Farm Equipment - 426 compression ratio

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426 compression ratio

Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=209685
Printed Date: 22 Mar 2026 at 8:11am
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Topic: 426 compression ratio

Posted By: 8070nc
Subject: 426 compression ratio
Date Posted: 12 Jan 2026 at 9:55am

This is a mental experiment and i should probally quit thinking so much. I have a DTA 466 with a p pump in a medium truck. It will start in 20 degree weather like its summer. Its my understanding that those engines are 17to one compression. Would it be possible to have a set of pistons made for a 426 allis to raise the compression and what problems could that solve ir create

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1984 80780
1957 D14
DES 300 with 25000 engine
616 tractor

Replies:

Posted By: DrAllis
Date Posted: 12 Jan 2026 at 11:03am

I'm sure you could have custom pistons made with a few less cc's in the bowl, raising the compression to improve cold starting. Higher compression ratios are harder on the crankshaft, conn rods and all bearings. If you took two identical engines (except 16 to 1 and 17 to 1) and loaded them at 100% load until something failed, the 16 to 1 will run more hrs. it's a compromise. To get one thing, you might have to give up something else. DT-400 series have larger diameter mains and rods bearings. M & W piston kits from years ago did start as good as an IH and some on here can attest to that. Don't think you can buy them anymore.

Posted By: NEVER green
Date Posted: 12 Jan 2026 at 3:35pm

Throw in some total seal gapless second rings and bam!!

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2-8050 1-7080 6080 D-19 modelE & A 7040 R50

Posted By: Kevin in WA
Date Posted: 12 Jan 2026 at 8:22pm

The M and W pistons for the 426 were the same CR as the stock pistons but started so much better because of a better seal at cranking speeds, and less drag which allowed them to spin faster, wish they were still available...

The M and Ws for the 301 were slightly higher at 16.3 instead of 16.25 or 15.5 in the 200 and up

Posted By: Lynn Marshall
Date Posted: 12 Jan 2026 at 9:42pm

It's been a long time, but didn't M&W offer two different sets for the 426? A standard set with a normal ring configuration and then another that had the top ring very close to the top of the piston. Were they an SP 426 or maybe called a total seal kit? I remember that top ring was special in design and was supposed to expand against the cylinder wall tighter as the power was applied. They certainly did improve starting and had practically zero blow by. I don't know if they were ever available for the big wrist pin engines.

Posted By: SteveM C/IL
Date Posted: 12 Jan 2026 at 9:51pm

Yes they were. Got a set in 220 and 8070. That top ring is a 90 deg cross section like angle iron and open to the top so compression forces it against liner. Top of ring is flush with piston top. Think it's called "headland"ring. It travels to top of bore...leaves no ridge.

Posted By: Lynn Marshall
Date Posted: 13 Jan 2026 at 7:50am

Steve, do you recall if they offered that ring design for the Deere or IH engines? I don't know if either of them really needed a compression increase. I also don't remember that design to be any huge horsepower gain, but it did improve cold weather starting. They were a more expensive kit compared to what Allis offered ar the time.

Posted By: tbran
Date Posted: 13 Jan 2026 at 7:58am

Dykes Ring, is a type of https://www.mopedarmy.com/wiki/Piston_ring" rel="nofollow - piston ring that has an "L" shaped cross-section with the back side of the "L" facing the cylinder wall. The purpose of this unique design is that the top part of the "L" is not sealed to the piston and is able to be acted upon by the hot gases produced during combustion. The force of these gases pushes the the ring against the cylinder wall to create a superior seal. The lower part of the "L" seals against the piston. This is often used on the top ring of the https://www.mopedarmy.com/wiki/Piston" rel="nofollow - piston and placed at or near the https://www.mopedarmy.com/wiki/Crown" rel="nofollow - crown of the piston due to its resistance to https://www.mopedarmy.com/wiki/Ring_flutter" rel="nofollow - ring flutter .These were designed in the late 1950's. Copied of course.

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When told "it's not the money,it's the principle", remember, it's always the money..

Posted By: Kevin in WA
Date Posted: 13 Jan 2026 at 8:56am

Yep they made a kit for the big pin 426, I have them in an 8050, 8070 and 4W220 , they are better in every way, the 301 kit was SP306, the small pin 426 kit was SP307 and large pin was SP 308

And yes they made them for other brands, I put a set in my neighbors 4020

Posted By: SteveM C/IL
Date Posted: 13 Jan 2026 at 10:44pm

Kevin answered your question Lynn. Yes they were more $ but I thought it worth it.