VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1933-1936 Piston clearance

This is getting to be an old story, 2 months or so ago, after final assembley of a 207 engine it seemed to be to tight in # 5&6 cylinders. The cylinders at the block seemed a lot hoter than the rest. The engine would start & run like new, it is all rebuilt' until it would get to around 170 and then run hard, I would then shut it off. My engine guy came and he said # 5 may be to tight. I moved the cab back, removed the grill & radiator & tore the engine down. The 5th cyl had .0015 clearance, No.6 had .002 & the rest .0025 to .003. Thought we found the problem. We honed #5&6 to .003 . Put it all back together and guess what? Still have the same problem, runs for about 5 to 8 minutes and then gets tight. Tried with out the 160 thermostat and seems to do the same. :confused: I even ran some acid thru the block. New rad ,New water pump. I am not sure about the Water pump baffle? I have no more IDEAS !

Do your rings have the correct end clearance? Spec is 0.004" to 0.014" end gap and 0.002 between ring and top of groove and tight with a 0.003" feeler gage.

In addition to the ring end gap, you could have seized piston pins, tight rod clearance, or, since the problem is in #5&6, failure to get all the packed rust out of the water jacket when the block was boiled out.

The next time you run the engine, when it begins to show distress, run your hand from the top of the radiator core to the bottom and see if the radiator is actually free flowing.

When we were checking the bore size we talked about ring specs. Pins are free and rod fit is to spec. The radiator is a new core. If the radiator did not flow wouldn't the temp keep climbing? With the thermastat in the temp goes to about 165-170 and holds. The time it takes,about 5 minutes, :confused: for the engine to get to that temp is about the same with or without the thermastat. Can the water passage ways in the block be inspected with the head off ? Someone said if the block was pluged acid might fix it.

easy money, I was able to inspect the coolant passages inside of my block with a AA-cell flash light that has a small bulb at the end of a flexible extension. To clean my cylinder block and head, I scored a hacksaw blade length ways with a small cutoff wheel in a Drimmel tool then broke the blade into two pieces. I then taped one end with electrician’s tape to form a handle and ground an angle at about 45 degrees with a sharp point on the other end. I used this tool to clean the rust and scale from the coolant passages inside the engine block and cylinder head for a 1934 Master by running it through the passages, the way you would rod out a radiator. It took a little time, but it worked very well. I removed most of the larger pieces of rust by inserting a magnetic retrieval tool through the holes in the block’s deck, the water pump baffle hole, and the freeze plug holes. Then I taped a small plastic tube to my shop vac’s hose and vacuumed the inside.

Hi Bill, Had you had trouble with your 34 over heating? Did you remove the water pump baffel or just work around it? I could remove the easy soft plugs and take a look what might be behind them. I'll let you know what I find. I would think a good engine shop would remove,inspect & replace all the soft plugs on a complete rebuild ! I should have ***** Thanks for the info :confused:

Yes easy money, it did overheat. The radiator was clogged with rust and mineral deposits, and it was also damaged by a departing generator pulley long before I bought the car. The leak caused by the pulley was sealed, but that resulted in a decreased cooling capacity, so I decided to install a new core. I was concerned about the rust that was still inside the engine eventually clogging my new radiator core, so I decide to pull the engine to do a rebuild and thoroughly clean the inside of the head and block before I reinstalled the radiator. I removed a lot of rust, but I am going to also install a Gano filter for added protection. I don’t have it running yet, but I am quite confident that the cooling system’s efficiency has improved. I did remove the water pump baffle and replaced it with a good solid brass one from either The Filling Station or Obsolete Chevrolet; I can’t remember which place.

To night I removed the radiator and the water pump.Then the carb and manifold. I pulled two soft plugs, one at the rear of the block and one at the center behind the manifold. It all looked real clear. Next chance I get, I think you can see in the block using a small mirror and a light. That's a big area around the cylinders for cooling! I'll pull the head next if I don't find any thing else. Thanks ( I feel like I'am being tested)

My thoughts are that it is not a cooling restriction problem.I would consider it normal for a fresh, tight engine to get up to 170 Deg. quickly.Its amazing as to how fast the older engines get up to normal temperature.At least half of the cylinder wall area is air cooled on a '34.Would be intresting to measure the temperture on the exposed air cooled area to see if all cylinders are the same.When you get it back together I could send you my non-contact temperature gun and you could get a reading on the lower cylinder barrels.Could also check for hot spots in the water cooled areas - comparing cylinders and cyl. head,etc..

