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Discussion Starter #1
More Photos of Top End Re-Teardown here:
As some of you may remember, I have a 1975 Honda CB360T that I tore down to the bare frame for a long restoration project, including a complete rebuild of the top end (pistons, piston rings, gaskets, oil seals, springs, and valves). The bike started up easily at the first start with some fine tuning needed with the carburetors among other gremlins. I rode the bike for about 500 miles so far, which should be enough to break in the pistons. However, since the beginning I have noticed the following symptoms:
  • Compression in both cylinders is reading around 150 psi. Adding oil only bumps up the pressure slightly. Specification for a fresh engine should be 170 psi.
  • The head gasket weeps oil after I ride the bike for more than a couple of miles. It is not a head gasket failure, as that would have puked oil left and right. This is more of a moist layer of oil on the top fin of the cylinders.
  • My left side exhaust muffler puffs white smoke when cranking the throttle.
At this point (500 or so miles), the piston rings should have been broken in, and the compression readings are indicative of borderline "rebuild required" per the Honda Service Manual.
This week, I pulled off the top end down to the pistons and noted the following:
  • Crispy black (oil?) buildup on the valve "hemispheres" on the head. Left side had more buildup than the right
  • Oily head gasket (no rips or tears, so it is intact)
  • Grimey oil buildup on the top of the left side piston
  • Piston rings look intact, nothing cracked or broken
  • Other than the top oily grime, the pistons look pristine, no scratching/wear on the sides
  • Cylinder walls still have the machine shop's cross-hatching
  • When I place the cylinder and head on a thick flat surface (countertop) and wiggle the sides, I notice a slight wobble/rocking, so the mating surfaces may not be 100% flat.
Here are the considerations that I have in mind:
  1. The machine shop that did my cylinders found that my bore was out of round and had to overbore to 0.25 per the manual specs. To my dismay, they did not send me any measurements of the cylinders to confirm that they did the job to spec. I also am kicking myself now, because I didn't do a piston ring clearance check to verify. I am going to perform a ring clearance check with a spare set of 0.25 rings that I will have on hand soon.
  2. I didn't "deck" the head and cylinder completely flat, which could account for the head gasket oil leakage and may contribute to some loss of compression?
  3. The machine shop may have done the cross-hatching too finely, which will seriously impact the seating of the piston rings. I may need to re-do the cross-hatching myself, as I don't trust the machine shop anymore. Does anyone have a link to a product that will work for the CB360? I know CMC has one for a chunk of change, but it is out of stock apparently.
  4. Honda Twins memeber TOOLS1 rebuilt my head with fresh valve stem seals and new valves. I guess I can perform an acetone test at the intake and exhaust ports to see if there is any leakage at the valve seating surfaces.
If you guys have any other tips/advice, let me know what else I should be checking. NOTE: I don't have an air compressor for a leakdown test, nor do I have a bore gauge.
 

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Discussion Starter #2
One other thought. Is it possible that the valve guides are in need of replacement? I know when TOOLS1 rebuilt my head, I didn't provide replacement guides, as I assumed they would be fine.
 

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Discussion Starter #3
I popped the existing 0.25 OS pistons off and I did see a tiny bit of scuffing on the skirts, but nothing too alarming. Correspondingly, the same sections of the cylinder wall showed rubbing off of the cross-hatching.
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Existing 0.25 OS piston rings have been removed and they look to be in pretty darn good shape. I did manage to score a spare set of new ones on eBay "just in case". I am going to perform a ring gap check this holiday weekend (once I can get some undivided attention to focus on the measurements) to backcheck the machine shop's work. TOOLS1 also gave me a good suggestion to check the piston to cylinder clearance with a 0.002" feeler gauge.

I placed the head on a machined flat surface (as you can see in the picture above), and it does have a tiny bit of "wobble", and will need to decked flat. I looked at the mating surface between the head and the flat surface at eye level and I was able to see light peeking through at certain spots, so I am going to address that with the "decking".

I have a 67mm silicone carbide 240 grit flex ball hone coming in the mail by end of the week to re-hone the cylinders. I was a bit unsure about what size to get, as the spec for a CB360 cylinder (OS) is 67.26mm, but from the product page, the hone is oversized for the intended bore, so I should be good.

What I am not looking forward to is the cleanup of Threebond and old gasket material on mating surfaces. I have my can of brake cleaner and rags at the ready.
 

