3 Chrysler heads compared. 70hp, 75hp, 85hp

[snwwlkr]

Top head in pics is the 70hp, next down is the 75hp, 85hp on the bottom. These were taken off of complete motors I have. Granted I cant guarantee they are the original heads, regardless these are my observations.

All 3 heads spec for the same plug- L20V
All three show the same size bore in the specs.
Blocks on all 3 appear to be interchangeable except for the head bolt size. All in-letting around the head bolting surface on block for water passages etc appears the same.

1-1970 70hp Chrysler mod# 707HA
-2 piece head with 1/2 inch sized bolt heads and small head bolt holes, 5/16 diameter.
-temp sensor located on block instead of head.
-must remove head to access thermostat.
-Part# on head= 370995 and 369 on side of head.

2-1982 75hp Chrysler mod#756H2H
-2 piece head with 1/2 inch sized bolt heads and small head bolt holes, 5/16.
-temp sensor is integral as one of the smaller 3 bolt heads.
-must remove head to access thermostat.
-Part# on head 370995 and just 370 on side of head.

3-1973 85hp Chrysler mod# 857HE
-1 piece head with 9/16 sized bolt heads and larger diameter bolt holes than the other two heads, 13/32.
-temp sensor is integral as one of the larger bolt heads.
-can access thermostat via a dedicated thermostat cover. Head does not have to be removed.
-Only part# on head I could find appeared to read #886

 

[tater76]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

Yep, lots of the older heads interchange with similar hp motors when it comes to Force/Chrysler OB'S. I currently run a 1985 Force 50hp head on my 1973 55hp Chrysler, and a 1989 85hp head on my 1974 75hp. Pretty interesting, and good to know if you melt/warp a head.

 

[snwwlkr]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

Thats pretty much why I posted this as I see it as adding to the data base of info here.... i.e., the 1 piece head like is on my 85hp, does the same one piece head appear on 70hp and 75hp motors or is it more for the higher HP motors?? And if so is it the small or large size head bolt. Also is there any difference in the displacement, compression or whatever of that 1 piece head?? Also to me the one piece head with separate thermostat cover seems like such a much better idea..

Along those lines of thinking.. Is there a "best" head for these three motors be it a Chrysler or Force head?

 

[tater76]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

Excellent question! I think the idea of a removable thermo cover is great, as well as a removable heat sensor. My 89 head on my 75hp has what looks to be a pressed in sensor? I would like to see a screw in type in case I have to replace it. (I have never looked at it very closely)
As for the bolt sizes, my 1974 75hp head used the same size as the 1989 85hp. I cannot confirm what heads will fit what motors besides what I have listed. However, it looks like the manufacturer decided to do away with the two piece design for a reason. Just for giggles I put my old 1974 75hp head on a newer 90hp block after I read your post, and it has the same bolt pattern, and seems to match up well, but I am not sure about cooling passages?

 

[fucawi]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

From what I see not convinced ..did you measure the head volume of each head? syringe oil into the space and hence measure volume..looks like different water ways on one ?

 

[snwwlkr]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

I know looking at the 85hp vs the other two you would really think that no way the blocks look the same. I thought the same thing at first. Included are two pics (first pic is 70hp, 2nd is the 85hp), one of the 70hp block and one of the 85hp block. Except for the head bolt size they look exactly the same. I even compared head gaskets and the fit each other perfectly and are the same.

I was in a hurry, couldnt find a syringe but I did do a water volume test in the heads and they appear the same. Need a syringe like you say to be more exact. There are very minor differences in the casting of the head chamber but basically VERY close.

I do wonder where the extra HP comes from if bore size is the same etc. But, from reading on here I somewhere was under the impression its mostly via porting??? Not sure if that correct. Maybe I need to get a syringe and volume check the head again.

Note.. I see my pistons on my 85hp motor are stamped ".020". I was told that motor was rebuilt and it does appear to have all new gaskets everywhere. I'm assuming that .020 means it was bored .020 over. The block is stamped like that also. If it is bored .020 over what can expect out of that as far as HP?

 

[Frank Acampora]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

In terms of heat rejection, the one piece head is not nearly as good as the others. If you look closely, you will see that the cylinder water jackets extend into the side of the head and that is the reason for the perforations in the head gasket. In later heads, these perforations also help to keep water from collecting behind the gasket and corroding the aluminum.

However, when the thermostat opens, the only extra cooling delivered is through the cast-in tube running down the head-- not much surface area there.

The two piece heads have the entire surface of the head covered in water when the thermostat opens and therefore coll better.

Later two piece heads had a separate thermostat cover, just like the Force engines did. Force heads will swap onto your engines.

Bolt size was either 3/8 on 35, 45, 55, and some 3 cylinder engines, or 5/16 most common and used on most of the bigger engines. If a 5/16 bolt engine is stripped, it can be re-tapped to 3/8 with no difficulty and all that is necessary is to drill out the head holes to clear the 3/8 bolts.

All three engines you described have the same bore and stroke. Horsepower is developed by changing port size and timing, different carbs, and changing the RPM where horsepower is developed. Compression ratios are different but I have never cc the heads. It simply was not important. I do believe it is a function of port timing with all heads being the same volume. Chrysler did make two different volume heads: One for 3.3125 bore engines like yours and one for 3.375 engines. The larger can fit on smaller bore engines with a slight reduction in compression, but the smaller head can not fit on big bore engines without being relieved to clear the piston baffles.

