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type IV compression loss on 3 and 4
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gatorjos
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PostPosted: Thu Jun 07, 2018 8:10 am    Post subject: type IV compression loss on 3 and 4 Reply with quote

My wife and I were cruising up I-5 toward Redding last weekend when she noted that she couldn't get the bus over 50mph. Headwind! I said, and the landscape is sloping while appearing level (a false flat!). She insisted something was wrong, so we pulled and switched places. I noticed the throttle didn't have the right pedal feel, so pulled over, adjusted the throttle cable (red herring #1?) and checked timing which was slightly advanced (9.5 instead of 7.5)

Off we went and the bus did seem to make better power and the gas pedal feel was normal. Then, she slowed down to 50 again. False flats and headwinds were a bit of a stretch at this point, and I could hear a whistling indicative of a header or head leak. The engine was idling great, but when I blipped the throttle in the back, blue smoke would dump out from the head at 3/4. Oh-oh...

Luckily, we were close to a friend's house in Chico so we limped 30 miles to his shop. I checked compression, and sure enough -- 35/40 psi on 3 and 4 while 1 and 2 had 135psi.

I decided to drop the motor and have a look.

It appears the head came loose somehow. A local shop had done a valve and ring job about 6000 miles ago and about 3000 miles in I re-torqued the head on the right side (which clicked off on all nuts), and foolishly I passed on checking the other head. I'll never know if that would have saved it, but oh well, here we are.

A solvent leak test had minor leaking from all valves, but overall things looked quite good. I brought the head back to the guys who did the valve job, and they are step cutting the head, putting in new exhaust valves, and lapping the intakes. The plan is to put the head back on, do some painting on the engine bay, and hopefully have the bus back on the road by the end of this weekend.

No, I did not check the case for pulled studs, that will be this weekend in Chico. I hope this is not the case.

On removing the head, I noticed the arrows on the cylinders point to each other, so either the engine builder or the local shop did not assemble the engine correctly. Question, do I correct this now that it is open, or leave as is? The current rings/hone job has 6000 miles on it, so it seems I might just leave it at this point. I wasn't planning on doing a ring job as well.

Any thoughts from the peanut gallery? I don't do this every day, so much appreciate advice.

The engine has about 45k miles on it from a fresh build with AMC heads.

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SGKent Premium Member
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PostPosted: Thu Jun 07, 2018 8:28 am    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

Sorry about the bad news. The thrust angles, and thrust surfaces are usually different. One side of the piston is usually designed so it is heavier than the other, and offset. If that is reversed then skirt and ring wear can be accelerated. That might be the blue smoke. A loose head normally would not cause blue smoke but worn rings, or a guide would.

The factory manual also states
Quote:
"To keep the necessary clearance from causing excessive rocking (and piston slap noises), the piston pin bores are offset slightly from the piston centerline. This offset, shown in Fig. 13-6, is the reason why the pistons must be installed so that the arrow on their crown points towards the flywheel."

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To correct it, assuming the wear is not excessive, then the mis-installed piston(s) will have to be rotated. If the cylinders come off or the rings pop out you need new rings and a hone on the cylinders. If you are a gambling man, there is a chance you can just rotate the piston in the bore without re-ringing it. It will be touchy because you have to lift the cylinder above the piston pin without pulling it off accidentally. If you re-ring it, do all 4. To be really frank - it is such a rookie move that the rest of the engine is suspect. Normally any engine builder will look at those arrows four or five times during the build to be sure they are in the right direction.
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gatorjos
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PostPosted: Thu Jun 07, 2018 9:04 am    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

SGKent wrote:
To be really frank - it is such a rookie move that the rest of the engine is suspect. Normally any engine builder will look at those arrows four or five times during the build to be sure they are in the right direction.


I was pretty disheartened when I saw that. At this point two people have touched the motor: the original builder and the shop that did the ring and valve job. I'm not sure the shop would have removed the pistons to do the ring job. I would think not, right?

Also, the scratched-in piston numbers are reversed -- 3 is where 4 is supposed to be. If someone was keeping track at some point, then they ended up in opposite places. the number do both face up, meaning the numbers were scratched in when the pistons were already installed incorrectly at that time.

Another note, the shop that did the valve job 6000 miles ago said the engine was built using aluminum pushrods which is incorrect for a hydraulic lifter motor, so they replaced them with steel pushrods. Is that correct?

