21 stud stroker engine.

Selwyn

Selwyn

Junior Member
Hello everyone,

I was wondering if there are people here who have experience with a scat stroker kit in an 21 stud flathead v8?

What are the things I’am looking at to get this working? Any modifications to the bock itself? And is it worth it?

Thanks!
 
One challenge is getting bored large enough to fit suitable connecting rods down through. Your 21 stud bore started at 3 1/16” you need 3 3/16” bore which can be a challenge with a 21 stud block. A 37 block maybe but I have never sonic checked one of those.
Mike at H&H can supply you a kit for this I believe and will know answers on block and bore size to accommodate rods.
 
Yeah, I'm running a Scat 4"stroke crank in a 37 block with 3 3/16 pistons; 255 cu in. The Scat crank as supplied, did not fit the block, I had to widen the front and centre main bearing journals. And not all the rod journals were centred to the cylinder bores. Numerous emails back and forth to Scat, notifying them of the problem, they didn't seem to care. Hopefully in subsequent manufacture of these things, they've taken on board the issues I raised and rectified them, I dunno....If I was to do it again, I'd use a late oem Merc crank, and modify to suit; you could even achieve a 4 1/8 stroke that way.
 
On the one that was built for a 35 Coupe to run the GAR. They bored all eight cylinders pretty much out of the block to put sleeves in by leaving a step at the bottom and using epoxy to install the 3-3/16" sleeves. An original 255 Mercury 4-inch crank was modified quite a bit to get it in there and work. I bet those bottom head bolts were close. I don't know what head gasket they used. It ran the whole course with no problems. I think they used the last 81A 21-stud block type with water pump block off plates so it would look original in the 35.
 
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Basically, if you want to retro fit a 255 crank into the 21 stud block, you'll need to perform the following operations; shorten the snout and reduce the diameter to suit those of a 21 stud crankshaft. Reduce the main bearing journals to 2.399 to suit the 21 stud block, BUT, you'll also need to widen the front and centre journals to match the width of your sample 21 stud crankshaft. 21 stud blocks have smaller dia mains but the front and centre mains are longer than the later 24 stud blocks. As the later crank runs a positive rear main seal, and the early cranks run a slinger, you'll need to modify the rear of your block to suit a rear main seal. Depending on the style of your existing rear main cap, this can be easy or more involved....Now, depending on what bore you're gonna run, you may need to grind the rod journals to the 1.999 dia and run 21A type rods, you'll need to run 81A style rod bearings, not the earlier flanged style. So there is a cost saving...Should you decide to grind to the 2" journal size, you could offset grind and achieve a 4 1/8 stroke. Or, and I don't know if the rods will clear the block, if you bore to 3 3/16, you could run 99A or 8BA rods. I've always run the 2" bearings and 21A rods, smaller dia gives a slower surface speed, which in my opinion is beneficial. Note though, that the piston speed is increased, and with the shorter overall length of the piston, it is more likely to 'rock' in the bore.
A lot of work, a lot of dollars if you're paying someone to perform the machining operations, is it worth it? you asked. Well I suppose it gives you bragging rights; 'I've got a 255 cu in 21 stud', but in my opinion, having done all this crap, is that a 221 cu in 21 stud, built with a few traditional mods, is far and away better than the bigger cube motors, on account of the rotating/reciprocating assembly being lighter, they are freer revving engines.
Would I do it again? Nah, it was an experiment, and whilst it is a fine running engine, it is nowhere near as exciting to drive compared to its predecessor.
 
If going the late Merc crank route (which is what I'd do) -> I would run the full-floater 21A or 91A rods and have the rod journals sized to match. If you're going to the trouble of reducing the rod journals to 1.999, you might as well offset grind the journals for another 1/8" of stroke. When doing this, I would do all the crankshaft and mains work first, then measure the exact compression height needed for your deck heights. Then I would square deck the block to even up both sides and I would order my pistons to exactly match the measured compression height -- based on the combination of the stroked crankshaft, 21A rods and the cleaned-up decks.

Also, depending on final bore size goals, I would FIRST locate metric rings that would work . . . which will determine your options for bore size. Metric rings make a huge difference in overall parasitic loss and can be found with much better materials than our ole "Grant style" cast iron rings. It is no big deal to order your pistons to match the metric ring groove sizes and bore diameter.
 
Having recently completed a 21 stud block with Scats 4 inch stroke crankshaft they make for the application here's what I did maybe this hopefully will help others. First the block was only over bored .060. the Scat crankshaft which was purchased last year actually fit the block correctly so no block modifications were needed. For rods I used 91A the Scat crank 1.999 rod journals have a fillet radius to get the rod bearings to float the outer edges of each bearing half needed to be modified to clear the outer journal radius. The custom forged pistons were the difficult part. The piston skirt length has to be determined to clear the counter balance surface from the wrist pin center line. The Scat crankshaft counter balance dimensions are naturally smaller than the Merc crankshaft. So the piston dimension to clear this application are needed for the pistons to be made. Overall once the pistons were made everything went together easily.
Ronnieroadster
 
Ronnie, thank you for verifying that Scat have fixed their crank to allow it to fit into the block. I was banging my head against a brick wall trying to get them to acknowledge they'd made [my] one wrong...
 
Given that SCAT seems to have fixed their crankshaft issues for a 21-stud engine, then I'd go this route - it will be a heck of a lot cheaper than doing all the custom work required to drop a later Merc crank into the block. I am a bit interested in the journal radius issue that Ronnie spoke about? It seems that the journals are a bit narrow or have a much bigger radius (which is a good thing) on the edges of the journals. My bet is that crank was designed for the SCAT 1.999 H-beam rods (which use Buick bearings).

The Buick bearings would not have the radius issue that Ronnie is talking about. What I don't know is if the SCAT H-Beam rods with the 1.999 journals have any clearance issue with the bottoms of the bores? Any information on that Ronnie?

Also, would be great if Ronnie told us who made his pistons? (Probably Ross).
 
Thanks for all the information so far! Ronnies set up sounds interesting to me. I’m curious as well why you (Ronnie) went for the 91a rods instead of the scat H-beams and where you got your pistons made.

Would also like to know what type of rod bearings went in? Are you able to show us some pictures of the build?
 
Personally I much prefer the 21A or 91A rods over Scat rods in any flathead that is not blown or a race engine as they are plenty strong and much lighter than the Scat rods. Way less expensive also. The floater bearings used with the 21A/91A rods are also very good and when set up correctly give zero issues and last forever. I always use NOS rods as I have a large stock of them.
Just my 2cents
 
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