Technical swapping out flat tops 7.5 pistons for domed 10.1 early hemi

it's been about two years since my 1965 dodge royal 4 door 241 hemi rebuild and it now has approx 1500 miles on it, not even broke in yet. ,,,engine runs great and I am thinking about installing a set of Ross forged high compression 10.1 pistons from stock 7.5 flat top pistons in a 1954 241 hemi.
question is , will my old stock rod length work with these new domed piston ?
I'm assuming they will because the dome is the only thing different protruding into the cylinder head chamber.
will I have to rebalance the rotating assembly again with new pistons ?
I'm pulling the engine out anyway and would like to make a change on it
specs are now >>>> It does have a step up on a reground neilson's cam in it already with ported and polished heads. what are your views ? I'm NOT looking to race the engine by any means but am looking to increase horsepower and the fuel mileage on the dodge.

you can see the full step by step Hemi rebuild full atricle in search of " rod knock in my 241 hemi" on the " forward look.com"
see pic at lower .
Thanks guys





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Reply

 

Thanks Ray !
these domed pistons would be custom made to order. so. I would need to give them exact measurements of each cyl bore to be precise. correct ?

 

I believe that Ross will make the pistons to fit. Not that big a deal. A little backwards from normal. But look at race cars. They swap pistons all the time without reboring.

 

I'm stopping at my machine shop that built the 241 hemi today to ask a bunch of question. I'll get back to you all.

 

Why don't you call Ross and ask them? Get the best answer there.

 

The rod length is determined by the pin placement on the new pistons. That is a call and ask question but I am sure that they make pistons for your application. Listen to @RichFox here.

Yes you will have to rebalance the new pistons will be a different weight than the old pistons. you will also have to clay it for valve clearance prior to running it. May have to cut or modify the pockets in the pistons. You may even want to opt for a different some and try and keep your compression down around 9.5:1 with those iron heads. I have heard people say that you can run higher compression on a hemi but I have not seen it proven yet.

You d have yet another option, with the lower compression that you are already running you could just huff it.

 

Compression is harder to build in early hemi engines because of the large chamber. It sometimes will cause piston to valve interference. But if you have it and give it to your piston manufacturer they will put the right size reliefs in the right places. And they have seen plenty of early Dodge engines already. You still have to check. Ross made pistons for my Bonneville Lotus 907, which was new to them at the time. I had to maintain bore size to fit the drop in sleeves and class size requirements. 4 valves, Crower one off cams. Big compression. Custom size wrist pins. Everything fit because I was able to give them the right information. "If you can measure it. You can make it" Brown & Sharp motto.

 

Good info above.
We have had pistons made to fit existing bores so it is not unusual, just different. But, if you find a need to open the bore, even a little bit, then consideration must be given to the ring packages. What will be available?

As to compression and valve clearance, the piston manufacturer will need to have plenty of info including cam specs so the relief can be properly cut but we have had no problem with 10:1 c/r in any of the EarlyHemi engines.

As to balance, the new forging will likely be lighter than the old casting and they will be very consistent one to the next...the castings, not-so-much. You should plan on another balance job.

.

 

73RR your word I will take on the compression and keeping it alive. Actually I would listen to Rich too. I have had too many people who have no real pedigree that have told me that and having not proven it myself I just blow them off. But your word I will take on it.

I appreciate your input.

 

Thanks Beener. We have had excellent results with 10:1 and this includes some very picky 'gramma-type' quasi-restos rebuilds. In fact, a couple of years back we did a 10:1 241 in a project for a local guy that was recreating his hi-school car. With a mild re-grind on the cam it turned out very sweet. Other than cam and compression it was essentially bone stock. One of these days I 'spose I should figure out the video thing on first-fires. As long as the ignition is properly setup there is little concern.

Gary

 

good info guys Thanks !
My machine shop engine builder said the say thing regarding the weight of the new and old pistons. newer pistons are lighter today.

