SOHC.co.uk Forums > CB750
Oil pressure issue / motor rebuild
AshimotoK0:
Feeling for you Dave. I blasted my 250 engines only the outside but only after totally sealing everything on a fully assembled set of cases with no internals fitted and then pressurizing the inside to check for leaks.
How about this for an idea ..a few of us must have quite a few old shells lying about, that may not be too worn. How about experimenting with them to try to ascertain where you have lost 'clearance', then you are not going to fk up valuable new shells. This is going to be totally frowned up on here but why not get some pieces of copper foil and make little gaskets for the centre bearing mating surfaces. Put your crank in but only nip up the centre bolts and see if you can regain your clearance, using Plastigage to measure it. I think line boring would be quite hard but it depends on who you know and what equipment they have, definitely cheaper to get replacement cases if you had to pay commercial rates for that job.
If you find they blasted the bearing housings that the shells fit into then IMHO I would think your cases are toast.
K2-K6:
It's a real shame to get caught into something like this when you just wanted a nice bike to ride. I does look like you are right regarding the loss of tolerance to the mains though. I know when we are involved in stuff of this age there are problems to be taken on but you'd not anticipate something so obscure and fundamentally wrong.
There is a process other than line boring that could realistically get those cases back which is engineering scraping. It was routinely done to get bearings into tolerance in some fields particularly when people would have been making up soft bearings for large machinery. It involves using something like a correctly sized mandrel to check for high spots with engineers blue, then physically scraping the high parts away and re-checking until the hole accurately matches the mandrel.
To expand on your method Ash but instead the opposite direction, you could linish the back of the shells with a very fine grit paper while miking them to relieve the space over an arc at the deepest part of the shells then check crank fit in them with engineers blue to bring them into tolerance and roundness.
MrDavo:
Thanks for your replies, here's a couple of pictures from the garage last night, the cases are stripped right down now, though I just have to remove the transmission.
Every picture tells a story:
There is a slight lip at the back corner on each side of the barrel overhanging the cases, A smart whack upwards each side did the trick - daylight at last!
You can see that the pistons are stamped '25' - the motor had just had a rebore, great news and it explains why there was no sign of wear in the barrels. Clearly someone spent a good deal of money on this bike, only for it all to go breasts upwards over such a tiny issue, resulting in most of the new stuff, which included brand new rims and spokes, being virtually zero miles, because of the crank FUBAR. Thats why its well worth doing right, IMHO.
I would like to save these cases if I can, for all the trouble the 'whatever' blasting has caused, the external finish, polished and then painted (I didn't realise until I chipped it trying to prise the barrels off) is super, and the cases look like new. I'd struggle to get another set that good, particularly as they would definitely not be going anywhere near a basting cabinet! IDK if the threads were masked, or recut, but they all look fine, unlike a set of BSA cases I once bought at an autojumble, when I was young and stupid,that had been blasted and turned into scrap.
I'd rather not go down the packing route, Ash, I'd prefer sound engineering to trial and error, and also the packing would have the effect of pushing the crankcase halves apart unless you made a copper gasket for the whole of the crankcase - It could leak like a sieve, not support the transmission bearings properly, or if it torqued down OK all be under stress. That reminds me, as Hondaman says, if you can line bore a 'seasoned' motor, that has already gone through many heat cycles, you get a smoother motor than one that was line bored when it was virgin metal, something that race engine builders use to their advantage.
The bearing carriers aren't factory finish any more, but very smooth. Its the centre line measurement that has been messed up - removing some of the circle has caused it to become slightly oval - they should be possible to line bore out to spec, as they are tight not slack - you can make a hole bigger, but not smaller! ;)
I could have a go at scraping as suggested by K2-K6, at virtuall zero cost, but trial and error with 10 shells to go at sounds a bit of a mare, and it would only take one error.
According to the HMC crank bulletin, bearing support 'A' tolerance (which would let me use my yellow shells with Ash's AAAAA crank) is 39.000 to 39.008 mm. If necessary the bore can be as big as 39.024, but obviously thats a set of different shells.
I'm going to send an email to Quasar engineering in Stockport http://www.quasarengineering.co.uk/ who fine tuned my Harley crank for not a lot of money, see what they quote / think before I make any irreversible decisions.
AshimotoK0:
It was only a suggestion Dave.. personally, I would get replacement cases but if you want to preserve numbers then that's possibly not the route to go. I assumed it was only the centre 'pillars' that were nipping up the crank so small bits of 1 thou copper foil may have just confirmed that the problem is mainly on those and not the outers, but you could probably ascertain that anyhow from a decent bore gauge measurement, wiith the crank out and the cases bolted up.
I checked the cases the crank I sold you came out of as below and apart from one outer half, all the shells are still there if you wanted to try used, not particularly worn SH shells to check whatever method you come up with is satisfactory without trashing 100+ quids worth of new ones.
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Trigger:
I would also bin those crank cases and seek a replacement. If the bearing surfaces are damaged, then all bearing housing will be beyond spec, this will include bearing and oil seal housings.
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