Inliners International Forums General Discussion Engines 261/235 parts interchange?

Well, it looks like the 261 block I got will work (see 261 max overbore question). However after having it hot tanked and the bores measured the machinist noticed the rear main cap is cracked. So my question now is will a 235 rear main cap work? I know I can find one of these in the junkyard. Also will a 235 crankshaft work in a 261? I'm guessing the 261 crank is weighted differently. I am going to take the crank in to get it turned and with my luck, something will be wrong with that too.

I think they are the same but have it line bored. The cranks interchange.I used to thing the 261's had bigger counterweights but was corrected.

correct. the cranks are identical. the main will swap. yes it should be line bored. the only difference between 235 & 261 is
1)the bore size,
2)261 had heavier rods,
3)58-62 261 had full flow oil filter,
4)261 had the steam holes in the top of the block & in the corresponding surface of the head.
any thing else will swap between the two engines & a 235 head will work on a 261 after it's drilled for the steam holes.
robert.

Robert is correct.

Before you have it align bored, have the top of block surfaced to clean it up.

When they do it, it will parallel the block face. AKA "squaring the block".

Good luck,

Also, the piston pin diameter is larger on the 261.

Thanx for all the info guys. I'm really a happy camper now. I knew the rods were different with the bigger pin but never would have thought that the cranks were interchangable. I had already planned on milling the deck surface to raise compression and I have a 848 head that I'm putting on.
SECOND QUESTION: What is the compressed thickness of a typical 261 head gasket? I want to bring the piston to zero deck but don't want it too close to the head. Or should I not worry about blueprinting it this much and just take off .020"-.030"? .040"?

Someone correct me if i am wrong but isn't hard to align bore the 235/261 because the crank jurnals are all different sizes?

Just take off what is needed to clean the block surface.

This is so the align bore (crank centerline) is parallel with the top of the block and has nothing to do with raising compression.

To raise your compression, do it with pistons.

Also you don't want a 'zero' deck height as these early rods stretch at higher RPM etc.

Good luck,

OF ED, You are correct. The stock journals are different diameters and need to be sized the same or corrected with the proper sized bearing for each journal after align boring.They are as follows:
Front Main: 2.6835 - 2.6845
Front Int: 2.7145 - 2.7155
Rear Int: 2.7455 - 2.7465
Rear Main: 2.7765 - 2.7775
Rod Journals: 2.311 - 2.312

Dear Drew;

The purpose of the 'align bore' is to "square the block" making the crank centerline parallel to the top, exactly.

I believe that a special jig is used in the case of the different bearing sizes. Afterward, stock bearings may be used.

The 'end user' then uses whatever undersize (if any) his crank is etc.

John, High compression pistons are not available for these engines and milling the top of the block will increase compression. I'm well aware of rod stretch that's why I'm just wondering how much I can take off. Also I beleive that "align boring" is to make all the journals round not to "square the block". The top of the block is milled parallel to the crank centerline thus "squaring the block"
So each journal is differnt size, great, sounds like more money at the machine shop.

Sorry to correct you Deuce Coupe, but McGurk made 2 versions of forged "Power Dome" high compression pistons. Hard to find them today - saw them only once on Ebay. Wayn mfg Co. offerd me a remake of those pistons for $600! but some guys had problems with that company.

align boring make the journals square to them self and to the block. If your "holes" not round anymore you have a serious problem anyway.

Duece,the typical 261 head gasket has a compressed thickness of about .055,Detroit gasket used to have one that was .042 compressed.
Using a 848 head,milling the block and head .030 each,will give a 261 around 9-1 actual compression with flat top pistons set no more than .010 below the block deck.You may have detonation with higher compression depending on cam grind.
All the stock style oversize replacement pistons I've seen place the piston down the bore .030-.040 at TDC.Do a mock up with the pistons in the bore before milling the block.Do pay attention to the piston to valve clearance on the intakes if milling the head and using a hi performance cam.I never had a interference problem,but some supposedly do.

Deuce, You may also have noticed the use of various .002 thick shims on the mains to obtain
proper bearing clearances which makes the fitting of bearings easier.
John, You are right to the extent of using various diameter cutters and using a simple .010/.010 crank which to me is fine for a performance street engine. I think align boring is over rated for the street. I was providing some numbers for Deuce in case he wasn't aware of them. Personally, I think having the crank
magnafluxed and balanced after machining is enough.

Drew, after 1958 they used a more precice equipment to do the blocks. No more shims where used in this engines. This also alowed the use of the neopren crank seals. So it depends on the year of the engine. Check the bearings with Plastigauge. If out of specs, have the crank regrinded and use oversized bearings.

Yeah, spend your money in a good magnaflux and balance job than in align boring.

Frank

Hi Frank !
Thanks for reminding me about the later machining
process. I'm always stuck in 1955! Hey, have a great time in Sept. at Head Banger. Post some pics when you can.Always enjoy them.

Several companys make pistons for these engines. Ross is probably the best.

You also need to talk more with people that know about these engines before you make a huge error(s) etc..

Good luck,

Dear Drew;

The block needs to be 'square' to have even compression. Not so much for stock, but 'high performance'.

In the 1950s & 60s QC on these 100 horsepower engines wasn't that great. Today; On a 'Hot Rod' one It's necessary to have all equal.

Imagine if the crank isn't straight, say closer on #1. Then each cyl. going back will have less compression that the one before.

Like a house with the foundation on a slope.

And yes, all cranks should be ballanced & magna-fluxed.

John, I agree with you on the need for extra quality machining on any hi-po engine. It's just that I don't think it's necessary on a street engine, even one slightly modified and especially an L6 like mine. Believe me, if I was ever racing an inline in any category, it would have the best build I could afford.

Jahns use to offer high compression pistons for 216-235-261 engines.1956 is the year Chevy quit using shims on the main bearings and started using finished inserts.You would think that they would have done it in 1955 with all the other changes.(TECH TIP) get a FACTORY SHOP MANUAL for the year of car or truck engine you are working on.Those aftermarket repair manuals don't have enough information and what they do have is not always correct.A FACTORY MASTER PARTS CATALOG is also a must.A HOLLANDER EXCHANGE MANUAL used by wrecking yards can also come in handy.
EvilDr235

I guess I wouldn't say pistons made 40-50 years ago are "available" I didn't even think about spending $600+ on custom made pistons, a huge error on my part. However I think I will stick with my original plan of milling the block deck and head surface to raise the compression.
I am still confused about "squaring the block" maybe it's just a different use in terminology. I would say align boring the mains is to make all the journals round, make them all the correct size, and make them all in alignment with each other. The deck surface is milled parallel to the crank centerline thus making the block "square" The cylinders are bored 90 degress to the crank centerline thus making them "square" That is what I would call "squaring the block" If the deck surface were at a "slope" then the deck should be milled at a slight angle (which would be parrallel to the crank centerline) making all the pistons at the same height.
Personally I would always align bore the mains if swapping main caps from another engine.
Thanx everyone for the specs on the mains and head gasket thickness. The shop manual I have for this engine doesn't list any specific specs but my 53 shop manual does and I didn't know if they were the same between 235s and 261s. I didn't mention it before but my block is a 59 so I don't have to worry about those main shims.