 

Gary . how about some prices on these pistons.... anyone ? I found one alot cheaper than Ross........ Venoila
I'm surprise to see all the manufacturers make HP pistons for chryslers and desotos but no domed high compression pistons for the 241 dodge ..I mean pistons that are on the shelf. . I would tthink there is a big demand for HP 241 pistons if they were available. what gives ? baby hemi is pretty desireable engine .
everyone likes a little more horsepower.

 

talked to race tec pistons . they said they could not make them because that piston is Too small .

 

I really don't agree with you about the demand for 241 pistons. I ran one in my altered 53 years ago. Haven't really run into a lot of them lately. I am pretty sure you are going to have to step up for custom pistons for your 241. You know all those people who said they cost more than a SBC? This is what they meant.

 

...good to hear that RaceTec is protecting our relationship.

Pistons on the shelf are the sets that get made 50-100 at a whack; there probably aren't a hundred 241 owners ready for a rebuild let alone 100 x the number of oversizes that are 'normal'. Be happy that customs are available.
I haven't saved a pic of a 241 Dodge piston but the 276 DeSoto is close:



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well the 241 hemi is out again . going to the speed shop next week.

 

disassembled the 241 down to the block today . ready to go the the speed shop tuesday. cleaned all the mating surfaces on the hemi , so that is all cleaned up ready to go.

 

here are the 241 hemi domed pistons 10.1

 

one piston, head gasket and head are at the speed shop. the head chamber cc'd at 70*. and he measured the valves . few more specs are needed on the engine and can the send the flat top piston to venolia piston co. and make the 10.1 domed pistons . movin right along.

 

has anyone out there ever installed these 241 hemi sanderson shorty headers on a 1953 - 54 dodge royal coronet royal of plymouth models ? dodge did not have much room on the drv side with the steering gear box there. my 54 dodge stock manifolds dump in the middle on the pass side and towards the rear on the drv. side. just wondering if I could modify these sanderson headers to fit around the steering gear box and starter. although they are suppose to clear the oem starter believe. but I think theses headers were designed for street rods.
here is a pic of the header for a 241 hemi . little different design than the chrysler hemis shorty headers .
what are your opinions ?
Thanks !

 

machine shop has all the specs required for new 10:1 domed venoila pistons . will ship piston to them tomorrow.

 

old flat top 7.5 piston is shipped to venolia for mock up 10.1 dome pistons .

 

The 10.1 domed pistons from Venoila should be finished making to my engine specs by next week or week after . can't wait.

 

Well I gave up on the option to install headers on a 54 dodge royal with a hemi.. just could not see how to modify some of the pipes on the headers and after the collectors to clear all the obstructions on the underside. Just not enough room underneath there. I gave it my best shot.

 

A little late to this one.

The flat 4 into 1 configuration merger fits way better. The stacked 4 into 1 configuration needs 2x as much room. If you are building your own you can easily shift the dump fore or aft.



Good news on your pistons! What did that run you ?

 

my domed piston have arrived from Venolia pistons today 10--14-15 . they look awesome . speed shop weighed the old and new pistons and the new pistons are very very close and just need to remove a little bit off and the engine will not required a balance . Nice ! $$$.
2 weeks to have the new custom pistons made up. Not bad !

 

I'm suppose to be getting my pistons back from the speed shop thursday . I cleaning up some parts and just wanted to varify with you hemi guys what direction the front and rear oil pan rubber seals face . I marked them when I took them off as " the raised lip goes towards the inside of pan .. does that sound correct ?
I looked all evening long on line to find a answer / or pic ...but no luck.

Geeee.......... My memory is really slipping after I just retired .
Thanks !

 

I believe you are correct on the oil seals. Most seals go with the internal pressure attempting to force seal out. like wind pushing sail. Also mention that most Mopar Hemi's don't require as much total ign. timing as Chevys. I believe 30 deg. total.

 

Could you post pictures of the 7.5 to 1 old pistons, and the new 10 to 1 Venolia pistons?

 

I sure will.. suppose to be picking up everything from the speed shop this afternoon. 10-22-15 .