The tightening up problem could be bearings too tight , pistons or rings.I would suspect the bearings.Did you turn the engine over by hand when the pistons were removed?Was there a lot of resistance?Was the ring gap checked...ie...rings placed in cylinders and gap measured?

Your present piston clearance sounds adquate for most pistons.

easy what type of pistons are being used in your engine alum. or cast who made them . all pistons are not equal meaning they may have the correct diameter but the specs are not correct a cast piston is about .015 less in the ring grouve to the crown and the cam is about .002 on each side of the pin area this differs with a alum. piston a frend of mine had a similar problem with alum pistons that were not made for the application he was using them for did not lock down but would not roll over when hot . modern alum. pistons are designed for a peticular engine and operating temp . i had a twelve cylinder cat that was designed to run at 200 f with a open water system a frend of mine had the same engine and did not like his to run that close to boiling so he lowered the thermostat temp. and the engine was worn out at half the hours . if you get away from the original design you have to redesign to use that product it could be simple or complex but in expansion and contraction it may be as simple as clearances

When the engine is close to 160, you can touch all but #5 , #5 you can feel is a lot hotter. Since I removed the soft plugs and found nothing in the water jacket I am sure it's a friction problem not cooling. I do have access to a temp gun. The engine turned over with fan after we honed 5&6 and made sure they (all 6 cyl) had around .003 clearance. All the rods had side movement and none the other way. Pistons are alum Jahns split-skirt nos from Vintage parts in Denver and the spects say .002 clearance. Rings are Wausau brand. I am not in a hurry to put the "Brat" back together, this is the second time apart and would like it to be the last. My engine guy said he fit all the rings, maybe he missed #5 ! He's been doing engine work for 30 plus years. I've mixed a little 2 cyle oil in the gas to make sure of oil on top at first running. I did turn the crank with no rods and turned real good. Thanks for the help. :confused:

If the engine is still apart, try this test.

Remove all the rings from #5 piston. Place the bare piston in the hole. Take one ring and place it in the hole and press down until it is aligned with the top of the piston. Now measure the ring gap. Do the same to the second ring. Oil control rings are not normally a problem; however check the two outer rings in the same manner as above.

You ruined my day when you said the pistons are JANS pistons. If you were around and followed the "School is in Session" you would know that my problem was due directly to the JANS pistons.

Have your mechanic measure the #5 piston in all locations for a cam grind and accuracy of construction.

Wish you luck. Problems take the fun out of motoring.

What oversize is the engine bored out to?Perhaps a good set of Genuine Chevrolet cast iron pistons are the answer.

"I am not sure about the water pump baffle"-it does have a large freeze plug type thing in the front of the block behind the water pump I hope-should not be able to look in the hole and see the front cylinder barrel.It must be there or engine will heat up in the warmer weather.

i try to stay away from the alum. pistons because of the previous post of design problems . cast pistons are available up to .060 over and even semis. wich brings up a other problem i had a while back that ray talkes about cam grind . a while back i had a engine that was bored .060 over had some piston slap the walls were straight but .006 piston clearance which produces slap cast pistons need to lock up on .002 to .0025 to be quiet i found a set of semi pistons made .070 and cam ground installed and engine ran at a idle for about seven or eight minutes slowed and stopped . while it ran it was really quiet and i was patting myself on the back thinking it was a sucess but it did not last long i got so good at taking these apart in 48 minutes i had all the pistons out found the problem the cam grind on #5 and #2 was not correct redid those checked the others reinstalled and have put about 1500 miles on ok . the problem with the grind on those pistons was it did not go around the skirt far enougf and that gets back to design when you talk about .0025 to .003 thou. it does not take much to make metal to metal contact . the theory of cast pistons is it grows about the same as the cylinder expands except in the pin area where the mass is greater and it expands a few thou. more. where alum. has a differant expanion ratio . hope this helps and you do not get down to the 48 minutes but you may all ready be there

Thats why I mentioned Genuine Chevrolet pistons.They are correctly cam ground out of round.

