The OHV Briggs can be made to perform very well, however they do have issues:
****Use the crankshaft out of a 12 hp flattie and the oil pan on the 28 through the 31’s, this allows the ARC balance weights to be used. Modification is required to do this to the AVS (see below)****
1.They will not tolerate (break cranks)any timing over 30 deg.-Use a degree wheel to check-do not go by the marks on the flywheel(assuming you get a ARC aluminum wheel)
2. Some 28 and 31 have the coil mounted off to the side-not lined up with the cylinder, this advances the timing-again use the degree wheel.
3. They will break the stock rockers-use EC's aluminum ($200? Or mine for $40, mine are modified stock ones guaranteed for one year)-
4. They WILL pull the rocker studs out of the head-put a plate under the head, use longer studs or use my Stud Bridge ($50.0O also adds rigidity to head to prevent head gasket from blowing)
5.Use a copper head gasket (EC or make your own)
6.AVS engines (identify by oil tube going into block, non AVS goes into oil pan) both 28 and 31 have some that are AVS
7.The AVS WILL break the timing gear, no aftermarket available at this time, (good chance it will also break the oil pan $80 and the cam $150, highly suggest you not build a AVS with a AVS crank in it).
8.You can take a 31 AVS and put a 28 CI flathead oil pan an crankshaft in it IF you weld the top and bottom and the oil fill hole in the block. This will allow you to run the small journal crank, brass syncro eliminators, ARC rod and piston. Block must be machined back down to get the oil pan to seal. I ran two of these engines this year in a Mod and a Pro-had no problems with them, won regional in the mod, was in 2nd at the Nats until a guy jumped the restart and put me on the ground. The deal with building the 31 is you get a factory bore and don’t have to pay $150 to bore a 28 (assuming you cant find a non AVS 31) A 31 CI WILL BEAT A 28 ALL THE TIME, EVERYTIME PERIOD! You need to be able to weld aluminum and access to a milling machine, if you do this make a jig to support the block so the machinist can clamp it on the milling table. I used a heavy peice of square tubing, used one of my broken cranks, machined it down, bolted it to the block through the flywheel side and made plate that bolted to where the head goes on the block. This keeps the block from warping when you weld it. You WILL get porosity when you weld the block, don't worry about it, gasket sealer will seal it.
Want to go fast and the engine live? Do this:
Cut the head .125
Use EC's valves, springs and retainers
Use Precision Cams Cheeta cam, Dyno has one also just as good
Small journal (non AVS) crank, ARC rod, brass eliminator plates.
Copper head gasket
ARC aluminum flywheel
EC's Carb, if you run Pro use the big Tilly (ask George)
No more than 30 deg timing
Roller rockers (EC's or mine)
Stud plate (or my bridge)
Don't try to use dual springs on valves-engine WILL break over 6500 RPM-
Don’t waste your time using a large journal AVS crank, that balances, they WILL break the timing gear.
Note" I no longer make stud bridges or rockers, use EC's