750 Block height and CR - Alfa Romeo Bulletin Board & Forums
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post #1 of 7 (permalink) Old 03-17-2017, 07:24 AM Thread Starter
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750 Block height and CR

I'm in the process of having pistons made for a 1958 Veloce motor. The supplier (Venolia) is asking for block height and I couldn't find that dimension in the literature that I have.
Also what is the stock compression ratio for this engine?

Thanks,
Jim
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post #2 of 7 (permalink) Old 03-17-2017, 10:16 AM
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They should know that Jim. Venolia has made Alfa pistons for a LONG time. Call yours STANDARD unless you have had machine work done to block and liners. Stock compression is listed in some sources as 9.7 for Veloce engines. HOWEVER this varried slightly when new, and more now as previous owners have probably milled your head at rebuild time. The only way to note this is to measure head thickness, or CC combustion chambers with comparison source data. Most builders ordering stock replacement cast pistons today go with 9.1 (normale) pistons, as actual compression may be higher from previous mill cuts.
Best is DIY compression ratio calculation done by CC'ing your chambers with your Venolia pistons, then maching crowns (metal removal) to arrive at a chosen ratio.
One word here. Do NOT further mill your 750 head (or 101-1300;101-1600;105-1750 or any race or modified head) as they are getting scarce. Ruin one and you will regret it. There are better ways to get them flat, or change compression.
My opinion only from my own experience.
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post #3 of 7 (permalink) Old 03-17-2017, 10:31 AM
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105-1300 Jr pistons are pretty much the cat's meow and I know is a common practice with a very robust result. You can buy the liners and pistons w/rings matched which saves that part of the equation after you make new pistons. Just worth about 2 cents. Probably save some money too which is a bonus. Less than $500. uncle
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post #4 of 7 (permalink) Old 03-17-2017, 11:58 AM
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Good, smart move!


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post #5 of 7 (permalink) Old 03-20-2017, 02:35 PM Thread Starter
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Gordon, Uncle,
Thanks for the replies. The head that was on the motor when I took it apart had been cut to 3.975" and stock pistons had been installed. The pistons showed evidence of hitting the combustion chamber circumference and the valves hitting valve pockets! The seats were sunk pretty far in and the valves had been switched to 45 degree ones I guess to get a little more life out of the seats.
I was able to find a replacement normale head with good seats needing only guides and a .020" resurfacing. I'm not cutting it anymore! and that's one reason for the Venolia pistons as I'm shooting for the 9.7CR. I did cc one combustion chamber on the head and also the volume around the piston dome in a liner, but my math must be wrong as I didn't get the expected number. Still working on it before I send a sample piston and measurements into Venolia. Here's a picture of two suspected 1300 veloce pistons.
Thanks,
Jim
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post #6 of 7 (permalink) Old 03-20-2017, 02:50 PM
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Below is a 10.7 compression ratio piston in a 79mm bore GTA engine. These are JE forged with crowns cut by JE to clear the big valves and yield <11:1 with the CC combustion chamber information provided.
These came from Paul Spruell.
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Last edited by Gordon Raymond; 03-20-2017 at 02:51 PM. Reason: typo
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post #7 of 7 (permalink) Old 03-20-2017, 11:05 PM Thread Starter
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Beautiful!
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