• Registered: 2009-12-26
  • Posts: 385

Topic: Compression issue

I've got a 56 Nash flathead 6 with compression issues and an oil leak. This is the first engine I've ever opened. I'm a complete novice. My only goal is to make it good enough to last one race.

Compression for cylinders 1-6 in psi is 35 35 70 55 90 0. I opened it up hoping to find a stuck valve or broken spring on cylinder 6. But I can't see any problems. All the valves open and close. They seem to seat okay. The pistons can be moved a tiny bit side to side when at tdc. cylinder 6 doesn't seem any worse than the others in this regard. There was a bit of scorching on the deck between cylinder 6 and the end of the block. The oil leak was also at the rear of the block although I have not found the source yet.

Is it possible that both are due to a gasket problem at that spot? If so, what else would I look for to confirm it? If not, what do I do next? I'd rather not take on a whole rebuild.

Thanks.

I hope to have the car ready for Thunderhill.

IOE is on my bucket list. So is doing some hot laps at Monaco. I figure i'll start with IOE.

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  • From: WPB Fl.
  • Registered: 2010-10-27
  • Posts: 755

Re: Compression issue

Quick question, did you get the engine to run before doing the compression test and subsequent  tear down or are these numbers field/barn fresh ?

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3 Reply by squidbreath (edited by squidbreath 2012-05-31 11:34 AM)

  • Registered: 2009-12-26
  • Posts: 385

Re: Compression issue

I drove the car home after I bought it. It was feeble and didn't have much power. I was concerned that putting it on the track as it was would be dangerous because it would be too slow. Also the oil leak wouldn't have passed tech.

The compression was tested on the engine while it was in the car and able to fire.

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  • Registered: 2009-12-26
  • Posts: 385

Re: Compression issue

The car: http://carerror.blogspot.com/2011/12/19 … arina.html

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  • Registered: 2012-01-06
  • Posts: 126

Re: Compression issue

The best way to trace a low/no compression problem is with a leak-down test. Leak-down testers are not particularly expensive, but you may not want to buy one. Also, you probably don't care all that much about the percentage of leakage. But all is not lost...reassemble the engine. With each cylinder at TDC supply the cylinder with about 100psi of air. You may have to lock the crankshaft to keep i t from rotating, but probably not. Now, listen in a few different areas for a lot of air escaping...#1: Exhaust manifold, #2 Carb or intake manifold, #3 Oil filler or breather. This lets you know what part of the system is leaking; #1 Exhaust Valve, #2 Intake Valve, #3 Rinigs. Lastly, a blown head gasket may leak to the outside, the cylinder, or the water jackets, cooling system.

You can usually use a compression tester hose to adapt your compressed air to the cylinders.

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  • From: Tustin, CA
  • Registered: 2010-12-30
  • Posts: 971

Re: Compression issue

In general wide variances from cylinder to cylinder are caused by worn valves.  I know spit about 50's flathead stuff but I suspect that 90lbs is probably pretty good for one of those motors so for that one cyl to have 90 tells me the rings might be ok.  The reason why they had lead in gas was to lubricate the valves and there may exist the chance that the valve seats in that thing aren't hardened so unleaded gas took it's toll on the valves.  Another thought is that modern oils don't have ZDDP and the cam got worn flat on some of the lobes.  If the valve doesn't open/much then no air can come in to compress and get a number.  I'd spin the thing over and see if you are getting approximately the same valve lift on each of the valves.  If you are, I'd figure out how to disconnect the valve springs, get some lapping compound and a suctioncup valve spinner thingie, and learn how to lap valves just like grandpa used to do it.

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  • From: Doin' Crime in 21229
  • Registered: 2009-05-01
  • Posts: 2,118

Re: Compression issue

Here's some of my drama getting an old flathead 6 to run in my '49 Plymouth:

http://www.tunachuckers.com/blog/index. … 1&pb=1

I'd clean the deck and head up really well, make sure the head surface is true and flat (a machinist should be able to do this for almost nothing, flatheads are super simple heads to plane) lap the valves, and put it back together with a new gasket.  There's tricks to reading compression readings, but since you're all apart now none of that really matters and you're going to have to just take the buckshot approach. 

Oil leak sounds like a rear main seal.

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  • JThw8
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  • From: Mullica Hill, NJ
  • Registered: 2009-06-10
  • Posts: 1,707

Re: Compression issue

are those numbers wet or dry?  Another issue with old flatties is they almost have to have wet compression.  I replaced a headgasket on my last Studebaker, and after it would not fire, I had fuel and spark so I knew something wasn't right.  Thought I botched the HG install then I talked to an oldtimer in the stude club who told me to shoot oil down the cylinders and try again.  A couple shots of oil in each was enough to get the compression back and she ran fine after that.  It just seems if they sit dry to long it breaks that oil seal and compression goes to hell.

