Topic: New Video - (Not A) Botched Bore Job?

[Imago]

I've recently had my '76 CB750K cylinders bored, (the cylinders were rusty from many years of neglect) to accommodate the "Cruizinimage" 836 pistons, but when I picked them up the fellow at the engine shop mentioned that they had put a taper in them to make it easier to install the pistons.

I'm worried now that they may have botched the job. I understand car engines can tolerate a taper, at their lower rpm, but motorcycle engines can not. How do I go about measuring these properly to determine what they've done?

I should mention I asked them to bore the cylinders with .0010 clearance on the outside two and .0012 on the inside two on information I received here.

Hondaman has already provided some great advice, if I was closer I'd certainly head on over... hell, I may anyway, he's only 12 hours straight south!

Is there a gauge I can buy or borrow?

[ekpent]

 Did they just put a little taper on the very bottom of the sleeves so the rings will slide in easier ? That would be a nice touch probably.

[martin99]

Are those the exact measurements you gave them?  One thou seems very tight to me, a taper could prove quite helpful on install. Have you checked piston to bore clearance?

[disco]

Are you sure that he doesn't mean that they put a 'lead-in' on the bottom of the bores to make the pistons/rings easier to slide in. It's quite common to put a chamfer or 'lead-in' in this location. I just had my barrels bored for the same CI 836cc pistons and so glad my machinist put a good chamfer on bottom of barrels. It made installation relatively easy. Still slow but would have been a nightmare with no chamfer.

Btw my machinist set mine up at 0.03mm (0.0012 in) He does these all the time & said that was a tight clearance I asked for  and I would have to be very careful with break-in over first 500-600 miles. It is much, much tighter than my K6 engine with 50,000 km on it. Turning over the engine (with no plugs in) is tight.

Sorry just saw Eka said essentially same thing. Great minds think alike 

[martin99]

I must admit I've never had a set of CB barrels bored, just going by experience of old Brits where the clearance is commonly around .004 or .005. Always learning.

[markreimer]

Are you sure that he doesn't mean that they put a 'lead-in' on the bottom of the bores to make the pistons/rings easier to slide in. It's quite common to put a chamfer or 'lead-in' in this location. I just had my barrels bored for the same CI 836cc pistons and so glad my machinist put a good chamfer on bottom of barrels. It made installation relatively easy. Still slow but would have been a nightmare with no chamfer.

Btw my machinist set mine up at 0.03mm (0.0012 in) He does these all the time & said that was a tight clearance I asked for  and I would have to be very careful with break-in over first 500-600 miles. It is much, much tighter than my K6 engine with 50,000 km on it. Turning over the engine (with no plugs in) is tight.

Sorry just saw Eka said essentially same thing. Great minds think alike 

How are you breaking that engine in? I had my 750 bored for second oversize to the same spec. I've got about 300km on it so far. No smoking, good power so far. I've been avoiding redline, keeping the Rpm's changing.

Also curious if the original posters barrels were tapered the whole way (??) or just at the bottom for install. I've never needed to use ring compressors on my cb's because of that nice tapered entry.


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[754]

I am sure he meant the bottom, he did say TO INSTALL PISTONS.
KMS had cheap bore guages last month 60 bux, A buddy bought one, we will try it out soon.
 Should be good enough to check for taper and out of round.
 How do your pistons feel in the bore, should be barely any play.

[MRieck]

Install pistons in sleeves with no taper cut....you'll love it.

[05c50]

I've recently had my '76 CB750K cylinders bored, (the cylinders were rusty from many years of neglect) to accommodate the "Cruizinimage" 836 pistons, but when I picked them up the fellow at the engine shop mentioned that they had put a taper in them to make it easier to install the pistons.

I'm worried now that they may have botched the job. I understand car engines can tolerate a taper, at their lower rpm, but motorcycle engines can not. How do I go about measuring these properly to determine what they've done?
Is there a gauge I can buy or borrow?

Are you sure they put a taper in the cylinders, or like others have suggested, they chamfered the bottom edge to help in piston installation? I just can't figure how tapering the cylinders would help.

You should be able to find another machine shop that could gauge the cylinders for you for a minimal cost. I doesn't necessarily have to be an automotive machine shop. You could probably get a pretty good idea if the cylinders are tapered by checking piston to bore clearance using feeler gauges both top and bottom. feeler gauges are cheap.

...Paul

[754]

You wont get a feeler guage in if its done to what they said.

[05c50]

You wont get a feeler guage in if its done to what they said.

I thought about that, but if the cylinder is tapered like he said, there might be enough clearance to get a gauge in the larger end.

...Paul

[Imago]

I am sure he meant the bottom, he did say TO INSTALL PISTONS.
KMS had cheap bore guages last month 60 bux, A buddy bought one, we will try it out soon.
 Should be good enough to check for taper and out of round.
 How do your pistons feel in the bore, should be barely any play.

Hmmm, time to check again... you may be right about it being tapered only on the bottom to aid installation.
Stay tuned... maybe I'll take a video for your viewing pleasure.

[Don R]

 It would be silly for a pro to taper an entire bore, not to mention difficult. I'm sure he meant at the bottom, my machine guy said and did that for mine.

