frame measurement question

[poorbiker]

I dont know so I`m asking. With a perfect condition frame, bottom rails level side to side and front to back with a straight surface, and with a plate bolted to rear mount and spaced properly to front mount. If you place a level on the plate will it be level with the frame, (rails)? Or will it have a slightly downward direction to the front, not level with the rear trans plate bolted in place. we will leave my frame out of this and assume a very straight frame..........thnx......

[Steve Little]

Hi. The areas you have listed on your frame, would have been straight when the frame was first assembled by HD.
But not when the frame left the factory. Confused??

Harley re-cut the engine mount, and trans decks after final assembly of the frame.
If you have a vernia, measure the area of the the engine deck to the washer surface under the engine mount you may find a discrepancy of ....say 0.025" from one side to the other.
The engine mounts (front and rear) seem to have been more effected and had more cut off during the brazed period, but still had distortion/contraction during the electrode welding.
Dont use the lower tubes as a base for frame alignment.
The lower tubing could do, loop-dee-loops, and it really has no bearing on the frame.... although home based tinkerers usually try to use it.

The fact is, when HD re cut the engine and trans decks, the original frame centre line is gone, forever.

The crucial areas are, the neck alignment to axle carrier, the engine to the trans, the trans to the axle carrier.
There is more I could write but I this is the basic answer.
If you need more, I,m happy to answer.
Regards Steve

[poorbiker]

Well thnx Steve,
And yes I do have more questions. As per a pic you posted in your frame restoration thread I installed a 1/2" x 8" x 12" long piece of tool steel I know to be true enough for flatness. I drilled two holes on my mill to mount it to the rear motor mount. I then measured down to the top of the front motor mount surface from the bottom of the plate. The distance was exactly 1.625" from side to side. Used feeler gauges to test if any gap was present, no gap of more than .001 from side to side. I made some steel round stock spacers on my mill and drilled/tapped and bolted them to the front mount and rechecked with feeler gauges, still good. Rechecked for fit to plate and clamped & tacked in place. Took a measurement from that plate to seat post, was a little more than 74 degrees with my rather crude angle finder, but close. Frame was level with and supported and clamped to a piece of 3/4" x 12" by 48" long piece of cold roll, I had a friend check it for flatness in his cnc and cut off any high spots. Its pretty true for what it is. Just to be doing cause I have never checked a frame like this before I put my Starrett precision level on the plate and adjusted it to level. I then put level on the trans plate and it showed close to plate level.
I then put level on the motor plate I made and it showed a downward direction from rear mount to front mount .080 or a bit more than 5/64". Bottom frame rails running true with plate front to back and side to side. Frame supported by four v-blocks tapped to frame support plate and then clamped. I expected the motor plate to be at least fairly level or at least to be near the trans mount plate level, its not. 5/64' doesnt sound like much over 12" and front motor mounting surface is square with the rear motor mounting surface. Am I being to anal, motor cases bolt up nice with only .020 shim on right rear mount hole being necessary. But even 5/64" is noticeable with the naked eye.

[Steve Little]

Hi poorbiker.
I read and re-read your post a number of times to try and get an understanding of what you wanted answered.
I could only perceive one question, which is in the last sentence.
So I will answer that as thoroughly as I can, by relating the experience we have learnt here.
When we cut the decks of any frame (new or restored) we have a fixture that ties the frame rigidly in the mill.
This is done by way of front transmission mount, and the upper section of the seat post tube.
This method allows the frame to be held "stress free", while we cut the engine decks.

The front of the frame is also supported by a couple of “stand jacks” under the front engine mount and some adjustable side braces to stop bounce/vibration during a cut.
The decks are cut to 1.625 for 1936-1947 or 2" for 1948 up.

During work hours, my workshop is kept around 19 to 21 degrees C. Cosy!! But it can drop as much as 10 degrees at night.
(Those of you wanting to work out the difference of C and F ... water freezes at 0 C).
After the decks of a frame have been cut, it can be taken out of the mill and sat on a bench and re checked just after the cut, and it is always perfect.
That same frame, can be re checked the very next morning and we have seen 0.018" out of alignment.
As an experiment we have sat them in the sun and achieved 0.028 miss-alignment. Why?
The front engine mount is sitting 10.5 inches away from the rear engine mount on spindly tubing that will move differently during heat variations.

The design of the vintage HD frames from 1936-1957 are not built to be very rigid.
It is after all, a cruising bike and not a sport bike.
The bare frame of a Big Twin Harley is divided into two large "flexible" triangles.

To be fair to the original designers, the top engine mount will divide the front triangulation into two when fitted tightly to the engine.
My reason for the lengthy description is to describe the movable/flexible nature of the HD frame, and although shimming is a sound "Fitter and Turner" practice, and it should be continued, I do not believe that a difference of 0.020" on reasonably level, surface decks of a flexible frame was ever the culprit of sheared off engine decks on HD cases.
Regards Steve.

[poorbiker]

Thanks Steve,
And in part you answered my question. I re-read my post and will form the question this way.
Frame sitting level, trans plate bolted in, a level set on trans plate and it shows level front to
back and side to side. A properly spaced motor plate installed and the same level was
put on top of motor plate should it not show the same on the level as it did on the trans plate?
Are the motor plate and trans plate going to be the same as far as level goes, or at least near.

[poorbiker]

I know there are other issues here, motor mount among them. However, mount is straight and true, amazingly, though it needs replaced.
This pic shows what I was trying to ask about. The difference between the motor plate and trans plate as far as level.

[Steve Little]

Hi. Whats your first name?
HD recut the engine decks after every frame was assembled and brazed or welded.
This was done to ensure 74 degrees and 4 minutes to the seatpost tube, and then cut the correct height difference for the front engine mount.
It was also done to cut any distortion out of the engine decks after the heat of brazing or electrode welding had done it's work.
The trans decks were not specified as being a mandatory re-cut.
If you would like to check your frame to see if the trans decks have been cut, measure both ends of the slotted 5th mount, and the thickness of both sides of the front trans deck....trans palte face to washer face underneath.
When I restore a frame here, I check this. Most often you will find it has a different thickness from side to side, meaning it has been recut.
To answer your question....( Are the motor plate and trans plate going to be the same as far as level goes, or at least near.)
Quite often we have to tilt the head of our mill forward a couple of minutes to achieve the 74 degrees and 4 minutes.
HD had to do the same thing, so unless the trans decks were re-cut to the same plane as the engine decks, the answer would be No.

[Steve Little]

I should mention that the levels should all be on the same plane as measured across both axle carriers. This is criticle to chain life.
But the engine can tilt forward a couple of degrees because the inner primary has enough slop in the mounting holes to accomadate this.

[poorbiker]

Steve,
Thank you very much for all the responses, I appreciate it very much. I believe you have answered my questions. I also appreciate you sharing the knowledge.
I will use the other info you provided to check a few more things on frame. Again, thnx for the info............................Dave