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I have a 1972 1.9 that has flat top pistons... I am going to install a big valve head on it...with a mild cam.... It does not seem that the cubic inches I would gain would be worth boring the block and installing new pistons....anyone have thoughts about this ?? Is the power gain worth the $$$ it would cost ??
 

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If your current 1.9 flat tops are in great shape and in spec, I wouldn't bother upgrading to 2.0 pistons. Gains are very small vs cash invested. Bigger valves are much better bang for buck investment at this point.
 

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Opel Rallier since 1977
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If your current 1.9 flat tops are in great shape and in spec, I wouldn't bother upgrading to 2.0 pistons. Gains are very small vs cash invested. Bigger valves are much better bang for buck investment at this point.
I am of the same opinion. <5% increase in cubes is not a lot. You would gain a couple of tenths of a point in compression ratio... so the change would be felt in the butt-dyno for sure. But it is a lot of work/cost if the present engine is in good shape.
 

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I have a 1972 1.9 that has flat top pistons... I am going to install a big valve head on it...with a mild cam.... It does not seem that the cubic inches I would gain would be worth boring the block and installing new pistons....anyone have thoughts about this ?? Is the power gain worth the $$$ it would cost ??
I’m not really the guy to ask but I saw a post from Bob who is the guy to ask saying that the gain is much less than you’d think, somewhere between 4-6 HP if I’m not mistaken. That’s just the pistons alone.
Hopefully someone will correct me if I’m wrong.
 

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RunOpel
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I'm glad you mentioned it Vincent. Last summer I pulled an original 1.9 from a 1970 Opel GT that had the high compression pistons. It appears to be in very good condition, although I have not taken it to the machine shop, so I don't know if there are any issues. If assuming there are no issues, I have been wondering if I should keep it at a 1.9 or do the upgrade to 2.0? My goal is to just have a fresh engine so I can have fun driving mostly around town with occasional longer distance drives. The current engine in my 1971 Opel GT is a 1.9 with low compression pistons. If I do keep it at a 1.9, will I see much of a difference from the low to high compression pistons?
I appreciate any and all opinions and advice :)
 

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Going form the low compression to high compression 1.9 pistons it a definite plus! Even better if it's combined with larger 2L intake valves. More air + more compression = more power!
 

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RunOpel
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That's what I was thinking Vincent. I'm just not sure what is involved in the rebuild. You mention the larger 2L intake valves. What else do I need to make this rebuild happen. I know the block, head and cam have to be taken to the machine shop. If the pistons are okay, do I stay with the original or get new ones? This will be my first rebuild, I'm a rookie.
 

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IMO the upgrade to 2.0 pistons is well worth it if you are rebuilding the shortblock anyway. Rather than .020” or .040” oversized 1.9 pistons, the 2.0 pistons are a nice (small/incremental) increase in both displacement and a marginal compression increase too.

That is, unless you have a worn out low compression 1.9, in which case the performance boost will be far more noticeable.

That said, when I was just starting out in Opels, my first Opel had a 1.9 that was .020” overbore with a big-valve 1.9 head, a CR-150 solid lifter camshaft, a Pacesetter header, an Ansa exhaust, and a 32/36DGV Weber.
My good friend John has an identical engine with identical parts and prep, EXCEPT it had TRW 265 Chevy forged pistons in it (.090” over). We both had 3.67 rear axles and 4-speeds.

So, my engine was 1917 cc’s and roughly 8.8:1 compression. His engine was 1989 cc’s and 9:1 compression. From a roll on the highway at 55-60 mph he would get the jump from the moment the throttles were opened and continuously pull away from me. Nothing crazy, but repeatable every single time.

So yea, IMO it’s worth it.
 

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Opel Rallier since 1977
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It is absolutely worth the compression increase! It is for exactly why Bob describes.... a big increase in low to mid RPM torque. You can be sure that it will be a completely different feeling engine going from the low compression (actually around 7:1) to higher compression (actually in the mid to upper 8:1 range). Also, it will counteract the lower dynamic compression ratio that results from a larger cam.

As far as the old 1.9L high compression pistons that you have, I would be ready to change them out. The rings may be shot, the ring lands in the pistons may be worn, bores may be out-of-round an tapered, etc. I simply don't rebuild an engine anymore without fresh pistons, rings and bore. This matter is one of the foundations of a good running engine.
 

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RunOpel
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Good advice, I appreciate your wisdom. The engine I currently have is not bad. Dose not burn oil and the oil pressure is 3.8 on startup and idles at 3.2 However, it does have 123,000 so its pep and get up is slowed (low compression pistons). It still has decent compression though. The plan is to keep driving it until I rebuild the engine I pulled. Then swap the engines so I would have a fresh reliable engine for a long time to come.
 

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Über Genius
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Don't bother rebuilding a 12 bolt head, IMHO
 

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RunOpel
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The one I pulled from a 1970 Opel GT is a 10 bolt. I'm curious why you wouldn't rebuild a 12 bolt?
 

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The exhaust valves seats were heat treated on the 12 bolt heads. They are notorious for cracking between #2 an #3 exhaust valves thus becoming junk head.
 

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RunOpel
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Ah good to know. Glad my spare and the one in the car are both 10 bolt :)
I figure I can rebuild one while the other is being used and just keep the cycle going by rotating when one gets worn out 😛 Its just time and money, right???
 

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Yes, you can drive one engine while you build the other!
 

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Opel Rallier since 1977
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Yep, have had 2 later heads. The first from '73 cracked a bit in #3; eventually replaced. The 2nd is a '75; baaaad crack out of#2. It is now a cam test fixture....
Some can be fixed but at some cost; we may have to start doing that sooner or later.
 

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Opel Rallier since 1977
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BTW, one other reason to rebuild is that a lot of engine bearings of that era get brittle and can chunk out. Dunno if the Opel factory bearings are bad for that or not. It is seen a lot in Mopars.... particularly in the cam bearings.
 
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