primary chain needle roller bearings

[UKROBK7]

Is there anything special about the 2 needle roller bearings (25x32x20) that fit inside the primary chain driven sprocket on the cb750?  They only rotate when the clutch is used?  I was looking at using good quality HK2520 ones in place of the relatively expensive honda ones, would there be any problems?

[AshimotoK0]

What's wrong with your existing ones Rob? I think they last pretty well. I am going to list a < 15k mile primary sprocket assembly on Ebay soon. I bought two gearboxes and 2 primary drives last year at great shipping expense from the USA - all UK ones seemed to be junk. One gearbox is spare too but its from a K2 I also have another 3 spare primary sprocket assemblies mileage unknown. The other gearbox I got was K1 and is less than 11k miles and it shows! its going in my K0

On the SOHC4 site a guy listed all of the part numbers for replacement bearings for the 750. If you guys are not visiting the SOHC4 site you are missing out big style on information. Trouble is there is so much of it!! i will try and get the info for you Rob

Cheers ashimoto

Found it Rob

http://forums.sohc4.net/index.php?topic=60026.0

Also for 550 four

http://forums.sohc4.net/index.php?topic=82355.0


Ash

[UKROBK7]

Hi Ash, nothing as far as I can tell but this k7 engine is having a full rebuild.  I stripped it completely to have an engine mounting weld repaired.  This has been done and looks great.  I have sourced all new bearings for the rest of the transmission and these are the only 2 that I haven't bothered to obtain yet.  I just didn't want to leave anything to chance after all the effort (and cost) that I am putting in on this!  The only thing I will not touch will be the bores as these are unmarked, still have their hone marks and measure up perfectly, I may treat it to new rings though.

Cheers

[AshimotoK0]

One thing that probably does age are the rubber cush  drives inside the transmission sprocket assembly - hardening . They are not available except from a German SOHC club who will only sell to members. You need to de-rivet to get at them too. My 400/4 transmission rubbers (available separately -  handened after about 8 years)

I got a couple of the ruibber elements  from a  NOS assembly from KOS racing in the States, as my mate runs an automotive rubber moulding company and offered to tool up to make them in Viton. I am just going to live with mine I think from my 11k transmission. As ChrisR says most restored bikes don't do many miles/per year anymore (apart from his!!)

Cheers

Ash

[K2-K6]

That point about perfect bores with hone marks.

My understanding is that it's new hone marks that effectively bed the rings in to get a good seal against compression and oil control and if you rub your fingernail over them they feel slightly abrasive. If they've lost this then you may end up with a less than ideal seal if using new rings.

If the originals are good and matched to the piston/bore on dis-assembly they should be ok to go again.

If not then a clean-up hone should be used for new rings as I understand it, what do others feel?

[UKROBK7]

I spoke to a specialist reborer about a clean up hone and new rings which I have always had done.  He said that in his experience on most air cooled engines he found that because most bores went oval and tapered it was better to leave well alone so long as the rings met specification, he found that honing the bores usually worsened the problem of getting a good seal.  I haven't measured my rings yet so as the original compression was good (the variation across the cylinders I now put down to having worn valve faces on some valves, seats are OK) I may put the old rings back in if they are OK.  I think this is one of those things that there will be a difference of opinion on!

Ash, good point about the rubbers hardening, I never thought about that.  You are right, I won't be doing many miles after rebuilding this, I will have to live with the hardening of the rubbers unless something comes up before I get started an assembly

Rob

[K2-K6]

It's interesting to get a perspective from somebody that is working on them all the time and certainly gives you more to make a decision on.

I guess when you're talking to someone using proper engineering equipment that they can see and measure to an accuracy beyond normally equiped mechanics workshops. He is correct in that the bores ultimately go oval in the plane of the con-rod as the piston really has no thrust imposed on the sides of the bore, also the effect of any unburnt fuel is to reduce cylinder wall lubriction more at the top of the barrel so giving a taper-type wear pattern.

I would assume that if you can see the hone marks evenly all round the cylinder wall then that proves you have low wear so far in it's life, which also adds up as the K7 has accelerator pumped carbs (I think) which allow them to run generally leaner and leave less unburnt fuel for washing the oil off the bores.

Sounds like it's in reasonable order.