B7100 Piston Installation

[Schmotown]

Thanks, I'll follow your advice on break-in procedure.

 

[AOW162435]

I am rebuilding the engine in my B7100. I know that the connecting rod is directional when installing but I do not see any markings on the new (or old) Kubota pistons. Attached is a picture of the bottom of the new piston which does have a number cast in one side and the opposite side is blank. Are they directional? Thank you in advance.

Following this thread. Unless I missed another thread that you started, what prompted you to rebuild?
I purchased a very nice ~900 hour '79 B7100 about two years ago. After thoroughly going through it, cleaning it, and refreshing a number of aesthetic things, I put it to work pulling a finish mower. During the first summer the tractor performed perfectly - no issues starting, no smoke or blowby. The next summer was fine until about midway, and then I started noticing blowby. This noticeably increased until the mowing season ended and I parked it. Somewhat dejected. The blowby was visible right after starting it, and had significant pressure when covering the tube with a finger.

I finally dug into this Spring, adjusted the valves, disassembled & cleaned the breather screen, and did a compression check (using the glow plug bores). The numbers were a very disappointing 340psi, 180psi, and 190psi.

For now the tractor is parked while I decide what to do. It's far too nice to put up for sale. I grew up with an identical 1980 model, so I'd like to eventually repair this correctly and continue to enjoy it.

Andreas

 

[Selkirk_D]

On my L2850 I found that the low compression was caused by carbon build up on the exhaust valve seats. I lapped in some new exhaust valves and it’s great again.

 

[Selkirk_D]

Wallowed shaft due to missing/ bad key. If there was still a decent fit, a new key and some green loctite sleeve and bearing retainer would make it stay on permanently.

 

[Schmotown]

I bought a "well used" B7100 for a project that smoked terribly upon start-up but mostly cleared up a few minutes after running. Before rebuilding the top end and rod bearings (no carbon build-up) my compression readings were 240, 200, 220. Pitting on the head and valves (see previous posts) from what I presume was a leaking head gasket prompted me to buy a Kumar Bros. head. When I start it up I'll report back.

 

[Schmotown]

I started my B7100 a few days ago and it smoked a ton of smoke with a heavy diesel smell. I expected this after a rebuild of the top end and it cleared up by 70% but still continued to smoke. I had the fan and radiator connected, looked for bubbles in the radiator that would indicate a leaking head gasket but no bubbles. There was no blow by at the breather.

As the tractor is still split I couldn't run it under load to seat the rings so I shut it down after a few minutes. There was also a "knock" that I didn't like but as the motor has new rod bearings, piston pins and a new head I'm wondering if the knock could be fuel related. As this tractor (and diesel engines) is new to me I think I will do the following to remove any questions if the knocking sound and the smoking are actually fuel issues:

1) Replace the injectors with rebuilts. from Oregon Fuel Injection
2) Replace one bad glow plug.
3) Check the injection pump timing.

I tend to overthink things. Maybe I should reassemble the tractor and just run it to see what happens.

Thank you in advance for your comments.

 

[North Idaho Wolfman]

Take a video of it starting and of it running.
Post the video on YouTube and give us a link to see it.

 

[Schmotown]

Okay, will do.

 

[Schmotown]

Here's the link to my start up: https://www.youtube.com/shorts/_DbDjDRl65U

 

[#40Fan]

I would start with a compression test. From the sound of it cranking in your video, one cylinder doesn't seem like the others.

 

[Schmotown]

After replacing the head and replacing the pistons, rings, wrist pins and rod bearings and before attempting to start I did a compression test on all 3 cylinders with the following results:
Cyl #1: Before rebuild 240 psi. After rebuild 280 psi. 300 psi with a bit of oil.
Cyl #2: Before rebuild 200 psi. After rebuild 291 psi.
Cyl #3: Before rebuild 220 psi. After rebuild 265 psi.

I then did a leakdown test and all 3 cylinders show less than 10% leakage which I believe verifies the tightness of the engine.

 

[RBsingl]

That combined with the smoke sounds like injector knock to me from an injector that is just dumping fuel into the combustion chamber. I think you are massively overfueling one cylinder from a bad injector.