[Imago]

Well, I'm feeling a bit sheepish... I think my engine shop has done exactly what some have suggested and put a taper at one end to simply aid in installation. I've taken a short video to illustrate. It seems they also did exactly what I requested in setting the outer 2 cylinder sleeve clearances at .00010 and the inner 2 at .00012, you can see the inner 2 cylinder sleeves are very slightly larger than the outer two. To 754's point, I doubt that I'd get a feeler gauge in beside any of them.

Learning, learning...

[MRieck]

As a side note I highly recommend decking a block...there are low spots all over the place in general. Not so much a concern with a composite gasket but no go when using an MLS head gasket.

[Imago]

As a side note I highly recommend decking a block...there are low spots all over the place in general. Not so much a concern with a composite gasket but no go when using an MLS head gasket.

Great minds think alike... see my bill, attached. They did the head too. $534.46 CDN is about $407.00 US. Not too bad for the work done?

My 836 head gasket is a "Cometic".

[05c50]

The prices seem pretty fair to me.......except 50 bucks for the taxes.

......Paul

[Imago]

The prices seem pretty fair to me.......except 50 bucks for the taxes.

......Paul

It's that darn free health care... 

[disco]

Think you have too many zeros in those numbers you're quoting.
.0010 & .0012 would be more like it.
I would be checking the piston clearance they seem to slide in almost too easily.

[Imago]

Think you have too many zeros in those numbers you're quoting.
.0010 & .0012 would be more like it.
I would be checking the piston clearance they seem to slide in almost too easily.

Yes, you're right... I checked my email to the shop and the numbers are .0010 and .0012.

When I first tried putting them in, they would hardly fit and seemed to get hung up easily especially when putting them in from the top. I oiled up the piston and put it in each sleeve about six times or so before I took that video. They are tight, I just got good at inserting the piston after it was lubricated. I think I'm good to go... we shall see. That first fire up will be a real nail biter.

[Terry in Australia]

Are those the exact measurements you gave them?  One thou seems very tight to me, a taper could prove quite helpful on install. Have you checked piston to bore clearance?

I agree Martin, tight clearances are fine for a modern water cooled engine, not so much for air cooled bikes like ours. I won't bore a cylinder with less that 1.5 thou clearance. Even then, a piston needs to be pushed thru a cylinder, it shouldn't slide through easily with any hint of "rocking" in the bore.

[drumstyx]

Are those the exact measurements you gave them?  One thou seems very tight to me, a taper could prove quite helpful on install. Have you checked piston to bore clearance?

I agree Martin, tight clearances are fine for a modern water cooled engine, not so much for air cooled bikes like ours. I won't bore a cylinder with less that 1.5 thou clearance. Even then, a piston needs to be pushed thru a cylinder, it shouldn't slide through easily with any hint of "rocking" in the bore.

With cast Pistons, the general recommendation around here is just a hair under 1 thou. Hondaman's book says between .0008 and .0010. I had my machinist do it to .0009...Seems to turn over and run just fine, but I guess we'll see

[rotortiller]

I won't bore a cylinder with less that 1.5 thou clearance.

I agree 100%. I always shoot for .002"or just under. Never had one seize or burn oil. These engines tend to run hot and the slightly larger clearance gives them a  better chance.

rt

[Terry in Australia]

I won't bore a cylinder with less that 1.5 thou clearance.

I agree 100%. I always shoot for .002"or just under. Never had one seize or burn oil. These engines tend to run hot and the slightly larger clearance gives them a  better chance.

rt

Yeah mate, if anyone thinks that a clearance of 2 thou is gonna turn their bike into a smoky mess, then they don't want to buy a Wiseco big bore kit, that's their recommended clearance. Once you bore a CB750 from 61mm to 65mm to accommodate 836cc pistons you're left with cylinder walls 2mm thick, so the last thing you want is super tight piston to cylinder clearances. Cheers, Terry.

[drumstyx]

I won't bore a cylinder with less that 1.5 thou clearance.

I agree 100%. I always shoot for .002"or just under. Never had one seize or burn oil. These engines tend to run hot and the slightly larger clearance gives them a  better chance.

rt

Yeah mate, if anyone thinks that a clearance of 2 thou is gonna turn their bike into a smoky mess, then they don't want to buy a Wiseco big bore kit, that's their recommended clearance. Once you bore a CB750 from 61mm to 65mm to accommodate 836cc pistons you're left with cylinder walls 2mm thick, so the last thing you want is super tight piston to cylinder clearances. Cheers, Terry.

Wisecos are forged and require larger clearances than stock or other cast pistons

[bochnak]

How do you even bore a taper? Most boring machines I have seen has a fixed single cutter that rotates and translates, providing a straight cut.

[754]

An automotive boring bar should stay straight.
 On a lathe you can bore a taper, a few ways.
But in fitting a piston, the boring is the first step, then you finish hone.. You can create a small taper when honing...either on purpose, or by accident if you are sloppy.
 
 Forged pistons are usually set up looser, they have different expansion rates.
The only reason to set up anything loose, it to be able to race without breakin...or maybe because you cant measure accurately.