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If you have 150 PSI on both cylinders I would suspect that the compression gauge is too large for an accurate reading on your small cylinders. I always check the valve guides for wear and the surface for flatness when doing heads. To measure your head surface you can use a straightedge ruler like the ones on a tri square and a feeler gauge. I always recommend resurfacing if I can get a 0.003" feeler gauge under the straightedge at any point. What torque wrench did you use to tighten the head bolts down with?
As for the cylinder honing. The new automobile engines use a very fine crosshatch, these old motorcycle engines with castiron piston rings need a pretty coarse crosshatch to seal. The flex hone Harbor Freight sells for $20.00 is great for doing these cylinders. I used to use one on my cylinders when I would get them back from a machine shop before I got into the business myself.
Oil on the top fin? Is this the top fin on the head or the cylinder?
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Discussion Starter #5
Hey James! I will address your questions one by one:

If you have 150 PSI on both cylinders I would suspect that the compression gauge is too large for an accurate reading on your small cylinders.
That may be the case? The compression gauge that I am using is a Carftsman set that I purchased from Sears back when they were "a thing".
I take it I should look for one specifically for motorcycles?

I always check the valve guides for wear and the surface for flatness when doing heads. To measure your head surface you can use a straightedge ruler like the ones on a tri square and a feeler gauge. I always recommend resurfacing if I can get a 0.003" feeler gauge under the straightedge at any point.
At this point I have already decked the heads flat using the CMC tutorial. I just verified that I can't fit a 0.003" feeler gauge anywhere.

What torque wrench did you use to tighten the head bolts down with?
I used a 3/8" drive Husky torque wrench. I used the star/cross pattern specified in the HSM, tightening a bit in a repeating fashion over multiple rotations until I achieved torque specifications in the HSM (21.7 - 24.6 ft-lb). For smaller torques, I used a micro-torque wrench, as that was under the torque range of the other wrench.

As for the cylinder honing. The new automobile engines use a very fine crosshatch, these old motorcycle engines with castiron piston rings need a pretty coarse crosshatch to seal. The flex hone Harbor Freight sells for $20.00 is great for doing these cylinders. I used to use one on my cylinders when I would get them back from a machine shop before I got into the business myself.
It may be that the machine shop made their crosshatches too fine. I intend to do a better job myself.

Oil on the top fin? Is this the top fin on the head or the cylinder?
Sorry, I should have specified. Oil was on the top fin of the cylinder. It was basically a thin layer of oil. No spurt/gushing, etc.

FYI: Condition of the cylinders (showing some light scuffing):
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With your help I went to measure the piston skirt (front/back) to wall clearance. I was not able to squeeze in a 0.002" feeler gauge between the cylinder wall and the front/back of the piston. Unfortunately that is the smallest feeler gauge I had. I wasn't able to find a set with a smaller one at an auto parts place. I may need to get a special order one from McMaster Carr or Grainger. I know that 0.002" is the typical clearance required, as smaller clearances are not recommended due to the thermal expansion under operation, correct?
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Put the feeler gauge into the cylinder and then put the piston in. This will make it a lot easier to do it. That scuffing looks like it might not have enough clearance.
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Discussion Starter #7
Put the feeler gauge into the cylinder and then put the piston in. This will make it a lot easier to do it. That scuffing looks like it might not have enough clearance.
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That is what I think may be the case. If the 0.002" feeler gauge is placed in the cylinder first followed by insertion of the piston, the piston slides in the cylinder up to the point where the piston skirt hits the feeler gauge and stops, held in place by friction. Is it worth trying to re-hone it to see if that buys any clearance?
 

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Its already run 500 miles with no signs of scuffing on the piston skirts, looks as if the skirt clearance is ok.
 

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Eh....there was some scuffing on the piston skirts and the cylinders. I sent the cylinders and the pistons to TOOLS1 to take a look. I don't trust that local machine shop any more.
 

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Looks more like dirt left on the bores during assembly than actual scuffing from lack of clearance.
 

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Did the machine shop tell you that you needed to wash the cylinders with hot soapy water?
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Yes they did. I had the cylinders in a hot soapy bath and scrubbed with a shop rag. After drying and oiling to prevent flash rust, I began the piston and cylinder installation process.
 

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I did not "document" every little step of the way, but this video generally shows what I have done with the engine.
 

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While I have everything apart and the cylinders/pistons are with TOOLS1, I took a look at the exhaust valves and ports. The bike ran on the rich side, despite having everything set to stock settings (stock airbox, stock mufflers, stock carbs, mix of aftermarket and stock jets).