Still that 0 is indicative of something worse.

And even if you have it at factory specs I'll warn you now, it will still be dangerously slow.

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  • Registered: 2010-12-29
  • Posts: 202

Re: Compression issue

What about stuck piston rings, from all that 1950's oil sludge?   if they're not moving freely, that could easily be the cause of low or no compression.

even if that's not the cause, it's probably not a bad idea to clean those grooves out.

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  • From: Los Angeles
  • Registered: 2010-08-22
  • Posts: 1,253

Re: Compression issue

mike944 wrote:

What about stuck piston rings, from all that 1950's oil sludge?   if they're not moving freely, that could easily be the cause of low or no compression.

even if that's not the cause, it's probably not a bad idea to clean those grooves out.

Almost all the rings on the Super Snipe engines (both '59 and '64) were like that.   There was ample evidence of blow-by on every piston.   A couple of the rings were even broken, and I broke a couple more trying to get them free.   What compression?

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11 Reply by Evil Genius (edited by Evil Genius 2012-06-01 11:33 AM)

  • From: Davis, Ca
  • Registered: 2009-04-30
  • Posts: 1,612

Re: Compression issue

Fred

       Are you bringing this to us for a cage?   Just axeing, as we are hella busy and would need to get you on the schedule...

      Oh, and on the weak compression.    Sounds like the head gasket may have been blown.  How does the bore in #6 look?     Pour some liquid (water, oil, brake clean, etc) on top of the piston and the valves and see how fast it disappears...   if it just drains away quickly, you've found a problem.   


     -John

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  • Registered: 2009-12-26
  • Posts: 385

Re: Compression issue

John,

It's Jeff not Fred. And yes, I plan to bring it to you for a cage. If I miss Thunderhill, it's not the end of the world. I'm not even sure if i'll be ready by then anyway. I'll call to schedule in a way that gives some lead time. It needs some chassis reinforcement too. At some point part of the floor was replaced in a flimsy looking way.

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  • Registered: 2009-12-26
  • Posts: 385

Re: Compression issue

Thanks to all for the suggestions.

i'm going to do the fluid drain thing John suggests to check the rings.
I bought stuff today to do the valve lapping.
I'll redo the main seals and gaskets.

It strikes me that i'll probably have to tear it down to do the valve lapping. Can't see how I can clean the lapping grit out otherwise.

“Never attribute to malice that which is adequately explained by stupidity.”

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  • From: Davis, Ca
  • Registered: 2009-04-30
  • Posts: 1,612

Re: Compression issue

I never get your freakin' name right, sorry...   It's (was)  wrong in our computer, so to me you're Fred... live with it....   

    That's about the 20th time I've done that.     

      I'm just going to start using 'hey you' for everybody in LeMons.  My little brain is just full up.

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  • From: Davis, Ca
  • Registered: 2009-04-30
  • Posts: 1,612

Re: Compression issue

squidbreath wrote:

Thanks to all for the suggestions.

i'm going to do the fluid drain thing John suggests to check the rings.
I bought stuff today to do the valve lapping.
I'll redo the main seals and gaskets.

It strikes me that i'll probably have to tear it down to do the valve lapping. Can't see how I can clean the lapping grit out otherwise.


  Apply a vacuum to the lifter gallery (I assume there are removable side covers where the lifters are) while lapping.. duct tape the end of a shop vac on there and rinse the lapping compound with soap and water while the vacuum is on...  It'll get most of the grit..  The little that gets through into the engine will not matter much in a 60 year old flat head.  Don't use something flammable to rinse the compound.. you'll blow up your shop vac.   

     Yes, I've done this before.. it worked, or, at least,  it was still working when I sold the car........

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  • From: Doin' Crime in 21229
  • Registered: 2009-05-01
  • Posts: 2,118

Re: Compression issue

Agree with John, valve lapping in head is possible and not too hard.  I did it in the Plymouth flathead 6 I mentioned above.  I didn't use the shop vac method though- I just doused the entire valve area in the side of the engine with brake cleaner when I was done.  I think you could probably use any kind of cleaner- but I needed a good degreaser to clean out all the oil sludge, too.  It should go without saying that the pan was dropped so all the crap just washed into a big catch pan on the floor.  It was dirty, filthy really, nasty, but it worked.

We affirm that the world's magnificence has been enriched by a new beauty: the beauty of speed. A racing car whose hood is adorned with great pipes, like serpents of an explosive breath- a roaring automobile that seems to run on machine-gun fire is more beautiful than the victory of Samothrace.  --Filippo Marinetti

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