Author Topic: '73 cb500 top end rebuild (Read 2482 times)

Schmurda · « **on:** May 05, 2015, 05:41:57 PM »

I had low compression across all four cylinders. After pouring a little oil in the spark plug hole and retesting, compression jumped way up. I've taken off the whole top end down to the crank case. The Pistons have quite a bit of carbon build up but they appear to be in good shape. The cylinders look good too. My plan is to replace the rings, gaskets, and lap the valves. Anything else I should be considering?

wowbagger · « **Reply #1 on:** May 05, 2015, 06:06:50 PM »

Check your cam chain length to see if it needs to be replaced.....or just replace it. I think it's like $30. Hone the cylinders and replace the valve seals while you're in there. While you have the top end off take a look down in the crankcase and see if the primary chain has been making contact with the oil galley on the lower case half. When the primary chain stretches that's its first point of contact. If it is then you might as well crack the case and replace the primary chain as well.

Schmurda · « **Reply #2 on:** May 05, 2015, 07:00:37 PM »

Thanks for the suggestions. I guess my real question is (since I'm ignorant about all of this) what else might I have to do to fix my compression problem.

wowbagger · « **Reply #3 on:** May 05, 2015, 07:04:27 PM »

Ah, measure the ring gap before just replacing them. The cylinder walls may have worn to the point that you need oversize rings.

calj737 · « **Reply #4 on:** May 05, 2015, 07:04:43 PM »

What you listed should easily solve compression problems if the cylinder walls measure within spec. If not, replacing pistons and rings after an over bore is required. Have a machinist measure the bores and pistons for best results.

HondaMan · « **Reply #5 on:** May 05, 2015, 07:07:53 PM »

You can 'sneak by' for a while with just new rings, but the bores are not round anymore, which is why your compression was failing. This is very common on these SOHC4 engines. Now that the metals have heat-cured, if you were to bore it at least a step size then it will stay round for many thousands of miles, and it will also give you back about 10% more HP than it had before teardown. Even a .010" (0.25mm) step bore is enough to accomplish this feat.

DON'T be tempted to replace the cam chain with a riveted one. Use an endless one if you replace it. However...most of the 500/550 engines that were not raced can get in excess of 80k miles from their lightly-loaded cam chain, so I'd suggest only replacing it if it is in bad shape or rusted, as you must split the bottom cases to replace it.

Schmurda · « **Reply #6 on:** May 05, 2015, 08:04:29 PM »

The cam chain looks great so I'll leave it alone. If I bore the cylinders will I end up needing overbore pistons or just new rings?

Also, being a neophyte in this world it might be more common than I know but I feel really cool having HondaMan reply to my thread.

That 70s Bike · « **Reply #7 on:** May 05, 2015, 10:09:08 PM »

Quote from: Schmurda on May 05, 2015, 08:04:29 PM

The cam chain looks great so I'll leave it alone. If I bore the cylinders will I end up needing overbore pistons or just new rings?

Also, being a neophyte in this world it might be more common than I know but I feel really cool having HondaMan reply to my thread.

Yes, the cyl's would be bored to the next piston oversize which I believe is what was given: .010" or .25mm
I think the machinist would also need the new pistons to match them to each bore. New rings also need to be filed (gapped) to each cyl.

Schmurda · « **Reply #8 on:** May 06, 2015, 05:05:59 AM »

Any suggestions where I should get new pistons?

calj737 · « **Reply #9 on:** May 06, 2015, 05:11:19 AM »

If it is determined you need new piston and ring sets they can be had through dealerships or CruzinImage via eBay has some decent values. You may find going to a larger bore cheaper by using stock 550 dimensions as the cost to bore 1 over is basically the same as a 58.5 bore.

But let a machinist measure the bores and piston to determine what's required. Then proceed-

HondaMan · « **Reply #10 on:** May 07, 2015, 04:06:42 PM »

One word of advice to remember clearly when taking these pistons to a machinist: if they are CAST pistons (as opposed to FORGED pistons), be DEAD SURE to tel them you want .0010" clearance (that's 'one thousandth of an inch'), NO MORE. Many, if not most, auto machine shops bore much more clearance than that, and you will shortly end up right where you are now if they do that to your new bores.

-Most pistons are cast pistons, unless otherwise specified.

If you end up with FORGED pistons (not always advisable for the neophyte!), then write back and we can talk more about it.

Schmurda · « **Reply #11 on:** May 11, 2015, 05:53:03 PM »

I'm think about getting these:
https://www.davidsilverspares.com/CB500K2-FOUR-1973-USA/part_233695/

My mechanic recommended going with something wider than .25mm because of a ring worn into the cylinder.

calj737 · « **Reply #12 on:** May 11, 2015, 06:17:48 PM »

Those will work well.

SOHC4 Cafe Racer Fan · « **Reply #13 on:** May 11, 2015, 08:20:16 PM »

Those will work fine but why not over bore larger if you are going to do it? It will not decrease reliability and there's no replacement for displacement. Or maybe you are stock-centric.

HondaMan · « **Reply #14 on:** May 11, 2015, 08:26:14 PM »

Quote from: Schmurda on May 11, 2015, 05:53:03 PM

I'm think about getting these:
https://www.davidsilverspares.com/CB500K2-FOUR-1973-USA/part_233695/

My mechanic recommended going with something wider than .25mm because of a ring worn into the cylinder.

Those are excellent! I've used them in the 500 and their brothers in the 550. Their skirts are micro-grooved, which retains just a tiny bit of oil for good cooling and extra power when hot.

Schmurda · « **Reply #15 on:** May 12, 2015, 01:45:20 PM »

Quote from: CB750 Cafe Racer Fan on May 11, 2015, 08:20:16 PM

Those will work fine but why not over bore larger if you are going to do it? It will not decrease reliability and there's no replacement for displacement. Or maybe you are stock-centric.

They also have .75mm and 1mm. I'm not sure what the maximum size I could get would be before I have to start changing something else.

flatlander · « **Reply #16 on:** May 12, 2015, 01:51:41 PM »

up to +1mm is within the measurements of honda oversizes, so nothing to worry about.

HondaMan · « **Reply #17 on:** May 12, 2015, 04:39:40 PM »

With the stock head gaskets, you can reach up to +1.5mm (this is called "S6", or sixth oversize) with a bit of careful work. Beyond that, you will need a 'big bore' head gasket. The "S1" is the +.025mm, "S2" is +0.50mm, etc.

Schmurda · « **Reply #18 on:** June 29, 2015, 07:35:38 PM »

So I put everything back together and...it won't start. My electric start has been dead so I've always used the kickstart. Now when I pump the kickstart the pistons don't move. I can move them manually via the bolt under the points cover. If the bike is in gear, pumping the kickstart will make it move forward. Any ideas?

calj737 · « **Reply #19 on:** June 29, 2015, 08:27:13 PM »

A bit of warming: dont use the bolt under the point cover to rotate your engine unless the plugs are out and you're timing the motor. That bolt is not rated for the strength needed, and you'll shear it.

News:

Author Topic: '73 cb500 top end rebuild (Read 2482 times)