Author Topic: K8 Engine Damage - cam shaft sprocket in pieces (Read 9112 times)

anotherCB · « **Reply #25 on:** July 20, 2020, 12:27:25 PM »

Thanks for all the input everyone. I gave it some thought but decided against rebuilding my engine. There are more things to consider that I haven't mentioned, the engine is painted in black, which is peeling off and needs to be stripped, that would add a lot of nasty work. Also the cylinder has a fin broken off. If I would do a complete rebuilt, I'd want a cylinder with all fins intact. This is not going into a pristine bike, it will go into the bike in my avatar, and probably won't be ridden more than a couple miles every once in a while, since I have the Airhead for vintage gatherings.

AND, I found a bike locally that was "running when parked", which I think I can get for a good price, so that may be the way to go. Will check it out tomorrow. I'll keep y'all in the loop and thanks again for the input!

anotherCB · « **Reply #26 on:** July 23, 2020, 11:04:32 AM »

Picked up a nice K7 parts bike locally. Complete except seat, tank rusted through so I left it behind, exhaust is trash. Has a nice Lockhard (I think) oil cooler on there, seems pretty rare from what I read. Will disassemble the bike and sell everything but the engine and the oil cooler. Oh and I will keep the airbox, tired of messing with pods or stacks. I'll work on this over the weekend and will post pics.

754 · « **Reply #27 on:** July 23, 2020, 12:26:33 PM »

Lockhart are not rare , but they are good.. a few sizes, and black, chrome , some have thermostat, some have threaded ports.... got a pic ?

anotherCB · « **Reply #28 on:** July 25, 2020, 02:17:14 PM »

Here you go.

anotherCB · « **Reply #29 on:** July 25, 2020, 02:25:24 PM »

Got some updates on my build. The engine from the parts bike is apart, there are a few things I need to address, mainly because the bike has sat for 27 years:

1) One of the pistons was a bit stuck. Some PB Blaster and light hammering got it free, however the rings are baked into the pistons. Maybe I can get them free, but I saw that a new set of pistons runs less than 100 bucks, is it even worth it? Should I go with new pistons instead?

2) If I get new pistons, do I need to hone the cylinders? There is a bit of a marking where the rings were stuck but otherwise cylinders look good. It's supposed to be a 15k miles engine, and you can tell. The cam looks like new and overall the engine most likely has never been open.

3) I decide to have the jugs, head and valve cover vapor blasted. They look like crap, they have this factory silver paint job, so I will take the valves out and lap them back in. What should I replace? Shaft seals at least?

4) Engine factory paint needs to come off. What's the best way to strip it? Just get a disk on adrill or use the aircraft paint stripper stuff?

I will post pictures soon, but progress is being made!

754 · « **Reply #30 on:** July 25, 2020, 05:56:24 PM »

Looks like a Derale cooler, how did you come up with Lockhart ?

Alan F. · « **Reply #31 on:** July 25, 2020, 06:26:01 PM »

I've got an adaptor plate just like that somewhere too, the cooler was Lockhart, but was damaged so I left it in the junkyard.

anotherCB · « **Reply #32 on:** July 25, 2020, 07:32:53 PM »

Quote from: 754 on July 25, 2020, 05:56:24 PM

Looks like a Derale cooler, how did you come up with Lockhart ?

I had never seen one before. In saw a pic in the net and guessed

PeWe · « **Reply #33 on:** July 25, 2020, 11:19:41 PM »

New oversize pistons as I answered earlier with links to cruzinimage. All work assemble an engine and cost in gaskets to end up in a smoker.

These pistons need a tight fit. Total clearance of 0.025mm ( 0.0001"). Not more.
https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

I have just got another set of those.
These if your bores are stock 61.0mm and no deep pitting.
Rusted bores can have a deep damage. Around 0.2mm steel is not much for rust to penetrate.

I had a rusted cylinder to my 61.5mm pistons. 2 sleeves had to be replaced due to deep rust and not perfect surface after bore. +1.0 mm pistons had been good for those rusted sleeves.

You need to inspect the rust. If you see/feel pits, +1.0mm kit.
https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-1-0mm-over-size-pistons-set-62-00mm/

bryanj · « **Reply #34 on:** July 26, 2020, 12:28:04 AM »

Oversize pistons cost same a standard and if its rebored everything is then perfect BUT the bore has to be done by somebody who understands Hondas as it has already been said the clearance is tiny and frightens car rebuilders, if they wont bore to what you say take it elsewhere

anotherCB · « **Reply #35 on:** July 26, 2020, 02:30:21 PM »

PeWe I saw your earlier post (and even printed it!) but at that point I was thinking about rebuilding the original engine. Now I am looking at the engine from the parts bike I bought, which had one piston stuck, but wasn't super hard to get out with PB Blaster. The bores look good, they even still have this cross pattern on the walls which I believe is an indication for low mileage (bike was supposedly parked in 1992 with 16k miles on the clock).

What if I just bought standard size pistons and left the jugs alone? Is there a very good chance that will work?

