8BA compression issues
- Thread starter mrlaser
- Start date
mrlaser
Senior Member
Forty CPE ,Thanks for helping me see the obvious. I'm afraid that in my hyper focus on the valve stem, I missed the "forrest for the trees" 
. I did as you suggested and (with a good light source) viewed the valve stem and it's relationship to the guide when the valve was closed ( with proper valve lash present). As can be seen, the slightly wider portion of the stem is not interfering with the guide. With that possibility eliminated, could valve spring preload be a factor? I had no way of testing the springs that I installed.
. I did as you suggested and (with a good light source) viewed the valve stem and it's relationship to the guide when the valve was closed ( with proper valve lash present). As can be seen, the slightly wider portion of the stem is not interfering with the guide. With that possibility eliminated, could valve spring preload be a factor? I had no way of testing the springs that I installed.Attachments
JSeery
Member Emeritus
Forty CPE ,Thanks for helping me see the obvious. I'm afraid that in my hyper focus on the valve stem, I missed the "forrest for the trees"
Then how did you determine the install height? You don't just stick the springs in there, you have to determine the spring height at the seat pressure you want and then shim to that install height on the valve assembly.
mrlaser
Senior Member
When the engine was rebuilt by the machine shop the cam,valves ,springs, and adjustable lifters were purchased by them. I do not know the source of the products. The intake valves are 4.829 in length. After lapping the valves and changing the valve springs to the shorter type shown in the Green Manual for the later rotator valves, the compression improved from the mid 40's to approximately 65 psi in all cylinders. With added oil there was an additional added 5 psi. At the time I did not check the height or set the proper spring pressure. I was aware of these steps but thought that ( as mentioned in Mike Bishop and Vern Tardel's book) this step was for a "performance engine". Being a novice at this, I incorrectly assumed that since my engine was to be a stock rebuild, I could simply put the springs in place without those two measurements. ( live and learn). I am now redoing the springs after making those calculations. I will report on the results when things are back together. The attached picture is of the intake valve and the initially installed taller spring on the left and on the right, the shorter spring used with the "51-'53 rotator valves which were in the engine originally. Thanks again for all your help.
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mrlaser
Senior Member
I have now finished checking valve install height and setting valve spring pre-load via a spring scale and shims. And while I did follow the suggestions offered by Cadillac512 regarding cam timing verification, I would like to be sure that my current assumptions are correct before reassembling the heads and intake. TDC was established by bolting a plate across number one cylinder marking the crank pulley in both the clockwise and counterclockwise direction. TDC was determined to be 1/2 way between those marks as seen on Marts video. It also coincided with the "bump" on the crank pulley being opposite the existing pointer. At that time both valves were closed and the proper valve lash was present on both valves on number 1 cylinder. Number 6 piston was at also at TDC at that point with the exhaust valve closing just prior to number 6 reaching TDC and as the engine passes through TDC the number 6 intake just begins to open. This is my novice understanding of "valve rocking". I have seen some mention of valve overlap in other threads, but I am not seeing any point during this transition where both valves on number 6 are open at the same time. I am wondering whether this is more of a phenomena seen with higher performance cams? The cam in my engine is of the stock variety. I am new to most of these issues and want to be sure that the cam timing concerns have been properly sorted before moving on. I greatly appreciate all of your suggestions.
mrlaser
Senior Member
I do hope that that any confusion was in my description. In the attached pictures, I believe that #1 is just below the thermostat on the passenger side of the engine, while # 6 is the second cylinder from the front on the driver"s ( left side of the engine)
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cadillac512
Well-known member
You have it absolutely correct. Your description of "valve rocking" and assumption that more performance oriented cams will make the lift at tdc while rocking more apparent is spot-on. Well done...I'll bet she fires up now!
Terry (out in the shop building my new 260 Merc flattie right now!)
Terry (out in the shop building my new 260 Merc flattie right now!)
Kurt in NJ
Well-known member
On post 70 , the picture of the valve it looks like the (if it is one that is lapped in) it looks like the seat is very wide
mrlaser
Senior Member
You're correct. When I was lapping that valve (#4) I was leaning over the '51 front fender with one foot in the air and fighting the heater motor.. At the time I didn't realize that the valve guide had slipped down into the gallery. It was only for a few strokes. After realizing what had happened, I covered the valve with Magic Marker and checked the pattern. It yielded the same narrow band as the others. I would have replaced the valve itself. But I don't know who the valves were originally purchased from. Thank you for catching that mishap
Bored&Stroked
Well-known member
Can you take a close up picture of a couple of the cylinder bores? Close as you can, with some light and good focus? I'd like to see what the cylinder walls look like. Thanks!