

K6 oil tank is a design flaw by Honda. It has NO vent as you have noticed. It can let over pressure out via the dip stick. If you use another aftermarket dipstick is will seal completely.
This will end up in oil weeping cases. I replaced my K6 tank with the older model last winter.
K6 case has bigger breather pipe, 1/2" hose, the old tank has smaller pipe. You need a reducer and have both hoses.

Mt bike has that plus a T- connection with an extra hose going upwards then down again. My 71.25mm pistons pump extra air.
I have the thicker heavy duty studs, no oil smoke.  Not before either with K6 oil tank.

Strange if oil will not return as it should
Sure the head/ base gaskets did not block the oil return holes?
Oil level in oil tank correct? Return hose to oil tank not kinked?

[disco]

Hi PeWe,

Yes I have an in-line adaptor to fit the two different hose sizes together.

Oil does return....just not fast enough. There are no blockages in the return galleries. The gaskets were installed correctly. As I said earlier, the return drainage holes in the barrel are 10.8mm. The Honda studs are 'waisted'. They are about 6mm where the oil flows around them. So there is 2.4mm clearance around the original studs. The HD studs are nearly 8mm and have about 1.4mm clearance. Maybe I'm barking up the wrong tree, but whist it's apart it won't hurt to enlarge the holes.

Can you do a simple test for me? Remove the tappet cover in front of No1 exhaust valve. Run the bike for 1-2 mins up to about 2,000-2,500 rpm. How high does the oil level get? With mine, oil was about to flow out the tappet cover hole. I had to shut it down.

(Terry, if you have HD studs, can you run same test?)

I think I'll add the tee & extra hose like you did. I'll go up & then finish somewhere down near to the drive sprocket. (Chain oiler)

[Terry in Australia]

Terry, you may be on something....

I should mention that I'm running a K6 oil tank on this bike. It doesn't have that circular device on back of it, that earlier bikes did.

Anyhow, I removed the hose of back of gearbox & tried to blow thru it. It offered some resistance. I then removed the cap/dipstick & easy to blow thru. Put cap back on & blew a lot harder. I could now hear some gurgling & air coming out inside the engine. I'm guessing that was coming out the scavenge side of the oil pump?

So is the cap/dipstick thingy meant to vent to atmosphere. Is that what the spring business & tabs under the cap are for? 'Cause mine sure ain't venting too good. I can't see how the later K6 oil tank can vent any other way but thru the cap. There's only 3 connections I think & 2 of those have oil running permanently thru them!

Woohoo, I'm finally right! But yep, I reckon that might be the cause of the engine smoking mate, I take it you don't have an earlier oil tank? Here's one that looks about right, and not too expensive. I've only got stock studs in my K4 engine, but I've had it running with a tappet cover off, and the oil doesn't build up at all. I wonder (but I don't know) if the lack of a vent in the oil tank has an effect on how fast the oil is returned to the oil tank? Hmmnnnnn..........

https://www.ebay.com.au/itm/Honda-CB750-SOHC-Oil-Tank/232400128515?hash=item361c208a03:g:X-MAAOSwAO9ZXt5g

[disco]

Well I don't exactly know what the cause is, so don't go popping the champagne (or bourbon) just yet! I did wonder the same as you're suggesting ie is crankcase pressure having an effect on oil drainage? I wish I had done the oil drain test with oil cap removed!

If you've got stock rods, then I would fully expect that you would have no oil drainage problems.

What I do know;
1. I've certainly got 3 three slightly bent rods
2. My oil drainage right now ain't as good as it should be
3. My K6 oil tank doesn't vent real good.

I do have a spare K3 oil tank on the shelf. However I'm gonna go with PeWe's extra breather for now. I saw Hondaman also referred to a similar mod in an earlier post too. I may swap oil tanks down the track a bit.

I'll get this fixed one way or another. But there's so many variables going on though I may never know the exact cause?

[Terry in Australia]

No worries mate, but you better re-read Pewe's post, he's changed his K6 oil tank for an earlier one. "I replaced my K6 tank with the older model last winter" (and then) "My bike has that plus a T- connection with an extra hose going upwards then down again". Cheers, Terry.

[disco]

Ah thanks Terry! Good pick up. I don't know why though. Doesn't the earlier type already have a vent from that circular separator gizmo?

[PeWe]

I have not opened the tappet covers lately. I adjust valves with valve cover off. I plan to soon change cam so next adjustment will be with cover off and a really cool engine

If the bike is standing on side stand with engine running I'm not surprised finding a lot of oil on valve 1 side if open the tappet cover.
The valve cover breather has a hose routed free not connected with something else?

[disco]

Bike on centre stand. I'm not a dufus.

[Yamahawk]

Here you go... just kidding lol
Charlie

[BPellerine]

well the old saying goes ''how do you eat an elephant?'',one bite at a time.you have most likely already checked this but if this is the bike with the modified oil jets,check that the cam towers are not being held off the head,allowing oil to escape under them,on the side that is building up oil,just a thought.bill

[ekpent]

 This is what was in my breather hose coming off the valve cover on a K5 I did some top end work on. Member Sean took it for its first ride and came back with the rear end covered in oil. Says I tried to kill him    Its on my check list now for all new to me bikes !
Seeds in a piston bore is also a very good sign to see 

[Yamahawk]

Mousetacular bore!
Charlie

[754]

Moused spectacular pic of the morning.
Sounds like clearly oil not running down.. did you mix any f head or jugs with k parts, by chance?

[disco]

Hi Bill,

Yes correct this is the ‘71 K1 head that has a custom holder for the later type oil jet. That was the first thing I checked. I thought maybe that I had too much oil being delivered to top end somehow. However there was nothing wrong. It was all held down tightly. The machine shop told me they made the custom holder, they had bolted the cam towers down with oil jet in place under them, and pressure tested too. No leakage.

Frank, definitely a K head. Jugs I think are also K, but not 100% sure on. The oil galleries all line up though?

[HondaMan]

Yep no alignment pin. The rings are free to rotate. The rings were installed correctly.

There's no appreciable wear. The cylinder still has the hone marks. The bike had probably done less than 100 miles on this engine. The pistons are a tight fit measured near skirt. I couldn't get a .0015" feeler gauge in there. I measure the bore at 61.48mm (2.420")

However, I can 'rock' the top of the pistons though. These CI Pistons do seem to have a lot of taper on them. Measured down near the skirt they are 61.45mm (2.419"). Measured up near the top ring they are 60.88mm (2.397").

Im going to get the valve guides checked tomorrow. Will update then.

All of these pistons are 1.0mm smaller diameter above the skirts, that's normal.

[disco]

Thanks HondaMan. I’ve since compared the CI pistons to a set of OEM Honda pistons. Both sets have about 0.5mm of taper between the skirt & top of piston.

[PeWe]

Any new findings and conclusions?
It seems for me very strange that 3 rods should be bent sideways.
Crank was properly placed in the case? It can move a little when working with the engine.