CSI type IV...
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PostPosted: Thu Jun 07, 2018 9:53 am    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

re pushrod - hydraulic is steel.

re pistons - Usually the piston is matched to the cylinder for clearance reasons. With worn pistons that need is less. As pistons wear there are three areas that come of concern. They can crack with high use. Uncommon but happens. The skirts collapse with use. If the collapse is minor a good machinist can whack them with a rawhide mallet and kinda put them back in shape. If they collapse a lot then they are worn out. Collapse means that the skirts bend inwards. That causes excessive rocking and piston slap. The ring lands wear out. That is common as the pistons age. Once the ring lands wear too much the rings start moving too much within the land. Quite often bargain builders don't check these things. Their approach is more it is what it is. From a quality engine builder's view it is more like putting new tires on a rusty bent rim. It will get you there but there is a difference. Myself I like to just replace the cylinders and pistons. However as parts become more unavailable there will come a day when that is not possible. I think right now I would try to rotate the piston and live with it, or replace the cylinders and pistons rather than pull all four and try to service them again.
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PostPosted: Thu Jun 07, 2018 10:21 am    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

Also...aside from the pistons being installed with the arrows/offset incorrectly....if those piston numbers are "scratched" or scribed into the pistons...find whoever did that and beat them with a large, rusty shovel.

Scratching anything into parts that have that much thermal stress is a great recipe for cracking and erosion at the scratch points. Ray
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PostPosted: Thu Jun 07, 2018 1:33 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

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Yeah, I know this is an elaborate set of torquing instructions, but I have
had zero issues with type IV heads loosening using this paper.
(mid 90's instructions,SIR failed to pay CA. L&I taxes, business closed)
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PostPosted: Thu Jun 07, 2018 2:32 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

Thanks for the replies everyone.

I like those torquing instructions. It makes sense to"sneak" up on the final value and keep working your way around. I do think I will retorque them all sheet some miles just to make sure.

I've been told both by the builder and the rework shop to not install head seals as the AMC heads allegedly don't need them. Does that ring true?

Also, since the head is being step-cut I understand I will need to add a shim at the bottom of the cylinder to offset the cut. I'll get to inspect the rings at that time, I guess. Maybe those are upside down too Rolling Eyes
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PostPosted: Thu Jun 07, 2018 2:55 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

gatorjos wrote:
Thanks for the replies everyone.

I like those torquing instructions. It makes sense to"sneak" up on the final value and keep working your way around. I do think I will retorque them all sheet some miles just to make sure.

I've been told both by the builder and the rework shop to not install head seals as the AMC heads allegedly don't need them. Does that ring true?

Also, since the head is being step-cut I understand I will need to add a shim at the bottom of the cylinder to offset the cut. I'll get to inspect the rings at that time, I guess. Maybe those are upside down too Rolling Eyes



None of the type 4 heads require the head to cylinder shim. Its not just an AMC thing. The factory removed them and had a Technical service bulletin. They should be removed from all type 4 heads. They had the same loosening due to compression issue and burn through with head damage issues on all type 4 models.

They simply addressed the TSB to the type 2 bus...because the 411/412 and 914 cars had been out of production for over 10 years when the TSB came out so were irrelevant to the TSB.

A large part of the torque change over the short time span as noted in that S.I.R. sheet...is because of the compressing of the head gasket. There were a couple of different designs over the years and some were steel and others were aluminum.

On the shim....no...if the head is being "step cut"....it means that they are shaving the quench area....to make a step that approximates the thickness that the original head gasket was...so you should not have to add a shim between cylinder and case.

If you were leaving the head gasket out and not having a step cut done....you WOULD then need a shim between cylinder and engine case to make the compression the same as it was.

Ray
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Tcash
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PostPosted: Thu Jun 07, 2018 3:31 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

Sounds like they are going to fly cut the head and then add the AMC step back into the head.
So yes, you need them to tell you how many thousands they flycut the head and add that amount of cylinder shims to the bottom of the cylinders.
Hell they should provide the shims for you.

Thnx mayor ratwell
Go down to head gasket
http://www.ratwell.com/technical/Heads.html

Good luck
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PostPosted: Thu Jun 07, 2018 3:50 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

A comment on the S.I.R. sheet. Section 4.E. states the 'cut-outs' should face up.
According to Bentley, that is true for Type 1 rocker arms, but the slots should face down on Type 4, with the bevels away from the head as stated.
I have to wonder if the case decks have been checked. I'm not a fan of the hardware on stock AMC heads, especially valve seats..
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PostPosted: Thu Jun 07, 2018 4:57 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

Tcash wrote:
Sounds like they are going to fly cut the head and then add the AMC step back into the head.
So yes, you need them to tell you how many thousands they flycut the head and add that amount of cylinder shims to the bottom of the cylinders.
Hell they should provide the shims for you.
Tcash


Yes, I believe that is what's being done but I'll clarify.

Good call on the Ratwell reading as well -- I'll go through that. thank you.