Jerry Rxman Griffin aka MuthaF'er · « **Reply #36 on:** July 26, 2020, 02:54:02 PM »

Why buy standard size pistons using your reasoning? You already have a set provided they are not damaged. I don't think a valve nick might be an issue unless it has created a crack to weaken the ring lands. Plenty of spares out there.

K7/K8 top end differences: K8 rings changed to 1.2, 1.2, with oil rings staying the same 2.5 at a particular S/N (not sure what?). Measure/check specific S/N part numbers ; valves have different keepers and grooves ; K8 has stronger F2/F3 valve springs so use those if good.

anotherCB · « **Reply #37 on:** July 26, 2020, 02:58:49 PM »

The rings are stuck in one piston, although I would probably get them out, that's why I thought new pistons. Heck, maybe just buy new rings and go??

I am asking around locally to find who can bore cylinders and how much it costs. Will make a decision then.

PeWe · « **Reply #38 on:** July 26, 2020, 11:29:14 PM »

If bores are OK, use the pistons you have.
If you need new pistons, bore +0.5mm is not expensive and you'll be sure about correct clearance if done by a shop that can do the tight clearance.
Cross pattern all over the bore is a good indicator. The part of the bore where rings travel.

anotherCB · « **Reply #39 on:** July 28, 2020, 06:51:51 AM »

Looked at the cylinders again, there is a little bit of damage, maybe too much to re-use as is. So I may go all out and do the oversize pistons and rebore. Talked to a very reputable local shop that specializes on these bikes and apparently can do tolerances to 2/1000 mm. What should I expect to pay to bore 4 cylinders 0.5 over from a reputable shop? Any ball park numbers?

anotherCB · « **Reply #40 on:** July 29, 2020, 06:47:48 PM »

Actually decided to try and hone before I have them bored. Got the cylinders, head and valve cover back from vapor blasting today. Lapping the valves and honing will be next.

anotherCB · « **Reply #41 on:** August 01, 2020, 04:39:16 PM »

Made some progress today. Decided to get into the bottom end as well, now since the cylinder, head and valve cover is vapor blasted, I want the rest clean also!
First I split the cases of my "old" bottom end, where I had the cam chain tear. Lot's of bits and pieces in the oil-pickup! I will use a few parts from that engine (ignition, starter, 2 covers), since they are better.
In next days I will dig into the other bottom end. Stay tuned!

bryanj · « **Reply #42 on:** August 01, 2020, 11:57:16 PM »

Make sure you keep the crank with cases and know exactly where each half shell came from or you are in for expensive and time consuming work

anotherCB · « **Reply #43 on:** August 02, 2020, 04:57:21 PM »

Quote from: bryanj on August 01, 2020, 11:57:16 PM

Make sure you keep the crank with cases and know exactly where each half shell came from or you are in for expensive and time consuming work

Yes, I am aware, but nevertheless thanks for the reminder! Have my fish tackle box with many compartments ready to be filled!

anotherCB · « **Reply #44 on:** August 02, 2020, 05:35:41 PM »

By the way, this is what I found in the oil pick-up of the bottom end where the cam chain ripped. Lots of bits and pieces!!

PeWe · « **Reply #45 on:** August 03, 2020, 07:26:44 AM »

Engine look really clean except for the metal debris.

anotherCB · « **Reply #46 on:** August 03, 2020, 07:56:45 AM »

Quote from: PeWe on August 03, 2020, 07:26:44 AM

Engine look really clean except for the metal debris.

Yes I agree. When I got it, oil was pretty new, however I changed it anyways to get it running, then changed it again to ride it....until the engine went poof!

Luckily the engine from the parts bike also had very clean oil in there, hopefully it's clean inside as well, will take that one apart next, to have the cases vapor blasted.

anotherCB · « **Reply #47 on:** August 04, 2020, 08:03:37 AM »

Even though I still have to get the cases vapor blasted, I started looking into sources for parts. I need all the usual gaskets involved in a complete top and bottom end overhaul. I will also hone the cylinders and now I believe I will be able to reuse the pistons, so just need new rings. I will also lap in the valves, probably should get new valve stem seals. Again, this is not the engine where the cam chain broke, it's the low mileage parts engine that sat for 25 years with one piston slightly stuck.

I looked at cruzinimage, cyclexchange and cb750supply. Somewhere I read that it's important to get better quality gaskets, especially since heads can't be re-torqued (without frame kit) once the engine is back in. Any pros and cons with any of them? Thanks!

PeWe · « **Reply #48 on:** August 04, 2020, 09:28:25 AM »

Honda's gaskets have good fitment and a good surface, make it possible to later open a cover and tighten it again.

Yamiya's are good too.
Gasket kit and separate oil seal kit.

I'm sure others can write about CycleX gasket kits.

O-rings needed too. I think CycleX include more o-rings that usually are missing in most kits.

anotherCB · « **Reply #49 on:** August 06, 2020, 08:02:45 AM »

Is there a difference in piston rings between the pre-76 models and the 77-78? Thanks!

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Author Topic: K8 Engine Damage - cam shaft sprocket in pieces (Read 9112 times)