VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Compression testing

Hi,
I'm new to this world of pre-'29 four bangers and new to this forum so please bare with me.

Question: Where can I get an adaptor to thread into my '28s 7/8"d spark plug holes and allow me to use my contemporary thread in compression gauge? :confused:

Thanks in advance....

Hi Babbit Head,

I have those adaptors and when you finish you can just leave them in the holes and use a modern spark plug.

Hi Ray,
The tours were awsome and it was a pleasure making your aquaintance. If I can get my '28 running as well as my '39 did at Lassen, you'll see me on more 4 cylinder tours.

Re spark plugs: We spoke at the recent meets about the desireability of the AC 78S (3/4" long). I'm just enough of an "authenticity freak" that I would have trouble using "modern" (oh, I hate that word) plugs. Comments?

Re timing mark: We also spoke of timing to a mark in the flywheel. Tell me some '28s are out there without a mark! I have been over that flywheel with a mignifing glass and see NO mark. Help! Help! Comments?

Hi again Whitney,

As the sparkplug situation for the 4-cylinder has been beat to death on this forum I'll just skim by. The objective is to obtain a 7/8 plug that has at least a 3/4 reach. For driving purposes the brand is not important. For judging you need an AC B or for the 28 an AC 78S (old). These plugs are getting more difficult to obtain and some cases the price is way out there. In view of that, obtain a set of the correct plugs for 'show and tell' and run the adaptors w/modern plug for daily/tour driving.

I only have one 28 flywheel off an engine at the moment. It was machined many years ago for the purpose of installing a new starter ring gear. However, the "mark" is still visable but I can't make out the identification marks on either side. Perhaps someone has an unmolested '28 flywheel off and can fill us in on the exact markings that would be visable through the timing hole in the flywheel cover.

-R

What timing mark on the flywheel are you refering to Ray?

Exactly where in the 1928 AB Instruction Manual or 1928 AB Repair Manual do you find any reference to any timing mark on the flywheel?

Rusty Fender,

Thanks a million for posting that picture. It is so good that I will point out some facts about it.

U ! C This is the Top Dead Center Mark. That is when both piston number 1 and piston number 4 are at the top of their travel. If you were setting the timing on a '28 Chevrolet with the use of a timing light this is where you would set the distributer WITH THE SPARK CONTROL IN THE FULLY RETARDED POSITION.

! 25 If the spark control lever was in the full advance position this is the picture you would see through the sight hole. This represents 25 degrees of advance. The engine must be under 1000 RPMs. At 1000 RPMs or approximately 22 MPH the mechanical advance is activated inside the distributer. This action can add 15 degrees of advance for a total of 40 degrees at/above 22 MPHs.

WK7 I must admit I do not know the meaning of this marking.

344100 This is the part number for an OEM '28 flywheel. Check the parts book and you will find that the REPLACEMENT flywheel is part number 344099. This flywheel fits '25 through '28.

CHEVROLET BOWTIE Indicates genuine Chevrolet

D-25-8 This is the casting date of this flywheel. The date is April 25, 1928

The hole drilled in the edge of the flywheel at 2:30 is to balance the flywheel.

Again, Many thanks to Rusty Fender for the picture.

Pre25Chev4,

You will not find any reference to the timing mark in either the Owner's Manual or the Repair Manual, however this is a feature the Antique Mechanics have observed from years of working with the old 4-bangers.

Yea - isn't it strange that Chevrolet took out any Distributor Timing instructions in its 1928 Service Literature.
I guess Chevrolet though that printing its new timing method once when the chage was made to the semi-atomatic distributor back in the July 1927 Chevrolet Service News was good enought?
Of course the Timing method illustrated in its 1928 Chevrolet Engineering book pg 65 shows the tolerence on timing was a +18 degrees to a -20 degrees - so manybe someone thought with this wide of a tolerance - timing was not all that important?

Babbit - when checking compression - block open the throttle and pull all the plugs, and do it after going for a 4-5 mile drive - then post your results.

Ray wrote, "344100 This is the part number for an OEM '28 flywheel. Check the parts book and you will find that the REPLACEMENT flywheel is part number 344099. This flywheel fits '25 through '28."

I don't believe that there is any difference in the parts originally supplied or from the parts system. In the 20s and 30s there are several numbers on cast items that differ from the number in the parts book. In most cases the difference is only 1 or 2. Another example is the '30-31 head with a part number 836375 and casting number 836373. There are many more.

Don't ask me why it was done but it was many times.

Actually, I found the description of "Semi Automatic Spark" on pages 43-46 of the Repair Manual quite helpful in timing my distritubor. Although they seem to leave that task as an exercise to the reader. I seem to recall U|C stands for "Upper Center"? Of course the flywheel will bolt on 6 different ways so the usefullness of the markings depends on whether the previous flywheel installer was paying attention.

Regards,
-R

ChevyChip ,

Woops! Did not intend to leave the impression that the parts were 'different' only that in MANY cases the OEM part carries a different number than the replacement part in the parts book. It is my opinion that Chevrolet did that to see if the mechanic was paying attention. (Statement made with tongue in cheek).

Rusty Fender

You are correct in that U !C stands for Upper Center. I used the 'generic' term. You are also correct on the 6-position assembly of the flywheel. Reminds me of when I picked up the components of a 4-cylinder that I had taken in to have balanced as a unit. The flywheel was unbolted from the crank and no index marks on either the flywheel or crank. Oh Well. They needed the training anyway.

A few thousand words on timing with a timing light. Use a 12 volt battery as a source of power for the light unless you have a real antique unit that is 6/12. The 'sight' hole is directly behind the heat stove on the passenger side of the engine. It may be necessary to remove the air cleaner in order to see the marks. When you stick your head into the engine compartment to observe the 'mark' be very careful or the 'ticking' sound you hear may in fact be the fan blade clipping your ear. Loosen the distributor clamp bolt and with the spark lever in the fully retarded position and a slow idle aline the U ! C mark in the center of the window. Tighten the clamp bolt, advance the spark fully and the engine should 'lope', ie sound as if it is not running smoothly. (A great big caution on the 4-cylinder engines. It is not wise to allow a 4-cylinder to idle for long periods of time. At idle the wrist pins do not receive any oil and pistons become very hot. Not a good situation. The wrist "piston" pins are oiled by the splash from the rods striking the oil in the trough only.)

Figure #44 on page #43 of the 1927 &1928 repair manual shows a pictorial depiction of the flywheel and a 25 *mark and different positions of the #1 piston and the automatic timeing advance.

Pre25Chevy4

For 'static' setting of the timing on the '27 & '28 Chevrolet, the explanation with pictures, on page 27 of the July '27 Service News, is the better description.

Chevrolet used the term "UC" and "TDC" interchangable which is confusing to most mechanics. That is the TDC for piston travel is not the same as the TDC position for crankshaft position because the Chev 4 engine [Mason Motor] by design uses a [about 3/16 to 1/4 inch] offset crank bore in relationship to the cylinder center line. The flywheel that are marked with a 1/4 timing mark line, this mark is lined up at the 12 o'clock position to indicate TDC.

The best static timing procedure is using a 6v test light to gauge when the points just begin to open. This is shown in CSN December 1929 and July 1930. Always recheck timing by turning crank two turn around to check if light just goes out at the flywheel marks.

CSN for October 1927 stated that 25 deg advance was not too much for starting the 27 engine and that for ordinary driving the spark lever can be ignored. But in the 28 Instruction Manual is told the owner to retard the spark for starting.

Ray, The repair manual is really good enough specially if you don't have the service bulletins! At least it is more clear rhan mud!