AMC heads were improved by Headflow masters down in Vista -- new valve seats, springs and valves, i believe.

thanks for confirming the head gasket thing Ray...
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PostPosted: Sun Jun 10, 2018 11:44 am    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

Got the head back from the shop-- they ended up replacing both exhaust valve guides and seats, and fly cut the step down a bit to make it flat.

However, when I got back to the engine, I found piston four (that's actually facing the right direction) had scored the cylinder wall. I removed the cylinder to see if I could hone it out, which I nearly successfully did...and then I dropped the cylinder and chipped two fins. Confused

I think that was fate trying to tell me to just get a new set of jugs and pistons as SGKent suggested already, so that's what I will do. Then I get to check the other head as well, which hopefully doesn't need anything since compression check was fine on that side.

Anybody have a good suggestion where to get quickest/cheapest Mahle cylinder set to NorCal?

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PostPosted: Sun Jun 10, 2018 11:49 am    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

after honing for a while

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PostPosted: Sun Jun 10, 2018 12:19 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

gatorjos wrote:
after honing for a while

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Yes...you need to get new cylinders. That level of scoring is NOT defect you can hone out. Its what you have honed out by a proper honing machine with cutting stones...and then a surface hone....to "cut" to the next oversize.

A better way to prove this to yourself ...other than all of the unfortunate things you have listed Wink ....is to now....measure the honed bore...and measure the piston. I think you will find that the piston to bore clearance is now excessive.
And....an even quicker check...push one of your rings into the bore with a piston and measure the ring gap. I think you will now find your ring end gaps are very excessive.
Every .001" you hone out of the bore adds about .003" to the perimeter distance of the circle that bore diameter represents.

For example....a 4.00" diameter cylinder bore ...if you think of that as a circle...has 12.56637" of perimeter. Bore that to 4.001" and that perimeter length becomes 12.5695...which is an increase of .0031".....which means your ring gap opens up 0.0031".

You hone out .002" and your ring gap opens up .0062".

New cylinder time. Ray
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PostPosted: Sun Jun 10, 2018 2:21 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

Quote:
Anybody have a good suggestion where to get quickest/cheapest Mahle cylinder set to NorCal?

you really can't use the old cylinders unless it is bored out to an oversize and then over sized pistons used which would be more expensive than a new full set. Last couple sets I bought were from bus depot. They drop shipped out of Los Angeles so you can have them in a day if you want. Pacific Parts International is the label that was on them as I recall. That is the warehouse. They were a mahle set. CALL BusDepot, rather than use their online ordering, and you will get them quicker. Ask for a deal and maybe they will knock an extra couple dollars off. They are good people.

It might be worthwhile at this time refreshing the engine bearings too. All the crank might need is a light polish, and if it needs more then it was time anyway.
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PostPosted: Sun Jun 10, 2018 6:06 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

If it did not run hot. The builder may have not cleaned the cylinders before assembly. The honing stones leave dust in the grooves. If not cleaned out the dust will wipe out the PC's.
It's up to the builder to clean the parts before assembly.

Good luck
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PostPosted: Tue Jun 26, 2018 3:30 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

Just a quick update:

I got the engine back together and it appears to be running well and making good power. Compression after assembly was : 160-160-135-150. Not sure why #3 was a little lower -- if deck height is off by .010 would that do it? The shop that did the overhaul originally rebuilt the head for free -- they fly cut the cylinder seat a smidge to make sure it was put in new seats and guides, and ground the valves. Did get a new Mahle kit for jugs and pistons.

While doing teardown on the other side I noticed they had done me the favor of dropping the 1/2 head and breaking off two of the fins and cracking a third when they overhauled the topend before. Bentley says that's a pass, so I put it back together like that, with kind of low expecattions for this motor.

Now however, the oil light has been coming on at times. It goes back off when I rev it so it may be real. I'm planning on installing a sender and gauge so I actually know what I'm looking at, but jeez... Fingers crossed it's some gasket maker flakes floating by the switch.
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PostPosted: Tue Jun 26, 2018 3:33 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

I did add this sweet CHT gauge as well:

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PostPosted: Tue Jun 26, 2018 6:02 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

as far as oil pressure, I'll blame SanFran hills & aggressive cornering.
I'd keep the crankcase full, (topped off, maybe a full four quarts) & monitor it.
(oil loss) Thanks for the wrap up.
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PostPosted: Tue Jun 26, 2018 7:27 pm    Post subject: Re: type IV compression loss on 3 and 4 Reply with quote

timvw7476 wrote:
as far as oil pressure, I'll blame SanFran hills & aggressive cornering.
I'd keep the crankcase full, (topped off, maybe a full four quarts) & monitor it.
(oil loss) Thanks for the wrap up.


I have run mine a full quart over for three decades at this point in time. Doesn't seem to hurt a thing, the oil all fits in the sump, maybe half way up the cam on full draindown.
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