I have used old stock aluminum pistons in my cars with no problems.It appears that the modern versions have the problems.

The pump baffel is there. I think the bore is .030 over. What was "school is in sission" ? What problem did you have with Jhans pistons ? What did you do to correct it ? Thanks Chevygene, do you have any nos .030 over pistons ?

I have some .040" over

I don't have any NOS but have several sets of NORS pistons. Have never had a problem with any of them in the past.

Ha chipper, Do you have .030 over pistons ? Are they cast or aluminum? With Rings? Price? See my profile and email me. Thanks

Hi Chevgene, I had the 207 running tonight and took it's temp just at the time it got tight. The front four at about 190 to 195, # 5 at 205 to 210 and # 6 at 200. The water jacket above the cylinders all at about 170, # 5 a little higher at 175- 178. 5 is behind exhaust pipe and might not get as mutch air. Wouldn't the cylindars below the water jacket be cooler because that is farther from the heat sorce,"combustion" Thanks

Chevrolet must not have thought that the lower half of the engine had adequate cooling or they wouldn't have gone to full jackets on the later 207s. I do realise that the later engines had considerably higher horsepower. Correct me if I am wrong, but isn't the lower part of the engine basically oil cooled? The temperatures don't seem to be too high but the pistons themselves might be higher.

Chevrolet must of had cooling borderline problems in the 34 up engines especially in the 1.5 ton trucks with higher loads and speeds. The 34-35 1/2 ton's had brass 2" thick cores with v type fins (33 were hex) and the 1.5 ton had copper 2" cores. They released a heavy duty 3" copper core radiator and then in 35 a cooling shroud option as shown in the April 35 Service News. Chevy made many changes increasing radiator hose dia., water pump increased flow, head changes to cool valves better and then in 36 full water jackets.

I'am about ready to replace aluminum with GM cast iron & GM rings. The way the pistons are made with a full round bottom "no cuts or slots" the only place the piston can go when it gets warm is OUT. I think the .0015 clearence per side, piston to cylinder, is gone with normal engine heat. Thanks

The pistons are cam ground out of round.That means that most of the expansion will be in the pin area to the front and back (when installed).The piston should pass thru the bore with a .002" feeler strip inserted on the skirt or side of the piston.Should be tight with .003" feeler.This should be done with block and piston at same temperature...around 70 deg. is best.The correct method is to use a narrow strip rather than a wide feeler gauge.At on time there were special piston fitting feeler strips about 1/4" wide made for this.I used to sell that at Diesel truck shops.Have some around yet but not sure of size.A regular width feeler will give a false reading.

If it were warm out I would take temp readings on the block of my '34.I know that the water cooled area will bw about the same as the temp. gauge reading.Don't recall taking a reading in the "air cooled" area.The 1935 was the last year for the short water jackets.Most cars changed to full length in the mid-'30's.The short jacket gave cooling only to the combustion chamber exposed area of the cyl. wall..The 1936 with full length jackets was advertised as making the oil temperature over 20 deg. less at high speeds.

It is true that the heat generation is in the top of engine. Some heat is rapidly transferred to lower parts by the hot gasses (convection) and more by radiation and conduction which is the slowest. It takes minutes to reach an equilibrium heat state at each engine condition.

The major reason for the full length water jacket is to have the full length of each cylinder be close to the same temperature. That will keep the diameters more consistant and help with engine life and oil burning.

The rule of thumb is that 1/3 of the heat went into power, 1/3 out the cooling system and the remaining 1/3 out the exhaust and from the engine directly to the air. When you increase the horsepower it means more heat to be removed. Trucks have another problem. Use higher horsepower at lower speed hence more heat with less air flow (to do the cooling). It ain't rocket science just practical sense.