The pinion access hole in the bottom of the chassis is a huge mistake by Tamiya. Including tape to cover the hole is a Band-Aid that just doesn't work. I designed a reusable plug. I printed mine on my home printer and I'll try it out this week. I have uploaded my design on Shapeways for sale.

https://www.shapeways.com/product/SE...t-chassis-plug

20170807_212150 by JatoTheRipper, on Flickr