Rodger

 

[Schmotown]

Thank you Rodger. I think I'll just replace all of them with new OEM injectors and give it another try. I'll report back when I get the new injectors.

 

[North Idaho Wolfman]

If they are Oregon fuel injection injectors, it's not an injector issue.
And you can not get OEM injectors for this motor, they don't exist.

You have a compression issue!
You should be closer to 450 psi and 325 PSI minimum.
Way too low of compression for it to start and run properly.

I would be pulling the front of the engine and verifying the timing is correct.
If it is then you just have to rebuild it completely to get it within specs.

One other test is the fuse test, to verify the head to piston clearance.
The procedure is in the WSM.

 

[Schmotown]

If they are Oregon fuel injection injectors, it's not an injector issue.
And you can not get OEM injectors for this motor, they don't exist.

You have a compression issue!
You should be closer to 450 psi and 325 PSI minimum.
Way too low of compression for it to start and run properly.

I would be pulling the front of the engine and verifying the timing is correct.
If it is then you just have to rebuild it completely to get it within specs.

One other test is the fuse test, to verify the head to piston clearance.
The procedure is in the WSM.

The injectors are not from Oregon Fuel Injection (OFI), they came with the tractor. According to OFI the injectors in my tractor have a Denso part number. I think it may be a good idea to get new injectors from OFI just to eliminate them as a potential problem.

I will pull the front gear cover and check the timing, I'll also do the fuse test with some fine soldering wire and report back.

Thanks to all that have replied to this thread.

 

[North Idaho Wolfman]

The injectors are not from Oregon Fuel Injection (OFI), they came with the tractor. According to OFI the injectors in my tractor have a Denso part number. I think it may be a good idea to get new injectors from OFI just to eliminate them as a potential problem.

I will pull the front gear cover and check the timing, I'll also do the fuse test with some fine soldering wire and report back.

Thanks to all that have replied to this thread.

OK I misunderstood a few posts back when you said replace injectors.
I still think you will have issues due to low compression.
Anything under 325 is going to have a hard time starting and running.

 

[Schmotown]

Agreed. Compression is way too low. When I installed new pistons, rings, etc. and found the cylinders and crank bearings to be right on specification I thought my compression issues were solved. After discovering low compression after the rebuild I did a leak down test on all (3) cylinders and they were all below 10% loss. I "think" the engine rebuild is in good shape.

A few weeks ago I tested the head to piston clearance with the piece of solder and I recall that there was too much clearance between top of piston and bottom of combustion chamber but I can't find my notes on the measured thickness of the fuse tests. I'll do it again today. As the pistons had the specified stick out above the top of the block and the head gasket is a Kubota gasket the only loss of compression that I can think of is the Kumar Bros. head. I even sprayed soapy water on the injector bases and didn't find any leaks around them.

I think if the "fuse" test is not within spec maybe I could have the head milled to bring it back into specifications. Does that sound reasonable?

My other issue is the knock that can be heard in my video. RBsingl (above) thinks it may be caused by a bad injector. I ordered (3) new injectors from Oregon Fuel Injection to ensure that the injectors are operating properly.

I am so far unable to remove the pulley and a splined gear on the end of the crankshaft to confirm gear timing. I didn't want to destroy the front crank seal if I couldn't remove the pulley so I heated the pulley and gear as much as I dared and with a suitably large puller and hammer it didn't budge.

Wolfman, you suggested that I pull the front cover to ensure all the gears are timed and I can probably accomplish that if need be with some serious heat and plastic explosives. Here's my question:

If the engine timing is correct (valves, pistons, crank) as would be done with a rebuilt gas engine wouldn't that eliminate the question of compression being low due to the injector pump being out of time? I'm not sure how those components all interact with each other. I do understand that their timing is critical for running the motor but are they critical while I'm chasing a compression issue?

Hope this post makes sense. Thanks in advance, I'm learning a ton, even at 77.

