SOHC.co.uk Forums > CB750

Oil pressure issue / motor rebuild

<< < (23/44) > >>

AshimotoK0:
Adding to my earlier post ..in a situation like this I would post the detail of your issues on the USA site ... problem is on there that you can easily get your post buried because of the no. of posts, so keep adding your own replies or 'bumps'. Then if you don't get a response / solution  after a few days, PM Hondaman with a link to your post. He usually responds by adding to the post and not a direct PM, which is the correct thing to do, as everyone benefits..... of course there is also Bryan, Trigger & Oddjob on here , who were in the trade and know a lot more than the likes of me.

Trigger:
Have you measured up the crank to make sure it is straight ? Can't remember how far you are away from me, as I find it hard to give a 100% comment without it in front of me.

MrDavo:
Thanks for your comments, I don't want to ask our American cousins just yet, from past experience with US car forums, they'd probably turn it into an oil thread :) I have read the relevant bits of Hondaman's book very carefully, though.

FWIW I have a theory, though its going to be hard to prove / measure.

I was thinking, like Ash, that the cases might not be a matching pair and so the line boring is screwed up.  However, both are stamped at the back with an 'M4', are there any other marks that would appear top and bottom?

It's too much of a coincidence that I got a freshly rebuilt engine, everything inside like new, with completely FUBAR mains shells, and absolutely nothing else wrong. Replace shells, torque it up and its solid. Maybe someone, a long time ago in South Africa, got to that stage before and either 'made' it turn with a big bar on the crank nut, or slackened things off a bit and flogged it quick, or even just trailered it to shows or stored it from then on.

As far as I can see the crank is straight and central, I just unipped the crankcase bolts , freeing it up again, and turned the motor over with a ratchet on the stator nut. As I did, I watched the crank at the ignition end turning perfectly in its new oil seal - if anything were bent you'd expect a slight wobble, and I trust Ash's judgment. It all turns over lovely until that last nip up to 18ft lbs.

Here's my theory. Remember how we were surprised that there's no oil stains, or any sign of that yellowish coating that the cases usually have? Well its ALL been blasted inside, we don't know with what media, but it was agressive ( it turned the polished forks and engine cases matt and rough to the touch -I Just read that vapour blasting doesn't, but IDK) INCLUDING the mating surfaces where the crank cases meet, AND where the crank journals are, look here:





Stupid muppets, that's a precision surface you ruined, critical to the fit of a crankshaft that is fitted to half a thou tolerances! IIRC that includes the bearing housings as well, I have no photos with the shells out, and its another teardown to look, but I will.

here's what we should have - a machined surface done at the same time as the crankcase mating sufaces, BEFORE the cases are line bored for the crank.



The perfectly circular line bored holes now aren't, if metal is removed at the join by media blasting they will become ever so slightly egg shaped - fine front to back but tight top to bottom. When I plastiguaged the mains I did it at 45 degrees to see clearly, I may get a different result at the top, I may have another go with a few threads in different places to see if the fit varies.

It would also explain why the mains were 100% toast but the rod bearings were fine, which was totally unexpected for an oil supply failure.

If this this the case (no pun intended) shimming wouldn't work, if you got the clearance right the case's mating surfaces would leak like a sieve. Re line boring by a precision engineer could work though, it needs to be 39.000mm, I don't see why the horizontal measurement would have changed for the worse, and if the cases are a tad closer together the other measurements can only have reduced.

The only other solution would be to find a good second hand set of K1 cases. There are later ones on eBay, but I'd be very wary of the subtle changes to the cylinders and gearbox in later models causing more hassle down the line.

The best course of action is to strip it down and try to measure the holes for round, whatever I do the huge elephant in the room is the fact that the barrels won't come off - they're going to have to whether they like it or not, only God knows how!

Trigger:
The numbers to match the crank case are a letter then 1,2 or 3 numbers (C190) as a example, these are located on the flat surface at the back.
When you have turned the crank, has it marked the shells in any way ?

MrDavo:
That might work Oddjob, I'll try it. There is the matter of the lower part of the rods being in the way, though, as the rods and pistons won't come out (in either direction) without the barrels being off (I have tried). It does sound doable with careful preparation though.

I've been (instead of doing some work) reading up horror stories such as a guy who had an vintage aircraft engine blasted (and saved by re-engineering, including line boring, but they are *&%$ expensive things) of course with those if you later break down you don't just walk home, you fall out of the sky first.

I just found this nugget - oh deep joy!


--- Quote ---That being said, I will caution against it for a more unlikely reason: future disassembly. A matte surface will bond with well over double the bonding force of a smooth surface. If you've ever removed a 20-year-old gasket from a machine, you know it's a bear to do. Cleaning the surfaces afterward is cumbersome at best. You make those mating surfaces matte prior to assembly and I would dare say those pieces are bonded for life.
--- End quote ---

  :'( :'( :'(

Navigation

[0] Message Index

[#] Next page

[*] Previous page