The right side was carbon-kissed and looked normal, while the left side had more grimey buildup of carbon with what appears to be oil (shiny black blotches). I used a pick to scrape off a few samples and determined that there were no metallic particles (whew) and crumbled between my fingers into fine black powder.

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I picked up an appropriate valve spring compressor and investigated the valve steam seals and the valve seats. The valve stem seals looked to be perfectly fine, but I replaced them anyway to be safe.

The exhaust valve seating surfaces still looked decent from TOOLS1's work on the head, but had some carbon buildup on the seating surfaces that should be cleaned off now that I have the head apart. I lightly lapped the valves just to clean off the buildup and performed a few leak tests to confirm. With no valve springs installed, both sealed up well from the acetone, with only a slight bit of weep. With the valve springs installed, I had no leakage over 2 hours of sitting. I also used compressed air to blow into the ports to verify that no bubbles were fizzing out into the flooded combustion chambers.

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PROTIP: Don't paint your head with aluminum engine paint before you do this acetone test. Any drips/leakage will instantly alter/marr the paint. You can see it in the top left corner of the above photo!

Additionally, I went to check out the valve guides. I recall reading on this forum about a "rule of thumb" method for checking the valve guide runout, where you pull out the valve so that the valve face is about an inch from the seating surface and wiggle it around to see if there is a perceptible movement. The right side (which had normal looking wear/buildup) had none, but the left (with the worse looking buildup) had a barely noticeable wiggle. When TOOLS1 rebuilt this head 2-3 years ago, his inspection determined that everything was in good shape and no need to replace the valve guides, so I am assuming it is OK? I wouldn't think that 400-500 miles on the rebuild would be enough to wear out the guides. Here is a video of the left side exhaust valve with a slight wiggle. Let me know what you think.

 

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You do the "wiggle" test at 3/8" If the buildup crumbled to a fine powder, it would be from running rich on that side. And you might not believe this, but you can be rich and lean at the same time. Usually, this is caused by over carburation and a lot of hotrodders have done this on their car engines by installing that big Holley double pumper carb on their small block Chevy, and most recently by guys installing the Mikuni VM 32-34 carbs on their 350 Hondas. I am betting that you have a vacuum leak and some sort of seal problem in that carb.
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Discussion Starter #17 (Edited)
You do the "wiggle" test at 3/8" If the buildup crumbled to a fine powder, it would be from running rich on that side. And you might not believe this, but you can be rich and lean at the same time. Usually, this is caused by over carburation and a lot of hotrodders have done this on their car engines by installing that big Holley double pumper carb on their small block Chevy, and most recently by guys installing the Mikuni VM 32-34 carbs on their 350 Hondas. I am betting that you have a vacuum leak and some sort of seal problem in that carb.
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Thanks! I am currently doing a disassembly and deep clean of the CB360 carbs and right now and will be double checking everything. I was scratching my head about that earlier because I am running stock exhaust, stock airboxes, stock carbs, a mix of stock and aftermarket jets (came with the set of CB360 carbs I have).
 

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Discussion Starter #18
Quick update: Picked up a set of bore gauges and measured the valve stem diameter and the bore of the valve guide to determine the valve guide clearance. All values were well within HSM specs, so I can rule that out. Somehow it always seems to be a "carb issue" :D
 

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Well, imagine that. I did my job correctly. Who would have thought?
I just checked out your cylinders and pistons. Your machine shop bored and honed the cylinders to an EXACT 0.002" clearance. However, I can see the eyebrows and remnants of the ridge at the top of the cylinders. You cannot clean up a cylinder with an overbore of only 0.25 mm. I will not even try to do a 0.25 mm overbore.
The scratches are from some sort of foreign matter in the oil or combustion chamber. I still think you have carb problems and maybe ignition problems as well.
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Well, imagine that. I did my job correctly. Who would have thought?
:ROFLMAO:

I can see the eyebrows and remnants of the ridge at the top of the cylinders. You cannot clean up a cylinder with an overbore of only 0.25 mm. I will not even try to do a 0.25 mm overbore.
Forgive my ignorance, but what does this mean?

The scratches are from some sort of foreign matter in the oil or combustion chamber. I still think you have carb problems and maybe ignition problems as well.
Yikes. Looks like I am going to have to drain the oil and pop off the engine side covers to see if I can find any culprits in the oil or oil filter.
 
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