 

[Duck Wrangler]

Wolfman, you suggested that I pull the front cover to ensure all the gears are timed and I can probably accomplish that if need be with some serious heat and plastic explosives. Here's my question:

Did you take your cam opening/closing measurements at the valve? Was the valve clearance set correctly - wherever you got the desired valve timings should have given the clearance, which isn't necessarily the same as what's used when running. If you had taken your readings with more clearance than the manual calls for then you'd see the valves opening late and closing early. That said, I'd expect the valve timing numbers to be quoted with the valve clearance set to something huge like 0.040 (1mm), which would give the opposite of what you're seeing. I'll also give a disclaimer than I don't know anything about this specific engine.

Finding TDC can sometimes be tricky, there's usually a few degrees of slop. If you pick an arbitrary number - say 0.250" - and measure 0.250 BDTC, mark it on your degree wheel, measure 0.250 ATDC, mark that on your degree wheel. TDC is exactly halfway between those two points. The few degrees of slop you might have when measuring when the piston gets to the top of the bore isn't going to be a deal breaker in this case, though.

 

[North Idaho Wolfman]

Agreed. Compression is way too low. When I installed new pistons, rings, etc. and found the cylinders and crank bearings to be right on specification I thought my compression issues were solved. After discovering low compression after the rebuild I did a leak down test on all (3) cylinders and they were all below 10% loss. I "think" the engine rebuild is in good shape.

A few weeks ago I tested the head to piston clearance with the piece of solder and I recall that there was too much clearance between top of piston and bottom of combustion chamber but I can't find my notes on the measured thickness of the fuse tests. I'll do it again today. As the pistons had the specified stick out above the top of the block and the head gasket is a Kubota gasket the only loss of compression that I can think of is the Kumar Bros. head. I even sprayed soapy water on the injector bases and didn't find any leaks around them.

I think if the "fuse" test is not within spec maybe I could have the head milled to bring it back into specifications. Does that sound reasonable?

My other issue is the knock that can be heard in my video. RBsingl (above) thinks it may be caused by a bad injector. I ordered (3) new injectors from Oregon Fuel Injection to ensure that the injectors are operating properly.

I am so far unable to remove the pulley and a splined gear on the end of the crankshaft to confirm gear timing. I didn't want to destroy the front crank seal if I couldn't remove the pulley so I heated the pulley and gear as much as I dared and with a suitably large puller and hammer it didn't budge.

Wolfman, you suggested that I pull the front cover to ensure all the gears are timed and I can probably accomplish that if need be with some serious heat and plastic explosives. Here's my question:

If the engine timing is correct (valves, pistons, crank) as would be done with a rebuilt gas engine wouldn't that eliminate the question of compression being low due to the injector pump being out of time? I'm not sure how those components all interact with each other. I do understand that their timing is critical for running the motor but are they critical while I'm chasing a compression issue?

Hope this post makes sense. Thanks in advance, I'm learning a ton, even at 77.

If the timing is off then the valves will be off that will lower the compression.
Are you 100% sure you have the valve lash set properly?

I can't see where milling the head is going to help you, it's pretty much flat as it is and the only volumetric issue would be the pre chamber and milling won't change that.

 

[North Idaho Wolfman]

Did you take your cam opening/closing measurements at the valve? Was the valve clearance set correctly - wherever you got the desired valve timings should have given the clearance, which isn't necessarily the same as what's used when running. If you had taken your readings with more clearance than the manual calls for then you'd see the valves opening late and closing early. That said, I'd expect the valve timing numbers to be quoted with the valve clearance set to something huge like 0.040 (1mm), which would give the opposite of what you're seeing. I'll also give a disclaimer than I don't know anything about this specific engine.

Finding TDC can sometimes be tricky, there's usually a few degrees of slop. If you pick an arbitrary number - say 0.250" - and measure 0.250 BDTC, mark it on your degree wheel, measure 0.250 ATDC, mark that on your degree wheel. TDC is exactly halfway between those two points. The few degrees of slop you might have when measuring when the piston gets to the top of the bore isn't going to be a deal breaker in this case, though.

There is almost Zero "Slop" on a Kubota engine.
If you have "slop" you have a serious issue.
Finding TDC on these engine is super easy.
The valve clearance is .008