Adding to any confusion is the fact that Chrysler used the same casting number for heads, blocks, pistons, and crankshafts. Example: The 140 is a 4 cylinder 3.375 bore X 2.875 stroke engine. The crankshaft has the same number and looks exactly the same as the 3.3125 X 2.80 stroke 120. The block simply has a paint mark on the bottom of the lower cylinder. The 125 is and even in the force engines have a bore of 3.3125 and stroke of 2.875 yet it looks exactly like the 120 and indeed except for pistons and head, almost everything else will swap.

The same head gasket was and is used on both bore size engines and MANY parts are swappable between all engines.

.020 over at this bore size will give less than one cubic inch displacement. you will not see any increase in horsepower.

 

[snwwlkr]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

I took a good look at the one piece head from a cooling standpoint and you are very correct there Frank. I definitely see the better cooling advantage of the two piece head..

That said I would say the disadvantage of the two piece is you have to pull the head and even separate the two pieces to access the thermostat. gaskets aren't cheap. Note link included Frank as I had already read and bookmarked your post about making cheap gaskets (reply #3 in that thread). This would work for the gasket between the two pieces. I will be looking for a two piece now with separate thermo cover.

http://forums.iboats.com/showthread.php?t=418679&highlight=head+gasket

The other disadvantage is the two piece head just invites another place to have a leak, be it internally or externally...

 

[vt750dc]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

i have a 1974 75hp and it has a 2 pc. head with a 2 bolt removable thermostat housing, and has a threaded temp. sensor

 

[fucawi]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

The purpose of the tube running down the head adjacent to the spark plugs is to bring extra cooling to the plug and hence prevent overheating of the centre electrode which causes pre ignition ....This pipe is common on many 2 strokes inc Yamaha with which I am also familiar

 

[snwwlkr]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

Frank, hope you read this.. You said "One for 3.3125 bore engines like yours and one for 3.375 engines. The larger can fit on smaller bore engines with a slight reduction in compression, but the smaller head can not fit on big bore engines without being relieved to clear the piston baffles."


How much compression would be lost if the larger head was used on the smaller bored/pistoned engine.. In terms of HP say on a 85hp???

Asking because I'm looking at buying a newer black force head off ebay that may or may not be the larger bore. My head has a un-fixable plug thread.. Spit out the plug on the river. It had been semi repaired by PO with a helicoil... I was hoping it might hold but to far gone..

 

[jerryjerry05]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

What are you planning on putting the head on?

 

[snwwlkr]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

1973 85hp Chrysler. Its the one piece head in this thread that is stripped...

What are you planning on putting the head on?

 

[Frank Acampora]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

You probably won't notice the difference, even if the head is from a 3.375 engine. I did it with a 4 cylinder engine and compression was only a couple of pounds different with no loss of performance on the water.

Some later model 85 HP Force engines from 1984-89 still had the 3.3125 bore. If it is that head, then there is no difference.

If you do see a difference and it becomes important, you can have the head milled .040-.060 and still clear the piston. Try it-- install the head without a head gasket. The piston will clear. This is how much you can mill the head.

 

[snwwlkr]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

First off... Thanks much for the answers, helps very much....Not to wear you out Frank but.. I'm pretty much with you now on the better cooling abilities of the two piece head so thats the heads I've decided to search for and use when needed. That said. between the Force type two piece heads, black with vertical fins, and the Chrysler type white head with spider web fins... Do you or anyone else think one is better than the other as far as cooling, quality, design or whatever?

 

[tater76]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

See my PM to you.

 

[Sea Stomper]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

I do wonder where the extra HP comes from if bore size is the same etc. But, from reading on here I somewhere was under the impression its mostly via porting??? Not sure if that correct.

View attachment 107411View attachment 107412

Much of the horsepower difference between the 75 and 85 is in the carburetors and specifically the carburetor throat size. The 85 throat size is visibly larger in diameter, letting in more air and jetted for more fuel. It's my opinion that when you bolt 3 each 85 carbs onto a 75, you get an 85.

 

[tater76]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

I have that exact setup, I pulled carbs from an 89 85hp Force and put them on my 75hp Chrysler. It runs well in the barrell, but I have not lake tested it yet. When I pulled both sets of carbs I could see no physical difference between the two. The jets may possibly be different between the two, but I cannot confirm this. There may be size differences between certain years? The only reason I swapped was because I bought a parts block with freshly rebuilt carbs. I am not under the impression that this will make my 75 into an 85. From my understanding it is absolutely the porting that makes the hp of the block not bigger carbs. When I get it in the water I will let you know Hopefully an expert will chime in on this

 

[snwwlkr]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

Hmmmm...I will measure the carb bores on my 3 motors asap and post my findings.. Probably tomorrow... "Sea Stomper".. Just to be sure, are you talking about the intake side of the carb bore, meaning, where the air comes in??

I will say though... If its as easy as changing carbs I would be looking for higher HP carb sets for sure.. Carbs can be bored also.. They can do it on dirt bikes. I know that for fact though not sure how much they can be bored...

 

[snwwlkr]

Re: 3 Chrysler heads compared. 70hp, 75hp, 85hp

Checked my 70hp against my 85hp carb throat size (the end of the carb the air comes into)... No difference between my 85 and my 70.. 1.5", checked with calipers.. Not saying it isnt possible my carbs got switched, replaced or whatever sometime since the early 70's.. Thats just what I found..